Study design and study population
We performed an observational study using monitoring data collected as part of the BD surveillance system of Zhejiang Province, China between 2012 and 2018. This system encompassed all births, including live birth, early fetal loss, stillbirth, and neonatal deaths within 7 days of birth, that involved a BD and occurred in 90 hospitals located in the 30 regions of Zhejiang Province, which covers one-third of the total number of births in the province. A total of 54,572 births associated with a BD were recorded in the system between 2012 and 2018. All the mothers included in the system underwent routine antenatal health care visits at least five to ten times during their pregnancies, in line with the antenatal health care regulations enacted by China’s Ministry of Health.21 Questionnaire surveys regarding socio-demographic data (including maternal age, domicile, and education), obstetric characteristics (including maternal parity, singleton vs . multiple pregnancy, and prenatal diagnosis), and birth data (including birth weight, birth outcome, and sex) were conducted by medical staff in each of the hospitals. Quality control was performed at the local hospital and provincial levels.
For the purposes of the present study, participants for whom data were missing regarding whether they had a singleton or multiple pregnancy (n = 62), period of diagnosis (prenatal or postnatal) (n = 40), the timing of screening (in which gestational week) (n = 548), the socioeconomic status of the mother (n = 36), maternal parity (n = 524), or infant sex (n = 166) were excluded. Thus, ultimately, 53,196 births were included in the analyses.