Introduction
It has been suggested maternal abnormal body mass index (BMI) before pregnancy is associated with several adverse pregnancy outcomes including preterm birth (PTB), abnormal birthweight and neonatal mortality.1-4 However, large cohort studies investigating the association between maternal BMI and adverse pregnancy outcomes have almost always been done in developed countries with high prevalence of overweight and obesity but low prevalence of underweight.5 Little reliable evidence exists from China or other developing countries where the prevalence of overweight and obesity is increasing but the prevalence of underweight is still high.6
Although the cause of adverse pregnancy outcomes is usually unknown, maternal age is the strongest known risk factor. The risk of several adverse pregnancy outcomes (such as PTB and miscarriage) is slightly elevated in the youngest mothers and then rises sharply in older mothers.1, 7-9 However, very little study has been done to investigate the association of pre-pregnancy BMI with adverse pregnancy outcomes according to the maternal age, which is vital for risk stratification and interventions tailored to subgroup population.
We aimed to clarify the association of maternal pre-pregnancy BMI with risk of several adverse pregnancy outcomes in a large population-based cohort study in China, and to quantify such risk by maternal age in order to provide accurate data for risk assessment and counselling in pre-pregnancies.