Researchers combined results from 24 studies involving over 52,000 pregnant women to understand how vitamin D affects two serious pregnancy conditions: preeclampsia (dangerously high blood pressure) and gestational diabetes (high blood sugar during pregnancy). The analysis found that pregnant women who took vitamin D supplements had about 42-45% lower chances of developing these conditions compared to those who didn’t take supplements. The benefits were strongest when women started taking vitamin D early in pregnancy, especially if they had low vitamin D levels to begin with. While these findings are promising, doctors emphasize that more research is needed before making vitamin D a standard recommendation for all pregnant women.

The Quick Take

  • What they studied: Whether taking vitamin D supplements during pregnancy can prevent or reduce the risk of preeclampsia (high blood pressure complications) and gestational diabetes (pregnancy-related diabetes)
  • Who participated: Over 52,000 pregnant women from 24 different research studies conducted between 1993 and 2025. Studies included women from various countries and backgrounds with different vitamin D levels at the start of pregnancy
  • Key finding: Pregnant women who took vitamin D supplements had about 42% lower risk of preeclampsia and 45% lower risk of gestational diabetes compared to women who didn’t take supplements. The protection was strongest in women who started supplements early in pregnancy and had low vitamin D levels initially
  • What it means for you: If you’re pregnant or planning to become pregnant, vitamin D supplementation may help reduce your risk of developing serious pregnancy complications. However, talk to your doctor before starting any supplements, as the right dose depends on your individual vitamin D levels and health situation

The Research Details

This was a systematic review and meta-analysis, which means researchers searched through medical databases to find all high-quality studies on vitamin D and pregnancy complications published over 30 years. They found 24 studies that met their strict quality standards. Two independent reviewers carefully checked each study to make sure it was reliable and extracted the important information. The researchers then combined the results from all these studies using statistical methods to see the overall pattern of how vitamin D affects pregnancy health. They separated the analysis into two types of studies: randomized controlled trials (where some women were randomly given vitamin D and others weren’t) and cohort studies (where researchers followed women over time and tracked their vitamin D levels and health outcomes).

The researchers followed strict international guidelines called PRISMA to make sure their review was done properly and registered their plan ahead of time with a database called PROSPERO. This helps prevent bias and makes the research more trustworthy. They used a tool called the Joanna Briggs Institute checklist to evaluate how well each study was designed and conducted.

This research approach is important because individual studies sometimes give different answers about whether vitamin D helps. By combining results from many studies with thousands of participants, researchers can see the true overall effect more clearly. This type of analysis is considered one of the strongest forms of evidence in medicine because it reduces the chance that one study’s unusual results will mislead us. The fact that they separated randomized trials from observational studies is also important because randomized trials provide stronger evidence about whether vitamin D actually causes the benefit

This review is considered high-quality because it followed strict international guidelines, was registered before completion, used two independent reviewers to check studies, and included only well-designed studies. The researchers clearly reported their methods so others can verify their work. However, readers should know that some studies showed more variation in results than others (measured by something called I-squared), which means not all studies agreed perfectly. The fact that they found consistent results across many large studies strengthens confidence in the findings

What the Results Show

In randomized controlled trials (the strongest type of study), vitamin D supplementation reduced preeclampsia risk by 42% and gestational diabetes risk by 45%. This means that if 100 pregnant women took vitamin D supplements, about 42-45 fewer would develop these conditions compared to 100 women who didn’t take supplements. The benefit was most powerful for women who had low vitamin D levels at the start of pregnancy and for women who had experienced preeclampsia in a previous pregnancy.

When researchers looked at observational studies (where they tracked women’s natural vitamin D levels without giving them supplements), they found that women with low vitamin D had a 29% higher risk of developing gestational diabetes. Interestingly, low vitamin D wasn’t clearly linked to preeclampsia in these observational studies, though the results were less consistent.

The timing of supplementation mattered. Women who started taking vitamin D early in pregnancy, especially in the first three months, saw the greatest benefit. This suggests that vitamin D’s protective effects may work best when it’s present from the earliest stages of pregnancy development.

The analysis found that vitamin D’s protective effect was particularly strong for women who were having a second or later pregnancy after previously having preeclampsia. For these women, vitamin D supplementation appeared especially beneficial. The studies also showed that vitamin D supplementation was safe, with no serious side effects reported. Different doses of vitamin D were used across studies, but even moderate supplementation appeared helpful

Previous research on vitamin D and pregnancy has been mixed, with some studies showing benefits and others showing no effect. This comprehensive review helps clarify the picture by showing that vitamin D supplementation does appear to help, at least based on the randomized controlled trials. The findings align with what we know about vitamin D’s role in immune function and blood vessel health, both of which are involved in preeclampsia and gestational diabetes. However, this review also shows that the relationship between vitamin D deficiency and these conditions is more complex than initially thought, since low vitamin D was clearly linked to gestational diabetes but not as clearly to preeclampsia

While this review is comprehensive, it has some limitations. First, the studies included used different doses of vitamin D and different ways of measuring vitamin D levels, which can affect results. Second, most studies were from developed countries, so the findings may not apply equally to all populations worldwide. Third, some studies showed inconsistent results (high variation), particularly for preeclampsia in observational studies, suggesting that other factors we don’t fully understand may play a role. Fourth, the review couldn’t determine the ideal vitamin D dose or the best time to start supplementation, since studies varied. Finally, researchers couldn’t fully explain why vitamin D appeared to help in randomized trials but the connection to vitamin D deficiency was less clear in observational studies

The Bottom Line

Based on this research, pregnant women or those planning pregnancy should discuss vitamin D supplementation with their healthcare provider. The evidence suggests vitamin D supplementation may reduce pregnancy complication risks, particularly for women with low vitamin D levels. However, the ideal dose hasn’t been determined by this research, so your doctor should help determine what’s right for you based on your individual vitamin D levels and health history. Starting supplementation early in pregnancy appears most beneficial. This research supports considering vitamin D as part of routine pregnancy care, but it shouldn’t replace other proven preventive measures like regular blood pressure monitoring and blood sugar screening

This research is most relevant for pregnant women, women planning to become pregnant, and healthcare providers caring for pregnant women. It’s especially important for women with known vitamin D deficiency, women who have had preeclampsia before, women with risk factors for gestational diabetes (such as family history or overweight), and women living in areas with limited sun exposure. Women in low- and middle-income countries where these complications are more common may particularly benefit. However, women with certain medical conditions affecting calcium metabolism should consult their doctor before supplementing, as should women taking other medications

If you start vitamin D supplementation during pregnancy, the protective effects appear to develop gradually throughout pregnancy. The strongest benefits were seen when supplementation started in the first trimester and continued throughout pregnancy. You wouldn’t expect to feel immediate changes, but the goal is preventing serious complications that typically develop in the second and third trimester. If you’re at high risk for these conditions, your doctor will monitor you with regular blood pressure checks and blood sugar tests throughout pregnancy

Want to Apply This Research?

  • Track daily vitamin D supplementation (dose and timing) alongside weekly blood pressure readings and any symptoms like swelling, headaches, or unusual thirst. This creates a complete picture of your pregnancy health and helps you and your doctor identify any concerning patterns
  • Set a daily reminder to take your vitamin D supplement at the same time each day (such as with breakfast). If you’re pregnant or planning pregnancy, use the app to schedule a conversation with your doctor about vitamin D testing and supplementation before or early in pregnancy. Log your vitamin D levels when tested so you can track whether supplementation is working
  • Use the app to track vitamin D supplementation consistency (aiming for daily intake), monitor blood pressure weekly if recommended by your doctor, and log any pregnancy symptoms. Create a monthly summary to share with your healthcare provider showing your adherence to supplementation and any health changes. If you’re at high risk for gestational diabetes, also track fasting blood sugar readings as recommended by your doctor

This research summary is for educational purposes only and should not replace professional medical advice. Vitamin D supplementation during pregnancy should only be started under the guidance of your healthcare provider, as the appropriate dose depends on your individual vitamin D levels, health history, and pregnancy circumstances. If you are pregnant or planning to become pregnant, consult with your obstetrician or midwife before starting any supplements. This research shows promising associations but does not prove that vitamin D supplementation will prevent preeclampsia or gestational diabetes in every case. Individual results vary based on many factors including genetics, diet, lifestyle, and overall health. Always inform your healthcare provider of any supplements you are taking