Reviewer's report
Title:Early maternal age and multiparity are associated to poor physical
performance in middle-aged women from Northeast Brazil: a cross-sectional
community based study.
Version:1Date:9 March 2015
Reviewer: Rebecca Sear
Reviewer's report:
1. Crude dichotomies of the two reproductive history variables are used (birth
before 18 or 18+; and up to 3 children or 3+). Exactly why did the authors chose
to dichotomise the reproductive history variables? Aren’t they losing information
by using relatively crude dummy variables, instead of using the full range of
variation in reproductive history (e.g. using age at first birth or number of children
as linear variables). Did they run analyses using different cut-offs, or more
categories of each variable? At the very least, the authors should present some
descriptive data showing the distribution and range of variation in both age at first
birth and total number of children. Preferably, the authors should also present
additional analyses (perhaps in supplementary material) using different variations
of their reproductive history variables, which provides support for their overall
conclusion that reproductive history is correlated with physical performance in
this sample
2. The authors assume in their models that socioeconomic position (SEP) might
mediate the relationship between reproductive history and physical performance.
But, as they note in their introduction, SEP might be a cause, rather than a
consequence of reproductive history. Lower SEP women have earlier first births
and more children overall, so how can they tell whether the relationship between
reproductive history they have found is not a spurious relationship caused by the
fact that SEP is correlated with physical performance? I recommend the authors
at least include some discussion of this potentially confounding factor, and
perhaps consider running some additional analyses (such as causal mediation
analyses) which tries to disentangle relationships between SEP, reproduction
and performance.
3. In the Results section, the authors refer to relationships being ‘significant’
when referring to the descriptive data presented in Table 2 (e.g. between age at
first birth and family income). But there is no indication of statistical significance
on Table 2. The authors say in the Methods section that they did run univariate
statistical tests on these descriptive data, so they should give some indication of
the results of these tests in Table 2.
4. Tables 3, 4 and 5 are rather confusing:
a. Table 3 presents the results of analysis of age at first and nulliparity on
physical performance. Firstly, I don’t understand why the nulliparous women are
included in this analysis but not in the analysis of parity, since nulliparity is an
indicator of parity, not age at first birth. Including nulliparous women as a
category in the age at first birth variable makes the table harder to interpret two
separate p values need to be reported (one for an overall difference between the
three groups of women [nulliparous, early age at first birth or late age at first
birth], and one for the difference between the two categories of interest – early
and late age at first birth). I recommend the authors either drop nulliparous
women from all analyses (they are, after all, a small and rather unusual group of
women); or run a completely separate analysis which tests whether nulliparous
women are different from parous women in physical performance. Then the age
at first birth analysis can simply present the difference in physical performance
between women who had an early or late age at first birth – this would be much
clearer.
b. Tables 4 and 5 flip the rows and columns compared to Table 3, so that the
physical performance variables are now given in columns rather than rows. This
makes it harder to compare results across different models. I recommend that all
three tables presenting results of regression models should have the same
format, so that it is easy to compare results across tables.
5. What exactly was the point of running the composite analysis presented in
Table 5? Was it to try and disentangle the influence of early age at first birth from
large numbers of children? If so, this is not discussed.
6. The authors conclude that reproductive history is correlated with physical
performance, but their results are sometimes rather weak. Only some of their
analyses show correlations between reproductive history and physical
performance, and some of these relationships weaken or disappear when
potentially confounding factors are controlled for. Effect sizes also appear to be
relatively small. I recommend the authors are rather more cautious about their
conclusion that reproductive history is correlated with physical performance,
because not all of their analyses showed this finding. I further recommend the
authors include some discussion of effect sizes in their Discussion, including a
discussion of whether these findings really are of great policy importance. Have
they really shown that women who early first births and high parity have
significant physical limitations compared to women with later first births and fewer
children? Or found they found a somewhat weak relationship, which is certainly
of theoretical interest, but may not have significant implications for policy?
Minor essential revisions
7. The introduction is very brief and does not discuss any literature on maternal
depletion, which is essentially what the authors are trying to show here. A
number of studies have tried to correlate reproductive history with mortality and
morbidity, though with mixed success. See Hurt, L. S., Ronsmans, C., & Thomas,
S. L. (2006). The effect of number of births on women’s mortality: systematic
review of the evidence for women who have completed their childbearing.
Population Studies, 60(1), 55–71 for a review (more recent papers are also
available)
Level of interest: An article whose findings are important to those with closely
related research interests
Quality of written English: Acceptable
Statistical review: Yes, and I have assessed the statistics in my report.
Declaration of competing interests:
I declare that I have no competing interests