ORIGINAL ARTICLE

Prenatal and Postnatal Exposure to Cell Phone Useand Behavioral Problems in Children
Hozefa A. Divan,a Leeka Kheifets,a Carsten Obel,b and Jørn Olsena
Background: The World Health Organization has emphasized theneed for research into the possible effects of radiofrequency fields inchildren. We examined the association between prenatal and postnatal exposure to cell phones and behavioral problems in young
children.
Methods: Mothers were recruited to the Danish National Birth
Cohort early in pregnancy. When the children of those pregnancies
reached 7 years of age in 2005 and 2006, mothers were asked tocomplete a questionnaire regarding the current health and behavioral
status of children, as well as past exposure to cell phone use.
Mothers evaluated the child’s behavior problems using the Strengthand Difficulties Questionnaire.
Results: Mothers of 13,159 children completed the follow-up questionnaire reporting their use of cell phones during pregnancy as well as
current cell phone use by the child. Greater odds ratios for behavioral
problems were observed for children who had possible prenatal or
postnatal exposure to cell phone use. After adjustment for potential
confounders, the odds ratio for a higher overall behavioral problems
score was 1.80 (95% confidence interval ⫽ 1.45–2.23) in children withboth prenatal and postnatal exposure to cell phones.
Conclusions: Exposure to cell phones prenatally—and, to a lesser
degree, postnatally—was associated with behavioral difficulties
such as emotional and hyperactivity problems around the age ofschool entry. These associations may be noncausal and may be due
to unmeasured confounding. If real, they would be of public healthconcern given the widespread use of this technology.
(Epidemiology 2008;19: 523–529)

Submitted 20 November 2007; accepted 11 February 2008; posted 5 May
2008.
From the aDepartment of Epidemiology, UCLA School of Public Health,
University of California, Los Angeles, CA; and bInstitute of Public
Health, Department of General Practice, University of Aarhus, Aarhus,
Denmark.
The Age Seven Questionnaire follow-up has received financial support fromthe Lundbeck Foundation (195/04) and the Danish Medical ResearchCouncil (SSVF 0646); financial support for this analysis was provided by
the UCLA School of Public Health, Research Innovation Seed Grant(4565963LK19914).
Correspondence: Leeka Kheifets, Department of Epidemiology, UCLA
School of Public Health, Box 951772, 73-320 CHS, Los Angeles, CA
90095-1772. E-mail: kheifets@ucla.edu.
Copyright © 2008 by Lippincott Williams & Wilkins
ISSN: 1044-3983/08/1904-0523
DOI: 10.1097/EDE.0b013e318175dd47

Epidemiology • Volume 19, Number 4, July 2008

E

xposure to radiofrequency fields is increasingly common,but the potential influence on health has not been thoroughly investigated, especially in children. Between 2003
and 2008, there were more than 900 million new cell phone
subscribers worldwide, with a total of more than 2 billion
subscribers.1 Fetuses and children may be more vulnerable
than adults to external exposures in general.2 In 2000, theStewart Report recommended a precautionary approach tothe use of cell phones until more detailed and scientificallyrobust information became available, especially for children.3 Numerous reviews, including 1 by the World HealthOrganization,4 stress the need for studies in children andon cognitive effects, because of the importance of cognitive abilities and learning in early development.
Most epidemiologic studies of exposure to radiofrequency
fields have focused on brain and acoustic cancers as outcomes5–11 or on subjective symptoms such as headaches.12,13 A
number of laboratory studies have examined physiologic effects
after short-term exposure,14 –18 but a variety of other outcomesare yet to be investigated, and none has included potentiallysusceptible populations such as fetuses and very young children.
Children are potentially exposed during fetal life bymaternal use of cell phones and then later in childhood when
they themselves become users of cell phones. Exposures
early in life may have particular importance because this isduring vulnerable stages of brain development.
There is limited information on the association between
radiofrequency field exposure during pregnancy and reproductive outcomes (spontaneous abortions, birth weight, sex
ratio, and congenital malformations), mostly from occupational studies. Occupational exposures are typically muchhigher than exposures from cell phone use. Some studies
have reported increased risk of spontaneous abortions andcongenital malformations, although these results come frompoorly designed studies.19
Since no established mechanism is known for radiofrequency exposure— except for what may be caused by an increased temperature in the exposed regions—it is impossible toexclude any health outcomes from consideration. Experimental
research indicates exposure might affect nonspecific neurologic
performance such as attention. In a preliminary cross-sectional
analysis of 13 year-olds in the MoRPhEUs study, differences incertain cognitive abilities related to cell phone use were observed(Rodney Croft personal communication, 16 December 2007).

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Divan et al

An increasing number of children are being diagnosed
with attention-deficit hyperactivity disorder (ADHD) as classified in the DSM-IV or hyperkinetic disorder in the ICD-10
classification. These behavioral problems include hyperactivity, impulsivity, and difficulty in concentrating. Known risk
factors for these conditions include sex of child, smoking
during pregnancy, family psychiatric history, and socioeconomic status (SES).20,21 To date, no epidemiologic studies
have investigated cell phone use as a possible risk factor for
behavioral outcomes with similarities to ADHD.
In this analysis, we explored the association of cell
phone use during pregnancy and during early childhood withbehavioral problems in children.

METHODSThis study was based on the Danish National Birth
Cohort, which recruited study participants from March 1996
through November 2002. A total of 101,032 pregnancies
were enrolled in the cohort.22,23 Mothers and live-born children constitute 2 fixed cohorts that are to be followed for
decades in a life-course perspective. Detailed information onlife-style factors, dietary habits and environmental exposures
were collected by means of 4 telephone interviews; 2 of thesewere conducted during pregnancy and 2 when the newborn
child reached 6 and 18 months of age.24
A new round of data collection from mothers that
focuses upon the child’s health status was initiated when thechildren reached the age of 7 years. This analysis is based on
the information collected about children born between 1997
and 1999. This Age-7 Questionnaire contained questions oncell phone use among children, as well as among mothers
during pregnancy. In addition, the questionnaire included
data on social conditions, family lifestyle, and diseases inchildhood, including behavioral problems as defined by theStrengths and Difficulties Questionnaire. Specifically, we
asked about mother’s use of cell phone during pregnancy (thenumber of times spoken per day, proportion of time the phone
was on), use of hands-free equipment during pregnancy(proportion of time) and location of the phone when not inuse (handbag or clothing pocket), and for children, current
use of cellular and other wireless phones.
Letters were sent to participants’ homes instructing
them about how to respond to the questionnaire via theInternet, as well as informing them that they may request aquestionnaire to return by ordinary mail using a prepaid
envelope. If they did not respond within a 4-week period, areminder letter was sent. If, at the end of a second 4-weekperiod, the Internet version still had not been completed, apaper-based questionnaire was provided via mail. Formats ofthe 2 questionnaire were identical.
This study was approved by the Danish Data Protection
Agency and by regional science ethics committees in Denmark as well as the University of California, Los Angeles
Office for the Protection of Research Subjects.

524

Outcome Measures
Behavioral problems in children were assessed using
the Strengths and Difficulties Questionnaire.25,26 Mothers
completed a list of 25 questions with scaled responses (verytrue, partly true, or not true) regarding their child’s behavior.
The assessment of disorders was based upon scores over aparticular group of questions with a priori defined cutoff
points. The questionnaire assessed overall behavioral problems or disorders, as well as specific emotional, conduct,
hyperactivity, peer and social disorders.
Based on the specific numerical score, children wereclassified as abnormal, borderline, or normal for overallbehavioral problems as well as for the specific outcomes suchas emotional, conduct, hyperactivity, or peer problems.27

Statistical Analysis
Comparisons were made between baseline characteristics (ie, sex of child, social-occupational status, and mother’s
psychiatric history and smoking) as well as prenatal andpostnatal cell phone exposure.
We used an ordinal logistic regression model (adjacentcategory logistic model) to estimate the odds of the outcomesof behavioral problems (2 ⫽ abnormal, 1 ⫽ borderline, 0 ⫽normal) in children according to combined prenatal andpostnatal exposure to cell phones, prenatal exposure only, andpostnatal exposure only. The model imposes the same odds ingoing from “normal” to “borderline” as from “borderline” to“abnormal.” Tests for heterogeneity were conducted to verify
this assumption. Regression analyses were adjusted for several
potential confounders (child’s sex, mother’s age, mother’s psychiatric history, social-occupational status, and smoking during
pregnancy). We computed odds ratios (ORs) and 95% confidence intervals (CIs).
To evaluate possible dose-response patterns, we considered proxies of prenatal exposure intensity (times per day
spoken, location of the phone when not in use, proportion oftime the phone was turned on, and use of an earpiece with cell
phone). For each specific characteristic of use, the reference
category was defined as the lowest possible category (ie, nouse, 0 –1 times per day spoken). For location of phone when
not in use, the reference category was “carried in bag” versus“carried in dress/pant pocket.”

RESULTS
Mothers completed an Age-7 Questionnaire for 13,159
children, which is about 65% of the eligible mothers who
were contacted through November 2006. Thirty percent ofchildren were using a cell phone at the age of 7 years, but
fewer than 1% used a cell phone for more than 1 hour per
week. About 11% of children were exposed to cell phones
both prenatally and postnatally (Table 1). Nearly half had
neither prenatal nor postnatal exposure, and another 8% (n ⫽1091) were coded as do not know or missing in the analysis© 2008 Lippincott Williams & Wilkins

Epidemiology • Volume 19, Number 4, July 2008

Cell Phone Use and Children

TABLE 1. Distribution of Covariates and Behavioral Problems Score by Prenatal and Postnatal Exposure to Cell Phone Use (n ⫽ 13,159)

Covariates
Sex of childBoy
Girl
Missing
Social-occupational status
High
MediumLow
MissingMother ever suffered from mental disorder or neurosis
Yes
No
Do not know/missing
Mother ever had psychiatric illness
Yes
No
Do not know
MissingMother’s smoking status during pregnancy
Entire pregnancy
Every dayLess than every day
Do not know frequency
Early in pregnancy only
Not a smoker
Do not know/missing
Behavioral problems score
Overall behavioral problems score
Normal (0–13)
Borderline (14–16)
Abnormal (17–40)
Missing
Emotional symptoms score
Normal (0–3)
Borderline (4)Abnormal (5–10)
Missing
Conduct problems score
Normal (0–2)
Borderline (3)Abnormal (4–10)
Missing
Hyperactivity score
Normal (0–5)
Borderline (6)Abnormal (7–10)
Missing
Peer problems score
Normal (0–2)
Borderline (3)Abnormal (4–10)
Missing

No
Exposure(n ⴝ 6471)

Prenatal
ExposureOnlya(n ⴝ 1895)

Postnatal
ExposureOnlyb(n ⴝ 2281)

Both
PrenatalandPostnatal(n ⴝ 1421)

Do NotKnow/Missing(n ⴝ 1091)

53.3
46.6
0.1

53.6
46.4
0.0

47.0
52.9
0.1

46.7
53.1
0.2

51.2
48.8
0.0

53.5
35.2
7.5
3.8

50.9
36.4
10.0
2.7

52.1
36.7
7.3
3.9

45.3
38.9
12.1
3.7

48.6
38.3
9.9
3.2

5.2
91.1
3.7

5.9
91.5
2.6

6.6
89.7
3.7

8.0
88.8
3.2

12.6
84.6
2.8

10.1
87.7
1.6
0.6

11.9
84.9
2.2
1.0

13.5
83.9
1.8
0.8

16.8
81.0
1.6
0.6

34.7
57.7
1.5
6.1

15.1
80.0
10.6
9.4
6.5
72.5
5.9

19.3
85.0
5.5
9.6
6.2
69.2
5.3

20.4
79.6
12.0
8.4
7.3
66.7
5.6

24.8
83.0
7.4
9.6
8.2
59.9
7.1

17.0
82.7
6.5
10.8
7.3
70.1
5.6

95.0
2.5
2.4
0.1

91.6
4.1
4.2
0.1

93.8
3.7
2.4
0.1

89.9
5.4
4.6
0.1

89.5
3.6
2.7
4.2

87.8
6.0
6.1
0.1

85.5
6.0
8.4
0..1

86.5
6.6
6.9
0.0

82.8
7.9
9.2
0.1

79.9
7.6
8.4
4.1

87.1
8.1
4.7
0.1

83.9
9.9
6.1
0.1

86.4
8.7
4.9
0.0

80.2
11.1
8.7
0.0

81.3
8.9
5.6
4.2

91.8
3.5
4.7
0.1

88.9
4.5
6.5
0.1

91.8
3.3
4.9
0.0

88.1
4.3
7.5
0.1

87.6
3.2
5.0
4.1

92.5
3.5
4.0
0.1

90.0
4.3
5.6
0.1

91.6
4.6
3.7
0.1

89.1
5.2
5.6
0.1

88.5
3.7
3.7
4.2

Values are given in percentage.
a
Mother’s use during pregnancy.
bChild’s use.

© 2008 Lippincott Williams & Wilkins

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For the combined prenatal and postnatal exposure,adjusted odds ratios (Table 3) were similarly increased foreach of the 4 specific behavioral outcomes. The odds ratios
were higher for prenatal exposure than for postnatal exposure, for each of the behavioral problems.
When analyses were stratified by the covariates theassociations between cell phone use and behavioral problems
were stable across the strata (Table 4). Associations withoverall behavioral problems in children did not vary when
considering the questionnaire administration format (paperbased or Internet-based) (data not shown).
Almost 85% of mothers carried their cell phone in abag during pregnancy rather than on their person or elsewhere, and nearly 82% reported not using an earpiece (datanot shown). In Table 5, nearly 56% of children withprenatal exposure had mothers who reported speaking 0 –1
times per day during their pregnancy and 43% reported
having the phone turned on at all times. For prenatal
exposures—regardless of postnatal exposure— odds ratios
for the overall behavioral problems score tended to begreater with higher potential for fetal exposure; however,

because of incomplete information regarding mother’s orchild’s cell phone use. For most characteristics and outcomes,
the percent missing or not known was small (0%–7%) andsimilar across the 4 exposure groups. One exception was thepercentage of mothers ever having psychiatric illness; nearly
35% of the 1091 who had incomplete information on exposure to cell phones answered “Yes” to this question. Nearly
90% of the children were reported as “normal” for all types of
behaviors. Children with exposure to cell phones (prenatally,
postnatally, or both) tended to have higher percentages ofborderline or abnormal scores for emotional symptoms, conduct problems hyperactivity and peer problems.
The highest odds ratios for behavioral problems wereobserved for children who had both prenatal and postnatal
exposure to cell phones compared with those who were notexposed during either time period (Table 2). For these children the adjusted OR for the overall behavioral score was
1.80 (95% CI ⫽ 1.45–2.23). For prenatal or postnatal exposure only, the adjusted odds ratios were 1.54 (1.32–1.81) and1.18 (1.01–1.38), respectively. Adjusting for potential confounders moved the results towards the null.

TABLE 2. Association of Prenatal and Postnatal Exposure to Cell Phone Use With Overall Behavioral Problems
Score
Postnatal Exposure
No

Prenatal exposure
No
Yes
Postnatal exposured

Prenatal Exposurea

Yes

Unadjusted
OR

Adjusted OR(95% CI)b

Unadjusted
OR

Adjusted OR(95% CI)b

Unadjusted
OR

Adjusted OR(95% CI)b

1.0c
1.77
1.0c

1.0c
1.58 (1.29–1.93)
1.0c

1.25
2.16
1.26

1.18 (0.96–1.45)
1.80 (1.45–2.23)
1.18 (1.01–1.38)

1.0c
1.74

1.0c
1.54 (1.32–1.81)

n ⫽ 12,068 with information about prenatal and postnatal exposure; n ⫽ 12,112 with information about prenatal exposure; n ⫽ 13,054 with information
about postnatal exposure.
a
OR for prenatal exposure adjusted for postnatal exposure.
b
Adjusted for sex of child, age of mother, smoking during pregnancy, mother’s psychiatric problems, and socio-occupational levels.
cReference category.
d
OR for postnatal exposure adjusted for prenatal exposure.

TABLE 3. Associations of Specific Behavioral Problems in Children With Prenatal and Postnatal Exposure to
Cell Phone Use
Prenatal Exposure Only

Behavioral problems
Emotional
Hyperactivity
Conduct problems
Peer problems

Postnatal Exposure Only

Both Prenatal and Postnatal Exposure

Unadjusted
OR

Adjusted OR(95% CI)a

Unadjusted
OR

Adjusted OR(95% CI)a

Unadjusted
OR

Adjusted OR(95% CI)a

1.23
1.39
1.29
1.36

1.12 (0.97–1.30)
1.29 (1.08–1.53)
1.21 (1.05–1.40)
1.27 (1.06–1.52)

1.13
1.00
1.06
1.11

1.06 (0.92–1.23)
0.98 (0.82–1.17)
1.02 (0.89–1.18)
1.08 (0.90–1.29)

1.50
1.52
1.69
1.51

1.25 (1.07–1.47)
1.35 (1.12–1.63)
1.49 (1.28–1.74)
1.34 (1.11–1.63)

Reference category is no prenatal or postnatal exposure to cell phone use.
aAdjusted for sex of child, age of mother, smoking during pregnancy, mother’s psychiatric problems, and socio-occupational levels.

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Cell Phone Use and Children

TABLE 4. Associations of Overall Behavioral Problems With Prenatal and Postnatal Exposure to Cell Phone
Use Stratified by Covariates
Prenatal Exposure Only
OR (95% CI)

Covariates

Postnatal Exposure Only
OR (95% CI)

Both Prenatal and Postnatal Exposure
OR (95% CI)

1.20 (0.92–1.58)
1.44 (1.04–1.99)

2.35 (1.81–3.06)
2.12 (1.51–2.97)

1.27 (0.90–1.79)
1.40 (1.04–1.89)
0.67 (0.36–1.27)

2.32 (1.63–3.30)
1.92 (1.40–2.64)
1.56 (0.95–2.55)

0.68 (0.33–1.42)
1.29 (1.03–1.60)

1.59 (0.84–3.01)
2.16 (1.72–2.69)

1.10 (0.71–1.69)
1.26 (0.99–1.60)

1.57 (1.02–2.41)
2.08 (1.62–2.66)

0.89 (0.59–1.32)

1.71 (1.18–2.47)

1.51 (0.68–3.37)

2.68 (1.24–5.80)

1.32 (1.01–3.37)

2.00 (1.50–2.66)

Sex of child
Boy (n ⫽ 6201)
1.89 (1.48–2.40)
Girl (n ⫽ 5856)
1.55 (1.09–2.21)
Social-occupational level
High level (n ⫽ 6259)
1.76 (1.27–2.46)
Medium level (n ⫽ 4359)
1.86 (1.39–2.51)
Low level (n ⫽ 1009)
1.22 (0.74–2.02)
Mother ever suffered from mental disorder or neurosis
Yes (n ⫽ 710)
1.12 (0.56–2.27)
No (n ⫽ 10,937)
1.83 (1.48–2.26)
Mother ever had psychiatric illness
Yes (n ⫽ 1424)
1.50 (0.96–2.3)
No (n ⫽ 10,349)
1.72 (1.36–2.17)
Mother’s smoking status during pregnancy
Smoked entire pregnancy
1.75 (1.22–2.51)(n ⫽ 2160)
Smoked early in pregnancy
2.21 (0.99–4.96)
only (n ⫽ 822)
Not a smoker (n ⫽ 8378)
1.70 (1.31–2.20)

TABLE 5. Association of Characteristics of Mother’s Cell Phone Use During Pregnancy With Overall
Behavioral Problems Score in Children With Prenatal Exposure (n ⫽ 3322)

Times spoken per day
0–1
2–3
4⫹
Missing
P for trend
Percentage of time turned on
0
⬍50
50–99
100
Missing
P for trendaNo. (%)

Unadjusted
OR

Adjusted OR(95% CI)a

Adjusted OR(95% CI)a,b

1873 (56.4)
777 (23.4)
347 (10.4)
325 (9.8)
—

1.00c
1.49
1.60
—
0.28

1.00c
1.33 (0.99–1.79)
1.51 (1.02–2.22)
—
0.61

1.00c
1.31 (0.97–1.77)
1.47 (1.00–2.18)—
0.62

397 (12.0)
500 (15.1)
954 (28.7)
1427 (43.0)
44 (1.2)
—

1.00c
0.70
1.20
1.43
—
0.15

1.00c
0.62 (0.35–1.11)
0.93 (0.58–1.48)
1.09 (0.70–1.70)
—
0.13

1.00c
0.62 (0.35–1.10)
0.91 (0.57–1.45)
1.06 (0.68–1.65)—
0.13

Estimates adjusted for sex of child, age of mother, smoking during pregnancy, mother’s psychiatric problems, and socio-occupational levels.
Also adjusted for postnatal exposure to cell phones.
Reference category.

bcproxies for intensity of mother’s phone use during pregnancy did not exhibit strong dose-response associations,and tests for trend were not statistically significant.

DISCUSSION
Use of cell phones during pregnancy was associatedwith an increased odds of behavioral problems in children inthis study. These results were unexpected and should be© 2008 Lippincott Williams & Wilkins

interpreted with caution. Observed associations are not necessarily causal. We have no known biologic mechanisms to
explain these associations, and confounding by unmeasuredcauses of behavioral problems could have produced theseresults. Furthermore, this is the first study of its kind. The highest
exposure group differed somewhat on several factors that might
relate to the risk of behavioral disorders in offspring: they wereof lower social-occupational status, mothers were more likely to527

Divan et al

have ever suffered from mental disorder or neurosis and to ever
had psychiatric illness, and mothers were more likely to havesmoked during pregnancy. Although the associations remained
after adjustment for these factors, and the associations wereactually stronger in the covariate strata associated with lower
risk of behavior disorders in offspring, the possibility of residual
confounding remains. Additionally, we did not have informationon other potential confounders such as history of psychiatricdisorders in fathers and history of lead exposure.28
Another possible explanation for the observed association might be the lack of attention given to a child bymothers who are frequent users of cell phones. It is also possiblethat behavioral correlates of maternal cell phone use, rather thanradiofrequency exposure, affect perception or reality of children’s behavioral problems. Thus confounding and othersources of bias may explain the associations observed.
The validity of our measure of behavioral problems
based on the questionnaire is supported by the fact that theprevalence of observed behavioral problems is consistent
with results from similar studies in which the questionnaire
has been used and validated.29,30 Additionally, we observed
associations in the expected direction and magnitude for otherrisk factors for behavioral problems, such as sex of the child,age of mother, social-occupational status, psychiatric historyof mother and smoking during pregnancy.20,21 We measured
mother’s reports of their children’s behavioral traits or patterns rather than clinical diagnoses such as ADHD.
While the questionnaire was designed to obtain historyof cell phone use, we do not believe that differential recall
bias explains the observed associations. The portion of thequestionnaire asking about behavioral problems preceded thequestions regarding cell phone use, but it is unlikely thatmothers would be influenced by the knowledge of theirchild’s behavioral status when providing data of phone use,as these behavioral problems have not previously been associated with cell phone use. A previous study has shown good
accuracy for the simple reporting of use versus nonuse of cell
phones, and reasonable reporting (underreporting of 10%) fortimes spoken per day.31 Self-reported duration of calls wasmore problematic, being overestimated by 40%.
Because pregnancy leaves a strong impression on
memory—allowing for mothers to recall events fairly welland actions during that time—we expect recall related to a
specific pregnancy to provide better data than recall without such a stimulus, and a number of studies have supported this view.32–35 Thus our estimates of exposure,which are based on cell phone use and number of calls,
may be better than for the case-control studies of brain
tumors published to date, although we do not expect thedata to be without error.
Obtaining actual exposure dose measurements to radiofrequency fields in a large prospective cohort is unrealistic.
Basing exposure data as collected by a well-designed ques-

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tionnaire is the only practical way of obtaining cell phone use
information for children in a large cohort study. It would be
useful to know how well reported use of cell phone bymothers approximates exposure to the fetus. Adequate models estimating the specific absorption rate to a fetus from cell
phones are still under development.36 Based on the distancefrom outside the body to inside the uterus, the exposure
reaching the fetus (either during conversation or when thephone is in a standby mode) is likely to be extremely low.
Although unlikely, thermal effects due to localized increases
in temperature should be considered, given that the temperature of the fetus is already about 0.5°C above that of themother37 and fetal heat dissipation to the mother (which
occurs mostly at the placenta) is not fully efficient.
The specific absorption rate for children is somewhat
higher than for adults due to differences in body size, shape,
tissue distribution, as well as permittivity and conductivity oftissues. In addition, the relative penetration is deeper for children, a logical consequence of the smaller head diameter. Nevertheless, use of cell phones by children in this group was so
infrequent and short term that the casual effect due to these
exposures seems unlikely, according to our present knowledge.
About 30% of 7-year-olds used cell phones in Denmark, albeit infrequently. Although the difference in cell
phone use by mothers and children between the 2 years ofenrollment (1998 and 1999) was small, prevalence of use wasincreasing and will likely increase further as the children age.
Many families will probably not have land-lines in the future,thus increasing frequency of cell phone use even more. In a
recent Swedish study, nearly half of 7-year-olds had access tocell phones and prevalence increased sharply with age, to98% of 14-year-olds.38
The immature nervous system is extremely vulnerable
to toxicants, which can result in behavior-related toxicities
that may not emerge until well into childhood, adolescence or
adulthood.39,40 Thus it will be useful to continue to follow
this cohort of children.
Examination of the possible effect of prenatal andpostnatal cell phone exposure on cognitive development andbehavior is best done in a longitudinal study. Our results need
to be replicated; they only suggest that cell phone use during
critical periods of brain development in pregnancy and earlychildhood could be a potential risk factor for behavioral
problems in children. We hope others will be able to pursue
this question in other cohorts of children. The observed
associations may be noncausal and due to unmeasured confounding; however, if they are real they would have major
public health implications.

ACKNOWLEDGMENTS
We thank the coordinator of the data collection, Inge
Kristine Meder; data analysts, Inge Eisensee and Kenn
Schultz Nielsen; and the participating mothers.
© 2008 Lippincott Williams & Wilkins

Epidemiology • Volume 19, Number 4, July 2008

REFERENCES
1. Geekzone. Number of cell phone users worldwide to increase to 2 billion
by 2007. August 8, 2003. Available at: http://www.geekzone.co.nz/
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