Researchers looked at over 127,000 pregnant women across 116 studies to understand how common vitamin D deficiency is during pregnancy. They found that vitamin D deficiency is extremely common—affecting about 7 out of 10 pregnant women in the first trimester, 8 out of 10 in the second trimester, and 7 out of 10 in the third trimester. Vitamin D is important for baby development, immune health, and preventing serious pregnancy complications like preeclampsia and gestational diabetes. This large review suggests that vitamin D deficiency during pregnancy is a major public health issue that needs attention, and doctors may need to screen and treat pregnant women more carefully for this problem.

The Quick Take

  • What they studied: How many pregnant women around the world don’t have enough vitamin D in their blood during pregnancy
  • Who participated: Over 127,000 pregnant women from 116 different research studies conducted worldwide. The studies measured vitamin D levels in blood samples taken during different stages of pregnancy.
  • Key finding: About 68% of pregnant women had low vitamin D in the first 3 months, 81% in the middle 3 months, and 70% in the final 3 months of pregnancy. This means vitamin D deficiency is extremely common during all stages of pregnancy.
  • What it means for you: If you’re pregnant or planning to become pregnant, getting your vitamin D levels checked is important. Talk to your doctor about whether you need vitamin D supplements, especially since deficiency is so common. However, don’t start supplements without medical guidance, as too much vitamin D can also be harmful.

The Research Details

This was a systematic review and meta-analysis, which is like a super-study that combines results from many smaller studies to find overall patterns. Researchers searched medical databases for all studies published through July 2023 that measured vitamin D levels in pregnant women during each trimester (the three 3-month periods of pregnancy). They found 2,627 studies initially, but only 116 met their strict requirements—meaning the studies had to clearly report vitamin D deficiency rates for each trimester separately.

Two researchers independently reviewed each study to make sure they were looking at good quality research. They used a special checklist called the Newcastle-Ottawa Scale to evaluate how reliable each study was. This careful approach helps ensure the final results are trustworthy and not biased by including poor-quality studies.

The researchers combined data from all 116 studies, which included information from over 127,000 pregnant women total. By pooling all this information together, they could see the big picture of how common vitamin D deficiency really is during pregnancy across different populations and countries.

This research approach is important because it gives us the most complete picture possible. Single studies might only look at a few hundred women in one location, which might not represent all pregnant women everywhere. By combining 116 studies with over 127,000 women, researchers can see if vitamin D deficiency is a problem everywhere or just in certain places. This helps doctors and public health officials understand how serious the problem is and whether they need to take action.

This study followed strict international guidelines (PRISMA) for conducting systematic reviews, which is a gold standard in research. Two researchers independently reviewed studies to reduce mistakes. The researchers assessed the quality of each included study using a validated tool. The study was registered in advance (PROSPERO), which prevents researchers from changing their methods after seeing results. The large sample size (127,290 women) and consistent findings across trimesters strengthen confidence in the results. However, the studies included were observational (not experiments), which means they show patterns but can’t prove vitamin D deficiency causes pregnancy problems.

What the Results Show

The analysis revealed that vitamin D deficiency is extremely widespread during pregnancy. In the first trimester (months 1-3), about 68% of pregnant women had insufficient vitamin D levels. This number increased to 81% in the second trimester (months 4-6), meaning deficiency actually becomes more common as pregnancy progresses. In the third trimester (months 7-9), the rate was 70%, still affecting the majority of pregnant women.

These percentages were consistent across the many studies reviewed, suggesting this isn’t just a problem in one region or population—it appears to be a global issue. The researchers used a statistical method to combine results from different studies while accounting for differences in how studies measured vitamin D and which populations they studied.

The high prevalence across all three trimesters is particularly concerning because vitamin D plays important roles throughout pregnancy. Early pregnancy needs vitamin D for the embryo to implant properly. Throughout pregnancy, it helps regulate the immune system and may prevent serious complications. The consistency of high deficiency rates across all trimesters suggests this is a persistent problem that needs attention.

While the main findings focused on deficiency rates by trimester, the research also highlighted that vitamin D deficiency is a widespread public health issue affecting diverse populations globally. The fact that deficiency rates were high across different countries and study populations suggests this isn’t limited to specific geographic regions or ethnic groups. The research suggests that current vitamin D intake and sun exposure levels for many pregnant women worldwide are insufficient to maintain healthy levels.

This comprehensive review brings together scattered research to confirm what smaller studies have suggested: vitamin D deficiency during pregnancy is very common. Previous individual studies showed high rates of deficiency, but this meta-analysis confirms these findings are consistent across many different populations and research settings. The findings align with growing medical awareness that vitamin D deficiency is a significant global health issue, not just during pregnancy but in the general population as well.

This study only looked at observational research (studies that observe and measure, rather than experiments where some people get treatment and others don’t). This means the studies show that deficiency is common, but they can’t prove that vitamin D deficiency directly causes pregnancy complications—only that they occur together. The studies included came from different countries and used slightly different methods to measure vitamin D, which could affect results. The review only included studies published in English, Spanish, and Portuguese, so research in other languages wasn’t included. Finally, the studies measured vitamin D at different times and used different definitions of what counts as ‘deficiency,’ though researchers tried to account for this.

The Bottom Line

Pregnant women should discuss vitamin D screening and supplementation with their healthcare provider. The evidence suggests that vitamin D deficiency is so common that many pregnant women would benefit from supplementation, but the right dose depends on individual factors. Current medical organizations recommend pregnant women get adequate vitamin D, though specific recommendations vary. Don’t self-treat with supplements—work with your doctor to determine if you need them and in what amount, as excessive vitamin D can be harmful. (Confidence level: High that deficiency is common; Moderate that supplementation prevents complications)

This research is most relevant for pregnant women, women planning to become pregnant, and their healthcare providers. It’s also important for public health officials who plan health programs for pregnant women. Women living in northern climates with less sun exposure, those with darker skin (which produces less vitamin D from sun), those who cover their skin for cultural or religious reasons, and those with limited dairy intake may be at higher risk. However, since deficiency is so common, all pregnant women should discuss vitamin D with their doctor.

If you start vitamin D supplementation based on your doctor’s recommendation, it takes several weeks to months to build up vitamin D levels in your body. You might not feel immediate changes, but adequate vitamin D levels throughout pregnancy support proper fetal development and may reduce risk of complications. Benefits are most important when vitamin D levels are adequate throughout pregnancy, so starting early (ideally before pregnancy or in the first trimester) is ideal.

Want to Apply This Research?

  • Track your vitamin D supplementation daily (dose and time taken) and log any vitamin D blood test results with dates. Note the trimester you’re in to correlate with deficiency risk periods.
  • Set a daily reminder to take your vitamin D supplement at the same time each day (such as with breakfast). If your doctor recommends it, log your supplement intake in the app to ensure consistency and help you remember doses.
  • Use the app to track when you have vitamin D blood tests scheduled and record the results. Monitor compliance with supplementation over weeks and months. Note any pregnancy-related symptoms or complications to discuss with your healthcare provider alongside vitamin D status.

This research shows that vitamin D deficiency is common during pregnancy, but it does not prove that deficiency causes pregnancy complications. Always consult with your obstetrician or healthcare provider before starting any vitamin D supplements during pregnancy. Do not self-diagnose or self-treat based on this information. Vitamin D supplementation should be personalized based on your individual blood levels, health status, and medical history. Excessive vitamin D intake can be harmful during pregnancy. This summary is for educational purposes and should not replace professional medical advice.