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.