Vitamin D assessment
All women participated in the study provided blood samples before 24 weeks of gestation to measure their 25(OH)D concentrations. Information was recorded on gestational age. A 200 μL blood sample was collected for each participant and placed directly into a 0.5 mL microtube after collection. Within 10 min of collection, samples were centrifuged at 3500 rpm for 15 min at 4 °C, and serum samples were taken after centrifugation and stored at -80 °C until assayed. Serum 25(OH)D concentration of each participant was measured using liquid chromatography tandem mass spectrometry (LC-MS/MS, Mass spectrometer, SCIEX4500MD) by the central laboratory of Guangzhou Golden Mile center.