To investigate the serum 25-hydroxy vitamin D concentrations and related factors in early pregnancy.

Plasma was collected in the first trimester from 23 396 pregnant women to investigate the vitamin D level, and its distribution and differences in different age, body maxx index(BMI) and seasons between primipara and multipara. Preterm birth was used as an indicator of clinical outcomes. Vitamin D concentrations were measured using chemiluminescence microparticle immunoassay.

25-hydroxy vitamin D level was 42.0(17.6-76.6)nmol/L in totally 23 396 pregnant women with early pregnancy, and 5% and 95% percentile sites of vitamin D level were 20.2 nmol/L and 70.7 nmol/L respectively. There were 18 170(77.7%) primiparas and 5 226(22.3%) multiparas, with the mean age of 30.0(24.0-38.0) years and BMI of 20.7(16.5-27.6)kg/m². The number of cases detected in spring (March, April and May), summer(June, July and August), autumn(September, October and November months) and winter(December, January and February) were 5 878, 5 554, 5 974, and 5 990, respectively, and the vitamin D levels were 40.0(29.3, 52.7)nmol/L, 46.2(35.6, 57.2)nmol/L, 43.8(33.1, 54.8)nmol/L and 37.2(26.9, 49.9)nmol/L respectively, with the difference in vitamin D levels statistically significant among the four seasons (P<0.001). According to BMI, all pregnant women were divided into four groups as BMI<18.5 kg/m², 18.5 kg/m²≤BMI≤23.9 kg/m², 24 kg/m²≤BMI≤27.9 kg/m², BMI≥28 kg/m^2, and the levels of 25-hydroxy vitamin D were 43.5(30.9, 56.9)nmol/L, 42.1(30.8, 53.8)nmol/L, 39.9(30.7, 50.4)nmol/L and 39.7(30.7, 49.4)nmol/L respectively with the difference statistically significant among the four groups. The levels of vitamin D detected in pregnant women with age<25 years, 25~29 years, 30~34 years, ≥35 years were 39.1(28.4, 52.3)nmol/L, 41.3(30.1, 52.9)nmol/L, 42.4(31.2, 54.1)nmol/L and 43.8(31.9, 55.7)nmol/L respectively and the difference was statistically significant (P<0.001). The levels of 25- hydroxy vitamin D in primary and multiparas were 41.6(30.2, 52.9)nmol/L and 43.5(32.5, 56.8) nmol/L with the difference statistically significant (P<0.001). Among women of different gestational age during childbirth, the differences in serum 25-hydroxy vitamin D in early pregnancy were not statistically significant(P=0.121). The severe deficiency of vitamin D in early pregnancy was defined as serum level of 25-hydroxy vitamin D less than 5th level. There were statistical differences in the probability of severe vitamin D deficiency of different ages, seasons and BMI between primiparas and multiparas.

Pregnant women of lower weight, lower age and primiparity have higher incidence of the severe vitamin D deficiency in early trimester of pregnancy. There is significant difference in 25-hydroxy vitamin D level among the different seasons (winter<spring <autumn <summer). Vitamin D level in early pregnancy is not associated with preterm birth.

25-hydroxy vitamin D; Vitamin D deficiency; Early pregnancy