Study population
We conducted a prospective cohort study in the Department of Obstetrics of The Eighth Affiliated Hospital, Sun Yat-sen University and Guangdong Women and Children Hospital between January 2019 and December 2020. The study design was approved by the local Ethical and Research Committees of the two hospitals. Pregnant women were recruited before 24 weeks of gestation. For most women, the expected delivery date was based on the last menstrual period date, while for a small proportion, this estimate was updated following an ultrasound scan. Those pregnant women would not be enrolled if they had any of the following exclusion criteria: multiple pregnancy, ischemic heart disease, stroke, peripheral vascular disease, diagnosis of diabetes, or/and hypertension before the current pregnancy. In addition, individuals who had a normal diet were selected; vegetarians and malnourished were excluded. Every participant gave full and informed consent to participate in the study. Initially, 1924 women were enrolled in the study. Figure 1 showed the filtering and processing flow chart of the study population. 9 women dropped out midway, 139 women were subsequently excluded because they were discovered to have exclusion criteria or had no blood sample to measure serum 25(OH) D concentrations, 2 women were excluded for implausible information, 8 women did not consent to the use of outcome data. Finally, there are 1766 women with available data for analysis. Personal medical histories were self-identified by the participants.