A retrospective cohort study of 2742 pregnancies in Deyang, Southwest China assessed vitamin D deficiency using the LC-MS/MS method. The median serum 25(OH)D concentration was 16.3 ng/mL, with 62.5% of pregnant women deficient (<20 ng/mL). Early pregnancy (≤12 weeks) increased the risk of deficiency 3.37 times compared to late pregnancy (≥25 weeks). Early deficiency was associated with gestational hypothyroidism (OR=1.32, P=0.048). Late deficiency was paradoxically correlated with a lower risk of preterm birth (OR=0.32, P=0.014). One-month D3 supplementation at 800 IU increased 25(OH)D by +28.4 ng/mL, which was significantly better than D2-800 IU (+14.6 ng/mL, P<0.001).[2]