In relation to other studies
Investigators leading several large, retrospective cohort studies, which
were mostly done in developed countries, assessed the association of
maternal BMI with several adverse pregnancy outcomes.3,
15 Sohinee and colleagues 3 used data from the
Aberdeen Maternity and Neonatal Databank (AMND) in UK, encompassing
24241 discharges from 1976 to 2015, and found a linear relationship
between increasing BMI and the risk of developing macrosomia, caesarean
delivery, while underweight women had better pregnancy outcomes than
women with normal BMI. Judith and colleagues [15]analysed singleton pregnancies of 436414 women in California and found
that increasing BMI was associated with increasing odds of adverse
outcomes. Obese women (BMI=30-39.9) were nearly twice as likely to
undergo caesarean (adjusted OR 1.82, 95%CI: 1.78-1.87) and twice more
likely to give macrosomia (>4000g), compared with abnormal
BMI (18.5-24.9). However, the association of pre-pregnancy BMI with PTB
(<37 weeks) was only found among underweight women (1.22,
1.16-1.28). Ram and colleagues 16 analysed data from
the Better Outcomes Registry & Network Ontario, Canada, encompassing
48780 singleton and 7860 twin births between 2012 and 2016, and found
that the risk of caesarean delivery increased with high maternal BMI in
both singleton and twin gestations, however, the risk of PTB
(<32 weeks) is only associated with underweight (adjusted RR:
2.10, 95%CI: 1.44-3.08). Some studies conducted in China use
self-reported and recalled pre-pregnancy BMI, or did not adjust for some
important confounders including history of pregnancy and pregnancy and
adverse pregnancy outcomes, both of which weakened the validity of the
association between maternal BMI and pregnancy outcomes17, 18.
The associations of maternal pre-pregnancy BMI with LGA, SGA and
caesarean delivery have been consistent among previous study3, 15, 16, 17, 18, 19, but not the association of
maternal pre-pregnancy BMI with PTB, shoulder dystocia or birth injury
and stillbirth. For example, some studies suggested that only
underweight was associated with PTB 15, 16, while
others suggested only obesity were associated with PTB18, 19. The findings of our large cohort study were,
however, inconsistent with previous study, which found that both of
underweight and obesity were associated with PTB. Evidence from a recent
meta-analysis suggested that maternal pre-pregnancy obesity associated
with an increased risk of shoulder dystocia (RR: 1.63, 95%CI 1.33-1.99)20, which was also inconsistent with our findings.
Another meta-analysis suggested that both obesity and overweight were
associated with stillbirth (OR, 1.27, 95%CI 1.18-1.36 and 1.81, 95% CI
1.69-1.93, respectively) 21, however, in our finding
we only find significant association of overweight with stillbirth. The
discrepancies of the association of pre-pregnancy BMI with adverse
pregnancy outcomes might be related to sample size, methods of research,
regions, and the various characteristics within the study population,
such as different prevalence of abnormal BMI, types and definition of
adverse pregnancy outcomes.
To our best of knowledge, our study is the first to investigate the
pre-pregnancy BMI with adverse pregnancy outcomes according to maternal
age, and the findings suggested that the associations differed according
to maternal age. A recent study used nationwide birth certificate data
from the US National Vital Statistics System to investigate the
association of pre-pregnancy obesity with PTB, also found that the
association of obesity with PTB differed according to maternal
age1. Both of these findings suggested that risk
assessment and counselling about pre-pregnancy BMI on adverse pregnancy
outcomes should be stratified by maternal age.