Interpretation
Several studies have focused on effects of anxiety on low birthweight
and premature birth, both of which have been recognized as important
markers for later mental health problems.5, 18,19 Mean
birthweight was lower for infants born to mothers reporting severe
symptoms of anxiety, and the lowest birthweight was found for exposure
to anxiety symptoms as reported in the 30thgestational week. During the third trimester in a pregnancy, the fetus
grows rapidly and triples its weight, and these findings suggest that
severe symptoms of anxiety later in pregnancy increase the chance of a
lower birthweight. The findings persisted following control for multiple
covariates, such as smoking and alcohol exposure, birth complications,
and the socio-economic status of the mother. These findings are also in
accordance with a meta-analysis performed by Ding et
al.5 which found that maternal anxiety was associated
with an increased probability of low birthweight. We also found maternal
anxiety in pregnancy to be linked to shortened gestational age in the
full cohort, with wider overlapping confidence intervals in the sibling
control analyses. After multiple imputations maternal anxiety at week 30
in pregnancy was associated with gestational age, as was found in that
earlier meta-analysis5.
There are several hypotheses regarding the influence of maternal anxiety
on fetal growth and gestation. One such mechanism involves changes to
maternal hypothalamic-pictuary-adrenal (HPA) axis
activity.20 That is, it is suggested that maternal
anxiety during pregnancy increases the production of stress hormones
such as cortisol and catecholamines.21,22 Animal
models have found these stress hormones to influence uterine blood flow
and immunologic functioning, thus potentially increasing risk of
shortened gestational length and lower fetal growth.23Another potential explanation for this link, could be through higher
risks of infections due to stress. Studies suggests that high levels of
stress may be linked to lower immune funcuion24 , and
increased infections during pregnancy that could shorten gestation.25
The second aim was to apply a sibling comparison design to compare
siblings that differed in exposure to maternal anxiety. This made it
possible to estimate associations without the potential confounding by
unmeasured constant family effects. Several studies have found
individuals high in stress and anxiety to be at risk of poor health
outcomes26,27 and to display higher-risk
behaviours.28 Thus, several confounders needed to be
taken into consideration, including shared-family confounding.
We found that exposure to maternal anxiety was associated with lower
birthweight, also when family effects were controlled in sibling
analyses. The association increased after controlling for smoking and
alcohol exposure. However, the effect became non-significant after
parity, birth complications, child’s sex, and preeclampsia were
controlled for. These associations were also examined for gestation age;
however unlike the case of birthweight, the different set of adjustments
did not change the magnitude of the associations. It is possible that
for the sibling analyses, these associations were not significant due to
a smaller sample size. In fact, after multiple imputations to account
for missing, maternal anxiety at week 30 in pregnancy was significantly
associated with shorter gestational age.
Self-reported exposure to maternal anxiety during pregnancy has
therefore found to be associated with an increased risk of low
birthweight and shorter gestation age. This finding is in accordance
with previous studies examining prenatal anxiety and birth
outcomes.29,30,31 However, the association for
prenatal anxiety and gestation was not significant in the sibling
sub-sample, this could reflect the fact that a low-risk sample, in which
few children are born preterm, was used and it could also reflect the
fact that perinatal care was provided for these women to reduce the risk
of preterm birth. However, it should be noted that the findings are
conflicting and some studies have found only small associations between
prenatal maternal anxiety and preterm birth30 and some
have found no such association.31