Strengths and limitations
There are several strengths to this study. First, the prospective nature
of this study reflects the direction of effects and enhances the
validity of measurements. In addition, the large sample size provided a
range of opportunities to adequately control for confounding factors.
This enabled examination of the association between maternal anxiety and
birthweight and gestation. That is, several confounders could be
included at different levels, such as those operating prenatally, as
well as those related to birthweight and gestation (e.g. birth
complications). Contextual factors such as maternal age and education
could also be included. Next, the unique sample included a large sibling
population, which allowed adequate control for family variables such as
maternal genetic effects. 11 A sibling comparison
design using a large cohort study can test whether associations differ
between children born after subsequent pregnancies to the same mother. A
discordant sibling design could also rule out all environmental
differences that may vary between families, which can’t be done using a
population comparison design.14 However, although the
sibling design is helpful to control for constant family factors,
caution is required as effects could also be a result of unmeasured
unshared environmental factors.15 The women
participating with multiple pregnancies might also represent a selection
bias: there is a possibility that the first pregnancy influences the
second, as has been suggested by Sjölander et al.,16and Frisell et al.17 It should also be noted that
important moderators that could influence maternal anxiety have not been
examined (for example, genetic x environmental effects). In addition,
maternal self-reporting was used to measure maternal anxiety. Social
stigma about mental health problems, and especially a desire to appear
healthy for the expected baby, could potentially lead to the
under-reporting of anxiety symptoms. However, in large-scale cohort
studies, the full range of symptoms can be found.