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.