Longitudinal Assessment of Vitamin D Status across Trimesters of Pregnancy

Abstract

Background: The evolution of vitamin D status across pregnancy trimesters and its association with prepregnancy body mass index (ppBMI; in kg/m2) remain unclear.

Objectives: We aimed to 1) assess trimester-specific serum total 25-hydroxyvitamin D [25(OH)D] concentrations, 2) compare those concentrations between ppBMI categories, and 3) examine associations between 25(OH)D concentrations, ppBMI, and vitamin D intake.

Methods: As part of a prospective cohort study, 79 pregnant women with a mean age of 32.1 y and ppBMI of 25.7 kg/m2 were recruited in their first trimester (average 9.3 weeks of gestation). Each trimester, vitamin D intake was assessed by 3 Web-based 24-h recalls and a Web questionnaire on supplement use. Serum total 25(OH)D was measured by LC-tandem MS. Repeated-measures ANOVA was performed to assess the evolution of 25(OH)D concentrations across trimesters of pregnancy and comparisons of 25(OH)D concentrations between ppBMI categories were assessed by 1-factor ANOVAs. Stepwise regression analyses were used to identify determinants of 25(OH)D concentrations in the third trimester.

Results: Mean ± SD serum total 25(OH)D concentrations increased across trimesters, even after adjustments for ppBMI, seasonal variation, and vitamin D intake from supplements (67.5 ± 20.4, 86.5 ± 30.9, and 88.3 ± 29.0 nmol/L at mean ± SD 12.6 ± 0.8, 22.5 ± 0.8, and 33.0 ± 0.6 weeks of gestation, respectively; P < 0.0001). In the first and third trimesters, women with a ppBMI ≥30 had lower serum total 25(OH)D concentrations than women with a ppBMI <25 (P < 0.05); however, most had concentrations >40nmol/L by the second trimester. Vitamin D intake from supplements was the strongest determinant of third-trimester serum total 25(OH)D concentrations (r2 = 0.246, β = 0.51; P < 0.0001).

Conclusions: There was an increase in serum total 25(OH)D concentrations across trimesters, independent of ppBMI, seasonal variation, and vitamin D intake from supplements. Almost all women had serum total 25(OH)D concentrations over the 40- and 50-nmol/L thresholds, thus our study supports the prenatal use of a multivitamin across pregnancy.

Keywords: 25(OH)D; 25-hydroxyvitamin D; pregnancy; supplements; vitamin D; vitamin D intake; vitamin D status.

FIGURE 1

Comparison of pregnant women's trimester-specific concentrations of serum total 25(OH)D according to ppBMI categories (n = 79). Symbols represent individual 25(OH)D concentrations, thick lines represent mean 25(OH)D concentrations for each ppBMI category. Overall P values refer to 1-factor ANOVAs performed to compare serum total 25(OH)D concentrations between ppBMI categories, for each trimester and after adjustment for season at the time of sampling. *Different from women with a ppBMI <25 kg/m², P < 0.05. ppBMI, prepregnancy body mass index; 25(OH)D, 25-hydroxyvitamin D.

FIGURE 2

Comparison of pregnant women's third-trimester serum total 25(OH)D concentrations according to their mean intake of vitamin D from supplements (n = 79). Circles represent individual 25(OH)D concentrations, thick lines represent mean 25(OH)D concentrations for each supplement intake category. Solid black circles represent women that did not take any vitamin D–containing supplement throughout pregnancy. Overall P values refer to the 1-factor ANOVA used to compare third-trimester serum total 25(OH)D concentrations according to mean supplement intake of vitamin D, after adjustment for season at the time of sampling and prepregnancy BMI. *Different from women with a mean supplement intake ≥600 IU/d, P < 0.05. IU, international units; 25(OH)D, 25-hydroxyvitamin D.