Why Vitamin D is So Important During the First Trimester

Low vitamin D levels in the first trimester of pregnancy are associated with higher rates of preterm birth and lower fetal length, according to a new study led by researchers at Penn State’s Department of Nutritional Sciences. This research provides evidence that early pregnancy or even the period prior to conception may be critical times to intervene in women with low vitamin D status to optimize pregnancy outcomes.

Vitamin D Status of the Mother in the First and Second Trimester

The study was led by Celeste Beck, who received her doctorate in nutritional sciences from Penn State University in 2023 and is currently a senior research scientist at Heluna Health, and Alison Gernand, Beck’s doctoral supervisor and associate professor of nutritional sciences at Penn State University. Their findings were recently published in The American Journal of Clinical Nutrition. “More than 25% of all pregnant or breastfeeding women have vitamin D levels below the recommended level,” said Gernand, explaining that previous research has demonstrated the effect of vitamin D on fetal skeletal growth, maternal immune function at the fetal interface, and placental development in pregnant women. Much of the development in early pregnancy requires vitamin D, so we conducted this study to better understand how vitamin D status in early pregnancy is associated with pregnancy outcomes.”

Most previous studies of vitamin D status in pregnant women have measured vitamin D concentrations in the second trimester or later, the researchers said. They explained that, to their knowledge, this study is the first to examine both maternal vitamin D status in the first and second trimesters in relation to longitudinal fetal growth and pregnancy outcomes. Researchers at Penn State University worked with colleagues at the University of Utah to examine blood samples from 351 women collected as part of the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be. This study was funded by the National Institute of Child Health and Human Development and recruited pregnant women throughout the United States between 2010 and 2013.

Higher Risk of Preterm Birth

Vitamin D was measured in nanomoles per liter (nmol/L) of blood. According to the Institute of Medicine, vitamin D insufficiency is defined as less than 50 nmol/L. When researchers compared the outcomes of women with vitamin D insufficiency (less than 50 nmol/L) to women with adequate vitamin D levels (greater than or equal to 50 nmol/L), they found no statistical differences in pregnancy outcomes. However, when the researchers compared pregnancy outcomes across a wider range of vitamin D concentrations, they found that pregnant women with vitamin D concentrations of less than 40 nmol/L in the first trimester were four times more likely to give birth prematurely than women with vitamin D concentrations of at least 80 nmol/L.

Despite the higher risk of preterm birth among women with low vitamin D status, the researchers cautioned that these findings were based on a very small number of preterm births in this study and recommended that additional, larger studies be conducted. The researchers also observed an association between first-trimester vitamin D concentrations and certain fetal growth patterns. Women with higher vitamin D levels had a small but statistically significant increase in fetal length.

When the researchers examined the mothers’ vitamin D levels in the second trimester of pregnancy, they found no differences in fetal growth patterns or pregnancy outcomes between women with lower vitamin D status and those with higher vitamin D status. They stated that more women in the study had healthy vitamin D levels in the second trimester, so their ability to detect results may have been limited. Nevertheless, they said further research is needed to gain a more comprehensive understanding of these results.

Have Your Nutritional Status Checked Before Pregnancy

“This study provides evidence that nutrition in early pregnancy – and even before conception – is critically important,” Beck said. ”Depending on their diet and lifestyle, individual women may or may not need supplements. However, a healthy diet is essential to promote healthy fetal development, according to the researcher. This study suggests that vitamin D levels – along with iron, folic acid and other essential nutrients in pregnancy – should be monitored and understood early on by obstetricians and women to promote healthy birth outcomes. The results suggested the potential benefits of taking a vitamin D supplement for women planning to become pregnant, the researchers said.

In this study, mothers with low vitamin D status were less likely to take a multivitamin. Women should therefore consider having their nutritional status evaluated by their obstetrician before becoming pregnant and discuss the need for vitamin D supplementation. This study provides evidence that vitamin D appears to be an important part of a pregnant woman’s diet.