MaEnd h his publica[io” is k tie public dmtin md, witi approfia[e mdit, may

be re~od”ced witi.ur p~tision.

—
*

.

NLS

Discussion

Papers

.
Women’s Employment During

Pregnancy and Following Bitih

,

.

Arleen Leibowitz, Jacob A. Klerman, and tinda Waite

The Rand Corporation

Februay 1992

.

TMs project was pamially funded by grmt number J-9:J-7.:00~.l.frorn.the DePifiment of Labor. BuEau of Lahr

Statistics, and gmnt numkr 2-5P50 HD12639-1 1 from the Natiorial.Institute of Child Heatth ~d Human

Development to the Rmd Poptdation Center. Opinions stated in this p-. do not necmsarily reprwent the

official position or policy of the U.S. Depafient of Lahr OFNIC”H”D”.
,

.

Women’s Employment During

Pregnancy and Following Birth

Arleen hibowiti

Jacob Alex Klerman

Linda Waite

Revised: September 1991

,~e RAND Corporation

1700 Main Street

San& Monica, CA 90406

213-393-0411

This research was ptiaUy fundd by Grant No. J-9-J-7-0091 frorn”the Dep_ent of

Labor, Bureau of Labor Statistics, md Grant No. 2-P50 ~12639-11 from the National

Institute of CMd Health and Human Development to the Mm Population Center. The

views expressed here_do gg~riecessfily represent the position of the Department of Labor

or NIC~.

. .

.

1
2

INTRODUCTION

Dufing the last three decadw, the “working mother” has become the norm rather than a

rarity. In 1960, fewer than one in five mothers with c~dren under age sk (18.6 percent)

were in the labor force. By 1987, tils percentage had tripled, reaching 57 percent (U.S.

Bureau of the Census, 1987, Table 624). Current participation levels for mothers of younger

ctildren are even more striking. Fifty-tiee percent of married mothers with cfildren 1 year

old or under are in the Iatir force (U.S. Bureau of tbe Census, 198~.

Previous research has consk-tenfly fo~d thatwomen with young children are less likely

to participate in the labor force than those with ody older ctidren ~lncer, 1962, Gronau,

1973; Heckrnan, 1974). Today labor force activity reaches Mgh levels soon after the birth of

a child, and many women interrupt work for ofiy short periods of time. Mthough hti the

new mothers have returned to work within a year after giving birth, the factom that affect tbe

timing witiln that year are not we~ understood. Similarly, the factors that influence how

long women wok during their pregnmciea have not been fuHy explored.

The analysis of women’s increased work effort during pregnmcy and rapid return to

work after childbirth caU for a research strategy using data that can distinguish among work

patterns by month of return to work rather than by year. ~Is pa~r uses panel data from the

National Longirudind Surveys of Youth ~S-Y) to examine whether perinataf lahr supply

is positively related to women’s rerd wages. We ~so expect to find a negative relation

between mothers’ work efforts and other household income.

~Is paper is orgardzd into five “sections: The first section reviews the previous work on

tis problem. The second section. outtines a conceptual model of employment choice, and the

third section describes the data and the empirical methods used. The foufi section presents

r=ults. Our conclusions are summarized in the find section.

LITERATURE REVIEW

Emly Labor Force Panicipation &W) is a good predictor-of labor force activity five to

ten years after the first birth, even controlling for severaf demographic and economic

predictom of LFP at the time of the first birth md for subsequent ferdhty (Mott and Shapiro,

1983). Mott and Shapiro suggat that either women differ in their tmderlying pmpenaities to

work, or fiat work during the time of the first birth indirectly stimulates market work by

increasing work expedience and hence market wage rates.

Even (1987)” expficidy addresses the intraperaond “cofielation in labor supply over time

in his hazard estimation of data from the 1973”National Suwey “of Family Growth. He finds

.

*

.
3

that the aggregate probability that a woman reenters the labor force after the birth of a child

falls rapidly as the length of the interruption rises. Even attributes this to tie combined

effects of structural duration dependence, unobserved heterogeneity, and observed

characteristics.

The work reported here differs from most of the previous work because it examines

continuous measures of lahr supply, ratier than labor force participation at a point in time

before or after the birth. Ufllke Even, we fit h~ard models to labor supply both during

pre=mancy and after birth, and &gin to address the correlation between the two decisions. fn

addition, our analyses relate to births occurring between 1979 and” 1985. These data

therefore cover tie recent period of very high labor force panicipation immediately before

and after birth.

BEHAVIORAL MODEL

This study investigates tie determinants of lahr supply behavior near the biti of a first

child. We assume that the choice of labor supply during tils period--as weff as the choiws of

the number and spacing of children and expendirore (of time and money) on children--

maximize a fifetime utility funclion subject to a fifetime budget constraint. Instantaneous

utility is a function of consumption, Iahr supply, and the point in time relative to the birth

of the child.

In general, the solution to the households problem for IaWr supply near the birth of the

first child can be expressed as a function of wages, w(r) ,“and tie reservation wage, r(f). The

woman works whenever

d(r) = w(f) - r(f)> O or simply w(t)> r(t)

.

~ls mervation” “wage is the value of time outside of the labor force. FomaUy, it is me wage

such that if the woman were offered it, she would be indifferent kttieen working and not

working. . .....
. .

4

/.

:t=

J

:i.

7

i“

i,..

... ..

-. .- .._ ..-

1 I I

=-

Resematimr Wage

“

I I I I I 1 I I I

-39 -26 11 to t2 +26 +52 +Iw

Prepmq Bti P06W

be b, Weeks

Fig. l—~]me Psth of Reservation Wage

Since the utifity of not working win vary with the point in pre~mcy or, later, tie age of

tie child, the resemation wage is a fiction of the childs age. As de~very nears, charrgea in

the value of time at home generate observed labor supply behavior. H@m 1 presents a

styfized illustration of the changes in tie vshres of marker md home time from the time of

conception, through defivery at to md finily, ~mm to work.

AS pregnmcy progresses, a womrm’s productivity on the job may f~l. ~IS dec~ne

occurs earfier in jobs that require strenuous physic~ activity md later in jobs that are less

physically demmtfing (See Desti and Waite, 1991).” me substantial physical changes

accompmying childblfi afso lower m--t productivity in the immediate postpafium pried.

We have, nevefieless, draw the wage as constit because it ia Megaf for employers to lower

a womm’s wages &cause she is pre~m~ mre~mcy Dlscriminatirm Act of 1978).1

,-

lfis aIso abshacts from real wage ~otih due b accumulating experience. Oer the short

intiwals under consideration here, this is unlikely h k a major distortion. At mme point, market

productiti~ reaches a mifimum and &#ns b ri= an. me relation ofthe new level afier return h

work relative ti the pre-intetiption” level is the subject of an ongoing literature (see Mncer and

Polachek,1974 Lfincermd ~ek, 1$s2).

.

-.
5

The value of time not-working tends to tise near birth. During pregnancy, additional

home time may be productive in avefing a premature biti After tie defivery, the presence

of an tifti”t raises the value of the metier’s time in the home (Gronau, 1973). Similarly, the

befief that the mother’s continuous presence during infancy is an imponmt input to child

qutity can be interpreted as m increase in the subjective productivi~ of time at home (see

Befsky, Lemer and Spanier, 1984)

Finsdly, tie physical strains of pregnancy and deEvery are significarrt. These strains

increase the value of the additiond mst that absence from the workplace Mows. As tbe child

progresses from newborn, tn toddler, to preschooler, the marginal productivity of the

mothefs time in the home faUs. Therefore, we illustrate the home productivity as eventufly

f~ing below the market wage (at tz), and the woman returns to work.

The empirical work that foUoys estimates reduced form relations for the determinants of

the date of leaving” work during pregnmrcy tI < to md the deternrinmts of returning to work

after delivery t2 z tO. In terms of tie decision problem ouflined above, the woman leaves

work during pregnancy when the reservation wage cast) rises above the market wage

r,= max(c T=fo s.r. d(z)= w(r) -r(z)> 0]

and returns to work after delivery when the reservation wage (first) f~s Wlow the market

wage:

f2= @n{r to< zs.r.d(z)= w(7) -r(z) CO]

Fi@re 1 relates to the p~blem for a given woman and emphasizes”the crucial role of the

market wage. We are interested in isolating the factors that shift .@e function d(t). Previous

studies in household production theory, female labor supply, and the medicaf/development

Hieramre suggest a rirrmber of variables.

The measures of home productivity include education, marit~ status, and family inmme

other than tie mother’s earnings. We hypothesize that education has a differential effect on

home productivity in the pre-.. and postnat~ periods. There is evidence that education

increases a woman’s productivity in child rearing (Leibowitz, 1974). However, there is Iitde

reason to believe that education increases ho”me”productivity before the birth. Thus,

education’s positive relationship with labor supply during pregnmcy (through its positive

effect on market wages) will be offset after the birth by its positive effect on home
. .

6

productivity. Therefore, we hypothesize that education wti be less positively related to labor -

supply in the posmatd period than it is in the prcnatat period.

Higher family income other than the mo~er’s earnings provides more goods witi wfdch

a woman can combbre her time, and therefore imphes a higher value of home time. The

effect of income in discouraging women’s labor supply has long been noted ~Incer, 196Z

Gronau, 1973). Net of other family income, marriage shmdd have a positive effeti on labor

supply after the child is born. We have argued in p~eviorrs work that marriage provides

access to additional @d care, both from tie husband and from otier members of KIs family

-i~wi~. Wtite, ~d WIEberger, 1987). In addition, marriage generMy precludes access

to NDC, which provides a source of inmme apart from a wommr’s own -rigs.

We ~so include a time trend h the analyses to account for the fact hat attitudes about the

productivity of women’s home time with ctildren and adjustment of la~r market institutions

may have been changing over the period studlcd.

Using this model, we wiU examine whether the panel data are consistent with any of the

alternative hypotheses regafl]ng early return to work. fiat it is ~sitively relaied to women’s

wages, that i~ stems from lower other-family-income, or fiat it is merely related to changes in

anitudea over tim~

;

DATA

The empirical analyses that fo~ow use data from the ~S Youth Cohort, which is a panel

study of a nationa~y representative sample of individuals 14 to 21 years old in 1979, the

initial survey year. The sample overrepresents blacks, Hispanics, and economically

disadvantaged non-black and non-Hispanic youths, relative to their proportions in the

population. We analyze the subssmple of 1372 women who had a first birth between 1979

and 1986 and who” w“orked during” their” pregnmcy. We study ofiy first births in order to

,

simpfify the problem by not comidering bfi intervals, which a smdy of higher order blfis

wmdd require. In addition, given the relative youth of the sample, we have considerably

more observations for first bbths than for higher order births.

In 1979, a basefine intemiew collected detailed background information on the sample

members. Anmsd interviews since that time have coUected infotiation on educational

attairmtent;~aritd status, fertility, and employment histories. Rates of sarnpIe retention were

about 95 percent in the early survey years and somewhat lower in more recent years.

me ~S-Y data contain a week-by-week WOA’history for each member of tie sample.

These data were developed from” annust questionnaires hat asked for beginning and end

dates for each job. h addition, respondents were asked the start and end dates of any gaps in
. .

7

work occurring wIlile they were employed by a given firm. They were told to list “(A)ny

periods of a ~ week or more during wtich (fiey) did not work for employer, not counting

paid vacations or paid sick leave” (&nter for Human Resource Research, 198@. Merging

Wls time-series with information on the bifidate of children mows us to construct a predelive~ work history.

Because we do not know the exact gestation age of the chdd at delivery, we do not

know the exact week in which the pregnancy began. We adopt the convention that all

pregnancies are ti term, lasting 39 weeks. To estimate the number of weeks worked during

the pregnancy, we count backwards from the date of birth. This incorporates some error.

However, 74 percent of deliveries occur between weeks 37 and 41 of the pregnancy

(Grrtrsnacher, 195Q.””

We define the events of interest as fo~ows. For work during pregnancy (wiWln the 39

weeka preceding delivery), we record the last week of work during pregnancy. Our sample

consists ofiy of women who have given birth, therefore tierc is no true censoring of work

during pregnancy. For work after delivery, we record the first week of work fo~owing the

birth. A considerable number of women had not returned to work by the time of the last

interview.

The independent variables have been constructed to proxy the value of rime in the

market, and at home, as described abve. The hourly wage rate is the reaf wage rate in 1986

dollars at the job held one year &fore detivery. For women who did not report m“ hourly

wage rate, we divide earnings per unit time by the number of hours worked in that unit of

time. We *O include a missing wage dummy. By construction, our sample onfy includes

women who worked during pre~ancy, so they should all have a wage. The ~S-Y,

however, did not co~ect wage data for women whose usual time commitment to a job was

less than 20 hours a week. In addition, we considered tbat””among women” for whom we had

wage data, that a calculated real wage of less than $1.00 an hour fin .198&dollara) was

misrepofied. For”bo.tb sets of women, we substituted $3.35 (the vdie of the minimum wage

in lg86) for women in the sample who reported wages under $1.00and set “tie missing wage

dummy to unity. Thus, the missing wage indicator dso carries the interpretation of low wage.

The income, effect is estimated by the coefficient on other income: The corresponding

variable is tie sum of income of the spouse in the calendar year prior to the birth of the

woman is unmarned, this is zero) and the woman’s unearned income in the year prior to the

birth (this includes rent, interest, etc.; set to zero if no such income).

We Aso include the following proxies for the reservation wage an indicator for mothers

who were Iess than 20 years old at the time of the birth, indicators for women w]th fewer
8

tian 12 years of schooting, and for women with 16 or mom years of schooting (women witi

12-15 years of schooling are the comparison group); an indicator for married spouse present

at the time of tie bIrrh. h addition, we include she cderrdar year in which the bifi Occurred.

Means and standard deviations of the variables ap~ar in Table 1.

Table 1.

Variable Means and Standard Deviations

Women with

M Women Gaps in Employment

Mean S.D. Mean S.D.

U Wage 1.669 .441 1.632 .431

Missing Wage .034 .181 .036 .186

Other Income ($000s) 14.893 .. 14.938 14.389 14.896

Education < H.S. .174 .379 .193 .394

Education 16+ .285 .451 .264 .441

Year of Birth (1979= 1) 2.346 2.057 “2.214 2.03 I

Age c 20 .210 .407 .232 .422

Married Spouse Present .664 .472 .648 .478

N 1372 1121

STATISTICAL METHODS

For tie sample of women who were working 39 weeks before the dehve~, we estimate

weibuU h~ard models where tie depcnderrt variable is the week of gestation at which labor

force participation ceased. This is an advance over previous statistical analyses that have

independerrtiy estimated labor force pafiicipation in a period 6 to 12 months ~fore the bifi

and in a period from O to 5 months before a blfi (e.g., Mott and Shapiro, 197~. We dso

estimate hasards for tie return to work following the birth.

Both hmrds are assumed to h IocWy of the weibti fom

.,.

with associated sumivor finctiom

.

,

S(cX,a,~) = exp[–expr=]

and density.
9

f(t;X,a,~) = h(cX,a,~)S(cX,a,~) = aexpra-’exp[-expfa]

.

,

where t is the time worked during pregnancy/not worked after de~very; X is a vector of

independent variables, and a and B and are pararnetem to be estimatti.

h what fo~ows, we estimate hazards over sub-pefiods from week a to week b. The

contribution to the likelihood of an individud who had not “failed’ (quit work during

pregnarrcy/retumed to work after childbirth) by time a, and failed at time c: a < c e b is:

h(cX, a, F)exp[-JJh(wX, a,P)dvl =

f(cX,a,P)

S(GX, a,~)

The contribution of &ose who do not fati in (a,b] is:

S(b;x, ex,p) exp[–~~h(wX, a,~)dv] =

S(q x, a,p)

Estimation is by maximum like~hood. The standard errors are computed using White’s

(19gO) robust formula.

RESULTS

We present our empirical .msults in four parts. We begiri with m examination Of the twO

raw hazards for weeks worked during pregnancy and weeks not worked after dehvery. This

examination reveals imporr~t conceptual and data problems. In the second and third

sections we discuss weibuH regression results for tbe determinants of each of the two hamrds.

The &Aysis through the fird section focuses on the margirrd distribution of weeks worked

during pregnancy and weeks not worked after delive~. The final section presents a

prtirninaw andysi: of the joint distribution of working times..

The Raw Hazards

As noted in the data section, the fineness of ,the ~S work histories Wows us to present a

considerably more detailed examination of work near pregnancy than previous research

which analyzes work over qrs&er years (e.g. Even, 1987j. Figures 2 and 3 present the raw
m

0.2

0.18

1:::::::

.

~ 0.16

& 0.14

g 0.12 =.

0.1

~ 0.08

o 0.06

.

3

.

0.04 . .

0.02 ..m. .===l=.l =m,=.,. ml. m,=:= ,.”=.==~.m

0.:,

0 5 “1o 15 20 25 30 35 40

Week of Pregnancy

Pig. 2—Hazard for Leaving Work During Pregnancy

hazards--the percentage of women Ieaving/retuming to work as a percentage of women

wortin~not worting up to that moment.

The hazards correspond quite closely to the hypothesized stresses of pregnancy md

postpartum recovery. Through the first two trimesters, the rate of leaving work drifts up

from shut I-12 percent a week to abut 2-1~ percent per week. During the fmd trimester,

as a womm’s sleep md moMlity are more seriously affected, fie hazard jumps from 2-lD

percent to over 15 Wrwnt Pr week.

The pattern immediately after childtirtfr ~so corresponds quite closely to physiologic

expectations. The hazard in weeks two through five is quite low under 3 percent of the

women return to work in those weeks. Starting in the sixth week, tie hazard rises sharply,

pea~ng ative 6“percent somewhere around tie eighth or ninth week.

.

*
T

*

0.08

0.07

0.01

0

11

o 10 20 30 40 50 60 70 80 go 100

Weeks after Childbirth

Fig. 3—Hazard for Return to Work After Childbirth

By the twenty-sixfi week, the bawd is again below 3 percent. Thereafter, it drifts slowly

down to a level we~ below 1 yrcent. We inte~ret this long slow dec~ne’in tie hazard after

about twenty-six weeks as an ~ffeti of heterogeneity. As women witi greater attachments to

tie labor force return to work; they leave behind a WOI of women with weaker and weaker

labor force attachment. Ttis is an isstse to which we wi~ remm in the fourth section of the

results.
1

0.9

m 0.8

8

= 0.7

~ 0.6

3

0.5 I~~

==~a. =

=.=

‘.. =K=

=8 .1.=,=

=1 . . .

■ . .

8..

=m

=mlm

.

.

.

=1

0 5 10 15 20 25 30 35 40

Week of Pregnancy

Fig. 4—Srrrvival Curve for StilI Working During Pregnancy

The suwivd curves, Figs. 4 md 5, plot the percentage of women stilI at wortiwho have

not yet returned to work in each week. They suggest that a tilrd of the women am still

working in tie week of .defivery. Furtfremom, over one-fifth of the women report that they

remmed to work in the week immediately following delivery and the overlap between the

two groups is substanti~. Table 2, wtich we discuss in more detail below, presents the joint

distribution of work during pregnmrcy and remm to work after. childblfi. The lower left

comer tabulates the overlap between those working until defivery mtd those who claim to

work in the week folIowing. delivery. Nearly ~ the women who report that they wofied in

tie fint week after biti dso Rport tiat they worked until the last week in pmgnmcy. That

ia, they appar to have continuous work expedience. It seems unfikely that 20 ~rcent of the

sample actu~y took less than a week off from”wok. The wording of the question reg~ding

gaps in employment instructed women not to count vacation or sick leave as a gap in

employment. Thus, many’ of the women reporting “continuou employment may have

retrrmed to their jobs after ta~rrg paid vacation, sick leave or maternity leave at the time of

de five~.

-.
*

n

1

0.9

G 0.5

,: 0.4

.~ 0.3

# 0.2

0.1

0

..

rirm

r.

m

‘.

‘1,

rm,=

ir,r,

‘I.,,,,r

“r’~i,a.,,i,,,,i,

"lm,~l,,ilirm,,mi,,.,,,,l.,.l.,., m,,,zi,,.,,, mi..,. *mi1m

7

0 10 20 30 40 50 60 70 80 90 100

Weeks after Childbirth

Fig. 5—Survival Curve for Not Yet Working After Childbirth

A special ~S-Y interview in 1983 Mows us to gain some insight into the behavior of

women who report wo~lng in the week immediately before and after defive~. In that year,

the ~S asked each woman whether her employer offered maternity leave, the date at which

she began maternity leave, and the age of her child when she returned to work. Comparing

the more specific data from the 1983 survey with responses to the tirrd work history

survey for the women who reported on hth questionnaires sheds tight on the interpretation

of the work Mstory data. For those women whose longitudinal work hismry data show that

tiey worked in the week immediately preceding and fo~owing birth, onfy 24 ~rcent re~rt

in the Materrdty Leave Survey in 1983 that their maternity leave began with delivery. Thus,

three-quafiers of fie women who re~fi in the work history that tiey were employed in the

week preceding delivery, actua~y begb their maternity leaVe earfier. me inconsistency may

actutiy be sm~ler than it appeam. A woman cotid work on Monday, take maternity leave

on Tuesday and deliver on Wednesday. In that case she would have worked in her “39W

week and have had her maternity leave begin kfore delivery. Nevertheless, tils is utiikely

to explain au of the cases in which women report that they were employed in the week of

defiveV and that their maternity leave began before defivery.
14

Table 2

Week of Leaving Work in Pregnancy and Week of Return to

Work After Delivery

~w Ilotiny i

Weeb hft in Wltiln

Pregnancy First Week 2-13 14-52 53-104 Total

Overti 70 0.26 2.42 6.37 8.41 17.47

1-13 Row % 1.51 13.87 36.48 48.14 100.00 -

Col % 1.39 8.35 25.76 30.85

OverW % 0.04 3.83 6.80 6.64 17.30

14-26 Row % 0.20 22.14 39.29 38.36 100.00

Col % 0.18 13.20 27.48 24.35

Overaff% 0.77 16.24 9.54 10.06 36.61

27–38 Row % 2.11 44.36 26.06 27.48 100.00

Col% 4.07 55.96 38.56 36.90

Overti % 17.90 6.53 2.03 2.16 28.61

39 Row % 62.57 22.81 7.09 7.53 100.00

Col 70 94.36 22.49 8.20 7.91

Totrd 18.97 29.02 24.74 27.27 100.00

Row 90 100.00 100.00 100.00 100.00

NOE The sample is timited to those who reported the birth of tie child at or before the

1985 interview. This should Wow us to obseme the fu~ 104 months fo~owing blrtb for

each child. The table incIudes three cases where the pattern of interviews implied that onfy

102 or 103 months were obsewed,.

Comparisons of the two data sources lead to similar conclusions for work after

pregnancy. Ordy 9. percent of the women who report in the work history data that they were

employed one week after defivery, acma~y returned to work in the wmk fo~owing de~ve~,

according to the 1983 data. Nevertheless, according to the 1983 intewiew, new mothers do

return to work relatively quic~y. Almost three-quarters have returned’ by two monfis, and

afrnost 95 percent by three months.

The discrepancy is concepmti. The matemity leave data specifically asks about time on

leave. The work ~story data which we use included probes spcificafly instructing the

woman not to count vacation or sick leave as time not worked. Thus, stricfly sp~ng, the

work history data reports the probabzfity~szard of “paid employment.” Paid leave is

counted as employment. The concept therefore corresponds to labor force participation,

except that tie unemployed are not included.

Because this conceptual problem affects one quarter of the sample, in the work that

follows we first analyze the probability of continuous employment, which is nearly

●

,

.-
. .

E

equivalent to taking some fom of paid leave. We then anafym fie time to return to work for

those who had some period away from work for whch they were not compensated.

Table 3 presents a logistic regression based on the Iongiturfind data of the determinmts

of whether a woman reponed working in tie 39th week of pregnancy (column 1) and

returning to work in tie week following delivery (column 2).2 As we detemined for a

subsarnple of these women who gave birth before 1983, most probably did take time off

from work. However, tie 1983 data dso suggest that tiey generally took off ofiy Emited

amounts of time. This would be consistent titi fimited amounts of vacation, sick leave, and

ruaterrdty leave.

Table 3

Determinants of Work Proximate to Delivery

Logit Coefficients and Standard Errors

Work ~ until Return in Week of Deliverv

Variable Baa s.e Beta se.

Intercept -3.065** 0.344 -3.234** 0.357

Ln Wage 1.045** 0.181 0.931** 0.192

Missing Wage 0.689* 0.352 0.444 0.409

Otier Income ($000s) : -0:009 0.006 -0.006’ ‘0.007

Education < H.S. -0.451** 0.211 -0.443** 0.23 I

Education 16+ 0.095 0.153 -0.048. 0.165

Year of Biti (1979 =“1) 0.066 0.036 O.1O5** ‘“ 0.039”~

Age <20 ? -o.54g* 0.214 -0.554* 0.238

Married Spouse Present ~ 0.282 0.176 0..288 0.190

NOTE: *.05 > p >.01

**.01 >=-p

The results indicate that women with tigher wages arc significarrfly more likely to work

to me end of pregrrmcy” and to return to work immediately after pregnancy. There is no

evidence of an income effect. Teenagers arc less fikely to retum”to work immediately after

pregnmcy. Relative to those with 12–15 years of schoofing, high school dropouts am less

fikely to wo& near delivery. RrtWy, the time trend in the second logistic regression suggests

hat employment in the week. immediately following cfrildbi~ is becoming more common

(holding eve~tirrg. else constant).

2fiom Table 2. we see that this lattir ~oup is essentially the group of women who worked

tithout a -p.
16

Work During Pregnancy

Table 4 presents maximum fiketihood =timates of the parameters of a weibu~ h=ard

model for leaving employment during the first 38 weeks of pregmmcy. Women reprtirrg

working into the 39th week are treated as censored at 39 weeks. It dso reports h=ards

computed separately for tie first two timesters and for the third trimester. The huard is

cleafly upward sloping. Over the entire ~riod, the point estimate for a, the stipe parameter,

is 1.90 with a stmdard error of 0.07. For a = 2, the weibti h=ard is a straight fine through

the origin. For a >2, it is convex. The convex pattern is cofilrrned by the piecewise h=ards.

For the first two trimeters, the hazard is sigrtificanfly but weakfy increasing (a = 127, with a

standard error of 0.05). For the finaf trimester, a = 9.95 with a standard error of 0.52,

cofilrrning the qutitative description of the raw h=ard given in the fimt pan of tis section.

Table 4

Last Exit From Work During Pregnancy

Hazard Regression Coefficients and Standard Errors

Weeks I to 38 Weeks 1 to 26 Weeks 26 to 38

Coeff. se. COeff. se. Coeff. se.

Intercept -6.140** 0.284 -4.272** 0.260 -35.680** 1.904

Ln Wage -0.340** 0.073 -0.377** 0.112 -0.419** 0.100

Mssing Wage 0.016 0.148 0.120 0.184 -0.344** 0.263

Other Income ($000s) 0.005 0.004 O.O1O* 0.005 0.001 0.004

~ucation < H.S. 0.223** 0.080 0.325 ** —O.116 0.126 0.126

Education 16+ -0.168** 0.063 -0.455** 0.129 -0.008 0.092

Year of Birth (1979= 1) -0.022 .0.015 -0.013 0.025 -0.047* 0.022

Age e 20 0.235** 0.077 0.295** 0.119 0.202 0.122

Mtied Spouse Present -0.216** 0.078 -0.479** 0.124 -0.019 0.106

Shape Parameter 1.900*” 0.070 1.270** 0.052 9.952** 0.518

*. 1372 1372 879

NO~. *.05 > p >.01

**.01 >= p

Sample includes onfy women who had a job during pre=mancy and who re~rted a job

leaving date in pregnancy (854 women).

We turn now to the effects of the covariates. Hgher wages significsnfly lower the huard

of leaving the job during pregnancy, increasing the length of time a woman stays at work.

Other household income ~ncome from a spouse and unearned income) has ordy a weak

effect smd ofly in weeks 1-26. figh school dropouts leave work sooner (over the entire

period and in weeks 1–26, but not significarrfly so in weeka 263.9), college graduates leave
. .

17

work later (again, over tie entire pefiod and in weeks 1–26, but not significantly so in w~ka

2&39). Flndly, married women leave the workplace later (again, the pattern is significant

over the entire period and in weeks 1–26, but not in weeks 2G39). We beEeve that this is

related to the availability in some states of -C during pregnancy for unmarried women. h

fomre work, we WM test this hypothesis by interacting tie marriage dummy with an indicator

,

of tbe availablfity of A~C during pregnancy.

It is interesting to note that the effect of covariates is concentrated in the first two

timesters. k the fid trimester, ordy the wage effect approaches smtistic~ sigrrificanm. ~Is

decline in significmce is not due solely to the decfine in the poptiation at risk from 836 to

410, which decreases the precision of the estimates; the Wint estimates themselves are

consistency sm~er in absolute value.

Return to Worh after Detivery

Using the sample of women who do not mtum to work immediately fo~owing defivery,

‘tie ovefi hazard of returning to work is f~ing (a= 0.80, see afso Table 5, column 1). This

overaU pattern masks two phases. As in the analysis of the raw hazard, after includlng

covariates the hazard rises sharply in the first quarter following defivery (a = 1,45) and f~s

off sha~ly thereafter (a = .51). We will return to the analysis of the effect of the sample

selection on the shape of the hamrd in the find section.

The change in h=ard, may be due to the increased use of maternity leave. There is a

strong wage effect throughou~ Higher wages cause women to return more quicHy. Also in

accord with neo-classicd Ia@r supply fheo~, there is a consistent and significant effect of

other income. High school drop-outs return more slowly. Those with some co~ege remm

more quic~y (though the significance of the latter. is weak). There is a weak time effect

concentrated in the first period. Women in later cohom remm to work more quicwy.

Su@ingly, women less than 20 years old return to work more quicMy (again the

sigrrificanm is weak). .FIdy, m~ed women ~tum to work more quickfy.
Table 5

Return to Work Following Birth

Hazard Regression Coefficients and Standard Errors

Weeks 1 to Iw Weeks 1 to 13 Weeks 13 to 104

Coeff. se. Coeff. se. COeff. se.

Intercept -3.722** 0.187 -5.163** 0.276 -2.234** 0.464

Ln Wage 0.330** 0.096 0.353** 0.126 0.260** 0.116

Missing Wage -0.409* 0.180 -1.042* 0.426 -0.190 0.200

Other hcome ($WOS) -0.024**. 0.004 -0.026** 0.006

Mutation c H.S.

-0.019** 0.006

-0.358** 0.099 -0.392** 0.149 -0.290* 0.120

Education 16+ 0.259** 0.092 0.168 0.119 0.311* 0.123

Year of Birth (1979 = 1) 0.015** 0.021 0.063* 0.027 -0.027 0.027

Age c 20 0.206* 0.097 0.069 0.143 0.282* 0.119

Married Spouse Present 0.470** 0.104 0.548** 0.143

Shape Parameter

0. 354** 0.134

0.803** 0.017 1.450** 0.059 O.51O** 0.076

N 1119 1119

NOTE

712

*.05 > p >.01

**.01 >=-p

Sample includes women who reported

repon to work in the week of delivery.

a job leaving date in pregnancy and who did not

Joint Distribution

We conclude tils paper with a preliminary discussion of tbe joint distribution of the two

h=ards. fn hls work on return to work after pregnancy, Even (1987) included as a regressor

the number of months prior to defivery that a woman quit work. Not surprisingly, it is a

good predictor of return to work khavior. Our analysis of the NLS-Y data suggest that

work before pre8rmncy is endogenous.

The relationship ktween early job-leaving in pregnancy md later return to work afier

defive~ is clear from the cross-tabulations reported in Table 2. The rows represent when the

E

women quit work during prewmcy, (1–13, l&26, 27-38, and 39 weeks respectively); the

columm represent when the womm reamed to work after pregnancy (1, 2-13? l&52, S= ,.

104 weeks). There is stiU considerable weight on the inverse diagonrd.

Among women who quit work in the first trimester of pregnancy, 15 percent (1.51 +

.-.. 13.87) returned to work in the first quarter y~r after tie birth. Of women who left work in

the second trimester, 22 percent (0.20 + 22.14) had returned in the first quamer. But among

women who’ worked 27 to 37 weefi into” tbdr pregnancies, my 45 percent (2.11 + 44.36)

returned to jobs in the first quaner after tie @rth.
. .

B

--- --- --- .-

Nm * ma Arooo

Weeks After Childbirth

●

✟

I

—0-12 ‘----13-25 .- . . ..-. 26-38 ------39+

Weeks Weeks Weeks Weeks I

Fig. 6—Hazard for Return to Work after Childbirth

Stratified by Weeks Worked During Pregnancy

Rgure 6 provides a complementary perspective. The four lines plot the h~ard functions

for rstum to work after delivery for each of four groups:3 those who left work in wwks l–

13, l&26, 27–38. and 39 weeks of pregnancy. The figure shows that women who work

latest into pregnancy return to work most quicHy after childtilfi. It is interesting to note

that W of tie ploq share a common shape. The hamrd begins at a low level in the fimt week,

rises to a peak near the end of the first quarter, and declines slowly thereafter. This result

suggests that the propnrdmrd hazard s~cifi cation used in the h=ard regressions is correct.

The joint behavior of tie two hazards is tie subject of ongoing resarcb.

4 DISCUSSION

Labor supply by pregnant women and recent mothers has expanded rapidly in the last

quarter centrrry, and particrdarly in the past 10 years. The expmion has been greatest for the

mothers of fie yo&gest children. This paper is dre first to examine data for the 1980s, a

period during which major changes in labor force behavior .of new mothers have occurred.

%e pIo@ have &en smmthed .ting the simple average of tbe five pints clowst ti e=h pint.
m

Three hypotheses have been proposed to explain this ‘increased’ work by pregnant women

and new mothers tie growth of women’s wages (subsrirution effect), the decfine of other

family income, and secular change. We find evidence that higher wages are associated with

increased work nmr chlldblrtb. The evidence for an income effect is somewhat weaker.

Finafly, bifi year as a measure of cohort and secutar change has ordy ~iited effects. Thus,

trends in LFP are consistent with a behavioral respome to rising women’s wages and stagnant

or f~hrg men’s wages over the period studied

Ufllke previous studies that use quarterly or amruaf interv~s since the blrtb, we have

anafyzed employment at wee~y intervals. As women remain at work longer in pregnmcy

and return to work sooner after blrtb, such finer intervals of data collection become

increasingly important. These more detailed data yield some interesting additiond findings.

First, w~lle we have found some evidence to support the implications of neo-classical

economic theory, the effects are not uniform. The effects am strong in some quarters around

Mrth and nonexistent in others. Analyses without detafled information on exactfy when

women exit Wd enter fie workforce will miss the time-varying effects Of theSe eCOnOmiC

variables. :

The low hazard of remm to work in the immediate post-partum period suggests that part

of the wage effect is “mediated through tie effect of maternity leave. As women move into

higher paying jobs they are more tikely to have maternity leave, sick leave, and vacation

time that Wow time away from work, as we~ as employment continuity. The various forms

of maternity leave allow women to presewe continuous labor force attachment while

remaining home during fie crucial early postpartum period. Such leave may be partitiarly

vafuable to high wage women, who have job-specific trtining.

Our future work will more completely anstyze the interrelation of maternity leave,

employment, and actual time at the job. In addition, we plan to continue to expIore fie nonmonotonicity of the huard and the time-varying effecti of the covariates in models that

exploit the correlation in tie unobsesvables, across the two deci$iom.

.*

.
. .

21

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b

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a=

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