Researchers studied 324 women carrying twins to understand how vitamin D levels affect pregnancy and baby health. They found that women with low vitamin D in the middle of pregnancy were more likely to deliver their babies early without medical reasons. Interestingly, the study didn’t find strong connections between vitamin D and other pregnancy complications or baby weight. The findings suggest that keeping vitamin D levels healthy during mid-pregnancy might be especially important for women expecting twins, though more research is needed to understand exactly how much vitamin D is best.

The Quick Take

  • What they studied: Whether vitamin D levels in pregnant women carrying twins affect the timing of delivery and baby health outcomes
  • Who participated: 324 women who were pregnant with twins. Researchers measured their vitamin D levels twice during pregnancy—once in the middle and once near the end—and tracked what happened during delivery and with their newborns
  • Key finding: Women with low vitamin D in mid-pregnancy were about twice as likely to deliver their babies early on their own (without medical induction). For every small drop in vitamin D, the risk of early delivery increased by about 15%
  • What it means for you: If you’re pregnant with twins, maintaining healthy vitamin D levels during mid-pregnancy may help reduce the chance of early delivery. However, this study doesn’t prove vitamin D supplements will prevent early birth—it only shows a connection. Talk to your doctor about vitamin D testing and supplementation

The Research Details

This was a longitudinal study, meaning researchers followed the same group of women over time. They collected blood samples from 324 pregnant women carrying twins at two different points: mid-pregnancy and late pregnancy. The researchers measured vitamin D levels in the blood and tracked what happened during pregnancy, delivery, and with the newborns. They used statistical methods to determine whether vitamin D levels predicted complications like early delivery, high blood pressure during pregnancy, gestational diabetes, and baby birth weight. The researchers were careful to account for other factors that might affect outcomes, like whether the twins shared a placenta and the babies’ sex.

Twin pregnancies are naturally higher-risk than single pregnancies, so finding ways to prevent complications is important. By measuring vitamin D at multiple time points and following women through delivery, researchers could see whether vitamin D levels actually predicted problems. This approach is stronger than just looking at one moment in time because it shows how vitamin D changes during pregnancy and whether those changes matter

The study included a reasonable number of participants (324 women), which gives the findings more weight. The researchers measured vitamin D twice during pregnancy rather than just once, which is more thorough. They also used advanced statistical methods to handle missing data and account for other factors that might affect outcomes. However, the study was observational, meaning researchers watched what happened naturally rather than randomly assigning some women to take vitamin D and others not to. This means we can see connections but can’t prove vitamin D supplements caused the benefits

What the Results Show

Among the 324 women studied, about one-quarter had healthy vitamin D levels in mid-pregnancy, while nearly half had insufficient levels and more than one-quarter were deficient. Women with vitamin D deficiency in mid-pregnancy were significantly more likely to deliver their babies early without medical reasons—their risk was more than double compared to women with adequate vitamin D. The study found an L-shaped relationship, meaning that as vitamin D dropped lower, the risk of early delivery increased steadily. Specifically, for every 1 ng/mL decrease in vitamin D, the risk of early delivery increased by about 15%. Interestingly, women whose vitamin D levels improved from mid-pregnancy to late pregnancy did not have better outcomes than women whose levels stayed low, suggesting that the mid-pregnancy period may be a critical window for vitamin D’s protective effect.

The study did not find significant connections between maternal vitamin D levels and other pregnancy complications like high blood pressure during pregnancy or gestational diabetes. Similarly, vitamin D levels were not associated with how much babies weighed at birth. However, other factors did matter for baby weight: whether the twins shared a placenta, whether the mother developed high blood pressure during pregnancy, and the babies’ sex all influenced birth weight

Previous research in women with single pregnancies suggested that vitamin D deficiency was linked to various pregnancy problems. This study adds new information by focusing specifically on twin pregnancies, which are different biologically. The findings partially confirm earlier concerns about vitamin D and early delivery but suggest that vitamin D may be less important for other complications than previously thought. The study’s focus on mid-pregnancy as a critical window is a new insight that wasn’t clearly established before

This study observed what happened naturally rather than randomly assigning women to take vitamin D or a placebo, so we cannot prove that low vitamin D causes early delivery—only that they are connected. The study measured vitamin D at only two time points, so researchers couldn’t track changes throughout the entire pregnancy. The study included women from a specific population and setting, so results may not apply equally to all pregnant women with twins. Additionally, the study didn’t measure other factors that affect vitamin D levels, like sun exposure and dietary intake, which could have influenced the results

The Bottom Line

If you are pregnant with twins, ask your doctor about checking your vitamin D levels, especially during mid-pregnancy. If your levels are low, your doctor may recommend vitamin D supplementation. While this study suggests vitamin D may help prevent early delivery, it’s not definitive proof. The evidence is moderate—it shows a connection but isn’t as strong as evidence from randomized controlled trials. Follow your doctor’s recommendations rather than self-supplementing

Women pregnant with twins should pay attention to these findings and discuss vitamin D with their healthcare provider. Women with single pregnancies may also benefit from vitamin D monitoring, though this study specifically looked at twins. Women with known vitamin D deficiency, limited sun exposure, or dietary restrictions that limit vitamin D intake should be especially attentive. This research is less relevant for non-pregnant individuals, though vitamin D is important for everyone’s health

If vitamin D supplementation is started in mid-pregnancy, any protective effect against early delivery would likely develop over the remaining months of pregnancy. You wouldn’t see immediate changes, but maintaining adequate levels throughout the second and third trimesters appears important. Benefits would be measured in whether delivery happens at full term rather than early

Want to Apply This Research?

  • Track your vitamin D levels (if tested) and supplementation doses in your pregnancy app. Record the dates of blood tests and the results in ng/mL. Note any supplements taken, including dose and frequency. This creates a clear record to discuss with your healthcare provider
  • If your doctor recommends vitamin D supplementation, set a daily reminder in your app to take your supplement at the same time each day. Log when you take it to ensure consistency. If you’re pregnant with twins, schedule a mid-pregnancy vitamin D test and add it to your app calendar as a reminder
  • Plan to have vitamin D levels checked at least twice during pregnancy—once in mid-pregnancy and again in late pregnancy. Track these test dates and results in your app. If supplementing, monitor for any side effects and note them. Share your vitamin D tracking data with your healthcare provider at each prenatal visit to ensure your levels are improving if supplementation was started

This research shows a connection between vitamin D levels and early delivery in twin pregnancies, but it does not prove that vitamin D supplements will prevent early birth. This summary is for educational purposes only and should not replace medical advice from your healthcare provider. Do not start, stop, or change any supplements without consulting your doctor, especially during pregnancy. If you are pregnant or planning to become pregnant, discuss vitamin D testing and supplementation with your healthcare provider to determine what is appropriate for your individual situation. Pregnant women should always follow their doctor’s recommendations for prenatal care and supplementation