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.