Interpretation
The causes of adverse pregnancy outcomes are complex and multifactorial. However, the associations of pre-pregnancy BMI with pregnancy outcomes could be explained by the uterine environment of the different weight phenotype. Compared with normal weight, underweight women have lower plasma volume and rennin-aldosterone response during pregnancy22, which may be associated with uteropla-central insufficiency and the increased prevalence of SGA. Previous studies speculated that inflammatory or intrauterine infection may be on the causal pathway between pre-pregnancy underweight or obesity and PTB23, 24, although increased prevalence of postpartum infective complications was not observed in several studies4, 23.
The associations of pre-pregnancy obesity or overweight with adverse pregnancy outcomes might be related to abnormal metabolism of fat. Obese women have higher levels of cord blood tumour necrosis factor α (TNF- α) and RANTES during pregnancy, which are known contributors to gestational diabetes mellitus and associated with an increased risk of LGA25, whilst LGA was associated with the increased risk of caesarean delivery, shoulder dystocia26, and stillbirth27. Overweight and obese women have increased insulin resistance in early pregnancy that becomes manifest clinically in late gestation as glucose intolerance and fetal overgrowth, which also are known risk factors for adverse pregnancy outcomes, such as caesarean delivery, shoulder dystocia and stillbirth28, 29. Furthermore, overweight and obesity is likely to gain more weight during pregnancy, which is known risk factors of several pregnancy complications30 (such as gestational diabetes mellitus, gestational hypertension) and associated with adverse pregnancy outcomes31. Overall, further studies are needed to uncover the potential mechanisms of adverse pregnancy outcomes related to pre-pregnancy BMI.