Pregnant women with low levels of vitamin D during the first trimester of pregnancy had more than four times the risk of preterm birth compared to those with higher levels of the vitamin, according to a study funded by NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development. Moreover, the researchers found an association between infant length at birth and maternal levels of vitamin D in the first trimester.
The study was conducted by Celeste Beck, Ph.D., and Alison D. Gernand, Ph.D., of Pennsylvania State University, and colleagues. It appears in The American Journal of Clinical Nutrition.
Background
Vitamin D is a nutrient that helps the body absorb calcium and is needed for bone health. It is also important for the functioning of nerves, muscles, and the immune system. In early pregnancy, vitamin D is involved in the implantation of the fertilized egg in the uterus and formation of the placenta. Furthermore, low maternal vitamin D levels have been associated with preeclampsia (a blood pressure disorder of pregnancy), gestational diabetes, and preterm birth.
Most previous studies have investigated potential links between maternal vitamin D levels in the second and third trimesters of pregnancy. Little is known, however, about the potential associations between vitamin D levels in the first trimester of pregnancy.
Results
For the current study, researchers analyzed data and blood samples from 351 participants in a larger study of first-time mothers. They measured maternal vitamin D levels at 6 to 13 weeks of pregnancy (within the first trimester) and again at 16 to 21 weeks of pregnancy (within the second trimester).
The study team found a positive link between vitamin D levels during the first trimester and infant length. Each 10 nanomole per liter (nm/L) increase in maternal 25-hydroxyvitamin D (a marker of vitamin D levels in the blood) in weeks 6 to 13 of pregnancy was associated with a 0.05 increase in fetal length z-score, which is a measure of fetal length based on growth standards.
The researchers also found a link between first trimester vitamin D levels and preterm birth. Participants with low vitamin D levels (less than 40 nm/L) at 6 to 13 weeks had 4.35 times the risk of preterm birth as those with higher levels (more than 80 nm/L).
The researchers did not find any links between second trimester vitamin D levels and fetal length, preterm birth, or other pregnancy outcomes evaluated in the study.
Significance
The authors called for additional studies of vitamin D supplementation during pre- and early pregnancy to determine its potential effects on fetal length and preterm birth risk.
“Testing for vitamin D status is not currently part of standard prenatal care in the United States. If you are pregnant, consider talking with your health care provider for advice at your first prenatal visit,” said Dr. Gernand. “This study provides evidence that vitamin D early in pregnancy appears to be an important part of nutritional health.”
Reference
Beck C, et al. Maternal vitamin D status, fetal growth patterns, and adverse pregnancy outcomes in a multisite prospective pregnancy cohort. American Journal of Clinical Nutrition DOI: 10.1016/j.ajcnut.2024.11.018 (2025)
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