Low vitamin D early in pregnancy tied to preterm birth, decreased fetal growth

Key takeaways:

• Higher vitamin D levels during the first trimester were tied to increased fetal length and lower preterm birth risk.
• Second trimester vitamin D levels were not associated with fetal growth or pregnancy outcomes.

Pregnant women with low vitamin D levels during the first trimester were four times more likely to give birth prematurely compared with women with adequate vitamin D levels, researchers wrote in The American Journal of Clinical Nutrition.

“Testing for vitamin D status is not currently part of standard prenatal care in the U.S.,” Alison D. Gernand, PhD, MPH, RD, associate professor of nutritional sciences at Pennsylvania State University, told Healio. “Clinical care providers should consider talking to pregnant patients about vitamin D during the first prenatal visit — asking whether they have vitamin D-rich foods in their diet and how much vitamin D is in the supplement they are taking.”

Beck and colleagues conducted a secondary analysis using data and samples from 351 pregnant women (mean age, 27.9 years) in the U.S. from the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be, a multisite prospective cohort study. Researchers measured serum 25-hydroxyvitamin D at 6 to 13 and 16 to 21 weeks’ gestation, fetal growth via ultrasound at 16 to 21 and 22 to 29 weeks’ gestation and neonatal anthropometric measures at birth.

Overall, 6% of infants were born small for gestational age and 8% were born preterm. Mean maternal vitamin D concentration was 68.1 nmol/L during the first trimester and 78 nmol/L during the second trimester.

One-fifth of women had vitamin D insufficiency, defined as less than 50 nmol/L, during the first trimester.

Every 10 nmol/L increase in vitamin D during the first trimester was associated with a 0.05 increase in length-for-age z score (95% CI, 0.01-0.1). Researchers observed no difference in fetal growth trajectories for length, weight or head circumference with vitamin D levels of less than 50 nmol/L vs. 50 nmol/L or greater during the first trimester.

In addition, researchers observed no differences in preterm birth risk, small for gestational age status or gestational age at birth among women with vitamin D levels of less than 50 nmol/L or 50 nmol/L or greater. However, women with first trimester vitamin D levels of less than 40 nmol/L had 4.35 times greater preterm birth risk vs. women with levels of 80 nmol/L or greater (95% CI, 1.14-16.55).

Vitamin D levels during the second trimester were not associated with fetal growth or pregnancy outcomes.

“This is a single observational study and it had a relatively small number of preterm births,” Gernand told Healio. “However, it was striking that we found those with low vs. high vitamin D levels during the first trimester had over four times the risk for giving birth prematurely.”

Gernand said more work is needed to measure vitamin D levels before someone is pregnant and track those levels into pregnancy, along with assessing the infant’s growth and other outcomes.

“Work such as this will help us understand if the problem originates from deficiency that starts before conception or in early pregnancy,” Gernand told Healio. “Supplementation trials that provide vitamin D during early pregnancy could help us understand whether improving vitamin D status could have a benefit.”

For more information:

Alison D. Gernand, PhD, MPH, RD, can be reached at adg14@psu.edu.

Reference:

• Wagner A. Vitamin D matters during first trimester, researchers find. www.psu.edu/news/health-and-human-development/story/vitamin-d-matters-during-first-trimester-researchers-find. Published Feb. 3, 2025. Accessed Feb. 5, 2025.