Study design and Participants
We undertook a retrospective cohort study of women with reproductive age who participated in the National Free Preconception Health Examination Project (NFPHEP) from 1st January 2013 to 31st December 2017, successfully became pregnant and then had pregnancy outcome in Guangdong Province, China. The NFPHEP has covered all rural counties/districts since 2013, with aim to decline adverse pregnancy outcomes through providing free health examination before conception, counselling services for reproductive couples. The study design, organization and implementation have been described previously7-10.
In the current analysis, we excluded women who did not measure weight and height before pregnancy; women with chronic disease (including anaemia, hypertension, heart disease, hepatitis B, epilepsy, thyroid disease, chronic nephritis, cancer and diabetes) as they were prone to receive medical interventions before and during pregnancy. Women with multiple births or without information on gestational week, birth weight, delivery method, shoulder dystocia or injury birth or stillbirth were also excluded.
The NFPHEP was approved by the Institutional Review Board of the Chinese Association of Maternal and Child Health Studies. Written informed consent was obtained from the participants before recruitment. The present study was executed jointly by Guangzhou Medical University and Guangdong Institute of Family Planning Science and Technology, in which the review boards determined that this study was exempt for ethical approval owing to the use of de-identified data.