Researchers studied 303 pregnant women to see if vitamin D levels at the start of pregnancy mattered, even when women took vitamin D supplements. They found that women who started pregnancy with healthy vitamin D levels had far fewer problems like preeclampsia (dangerously high blood pressure), gestational diabetes, and early labor—even though women with low vitamin D caught up by mid-pregnancy. This suggests that having enough vitamin D before you get pregnant, not just during it, might be really important for a healthy pregnancy.

The Quick Take

  • What they studied: Does having enough vitamin D at the very beginning of pregnancy protect against pregnancy problems, even if you take vitamin D supplements later?
  • Who participated: 303 pregnant women aged 16-50 in Mexico, split into two groups: 151 with low vitamin D at the start and 152 with healthy vitamin D levels. Both groups took the same vitamin D supplement (4000 IU daily).
  • Key finding: Women who started pregnancy with healthy vitamin D levels had about 5 times fewer serious complications. For example, only 1.3% developed preeclampsia compared to 10.6% in the low vitamin D group. The protection held even though the low vitamin D group caught up by mid-pregnancy.
  • What it means for you: If you’re planning to get pregnant, getting your vitamin D checked and corrected beforehand might be more protective than waiting until you’re already pregnant. However, this is one study and more research is needed before changing medical guidelines.

The Research Details

This was a retrospective cohort study, which means researchers looked back at medical records of pregnant women who received care between 2022 and 2024. They divided the women into two groups based on their vitamin D blood levels measured in the first trimester (the first 3 months of pregnancy): those with healthy levels (30 ng/mL or higher) and those with low levels (below 30 ng/mL). All women received the same standard vitamin D supplement during pregnancy. The researchers then compared how many pregnancy problems each group experienced.

The researchers used statistical tests to compare the two groups and identify which factors predicted pregnancy complications. They controlled for other differences between the groups to make sure vitamin D was the main factor causing the differences in outcomes.

This study design is valuable because it looks at real-world pregnancy outcomes rather than just lab measurements. By comparing women who received identical supplements but started with different vitamin D levels, the researchers could isolate vitamin D’s importance. The fact that women with low vitamin D caught up by mid-pregnancy but still had more problems suggests that early pregnancy vitamin D status is particularly critical.

This study has several strengths: it included a reasonably large number of women (303), the two groups were similar in other ways, and the researchers used proper statistical methods. However, because it looked backward at medical records rather than following women forward, and because it was done in one hospital in Mexico, the results may not apply equally to all populations. The study also couldn’t prove that low vitamin D caused the problems—only that it was associated with them.

What the Results Show

Women who started pregnancy with low vitamin D had significantly more pregnancy complications even though they took vitamin D supplements. The most striking differences were: preeclampsia (dangerous high blood pressure) occurred in 10.6% of the low vitamin D group versus only 1.3% of the healthy group. Gestational diabetes (high blood sugar during pregnancy) affected 24.5% of the low vitamin D group compared to 8.6% of the healthy group. Preterm labor (delivering before 37 weeks) happened in 5.3% of the low vitamin D group but 0% of the healthy group.

Interestingly, women with low vitamin D did catch up—their vitamin D levels increased to normal by the second and third trimesters after taking supplements. However, women who started with healthy vitamin D maintained even higher levels throughout pregnancy. Despite the low vitamin D group catching up, they still had the higher complication rates, suggesting that early pregnancy vitamin D status matters most.

The researchers used advanced statistical analysis to confirm that having healthy vitamin D in the first trimester independently protected against these complications. Women with sufficient vitamin D had about 79% lower odds of developing serious pregnancy problems compared to those with insufficient vitamin D.

Beyond the major complications, women with low vitamin D also had higher rates of urinary tract infections (14.6% vs. 4.6%) and bacterial vaginosis, a vaginal infection (13.2% vs. 3.9%). These infections, while usually treatable, can complicate pregnancy and require antibiotics.

Previous research has suggested that vitamin D deficiency during pregnancy increases risks, but this study adds important new information: it shows that the timing matters. Starting pregnancy with adequate vitamin D appears more protective than catching up later with supplements. This aligns with growing evidence that vitamin D plays important roles in early pregnancy development and immune function.

This study looked backward at medical records rather than following women forward, which limits what we can conclude. It was conducted in one hospital in Mexico, so results may differ in other populations with different genetics, diets, and sun exposure. The study couldn’t prove that low vitamin D caused the problems—only that they occurred together. Additionally, the study didn’t examine whether higher doses of vitamin D supplementation might have provided better protection for the low vitamin D group.

The Bottom Line

If you’re planning to become pregnant, consider getting your vitamin D level checked beforehand. If it’s low, work with your doctor to bring it to healthy levels (typically 30 ng/mL or higher) before conception. This appears to offer better protection than waiting until you’re pregnant to address vitamin D deficiency. Once pregnant, continue taking vitamin D supplements as recommended by your healthcare provider. Confidence level: Moderate—this is one good study, but more research would strengthen these recommendations.

Women planning pregnancy should pay attention to this research, especially those at higher risk for vitamin D deficiency (those with darker skin, limited sun exposure, or dietary restrictions). Women already pregnant should continue their vitamin D supplementation as prescribed. This research is less relevant for non-pregnant individuals, though adequate vitamin D is important for everyone’s health.

If you have low vitamin D, it typically takes 2-3 months of supplementation to reach healthy levels. Ideally, you’d achieve this before getting pregnant. Once pregnant, the protective benefits appear to apply throughout pregnancy, though the early weeks seem most critical.

Want to Apply This Research?

  • Track your vitamin D supplementation daily and log your vitamin D blood test results when available. Set a reminder to take your supplement at the same time each day and record any pregnancy-related symptoms (unusual fatigue, swelling, blood pressure readings if you have a home monitor).
  • If planning pregnancy, schedule a vitamin D blood test with your doctor before conception. If levels are low, start supplementation immediately and retest in 8-12 weeks. Use the app to set a daily reminder for your vitamin D supplement and track when you take it.
  • Log vitamin D supplement intake daily. Record vitamin D blood test results in the app when you get them. If pregnant, track any symptoms of pregnancy complications (unusual swelling, headaches, blood sugar symptoms) and share this data with your healthcare provider at appointments.

This research suggests an association between vitamin D levels and pregnancy outcomes, but cannot prove that low vitamin D causes complications. These findings should not replace medical advice from your healthcare provider. If you are pregnant or planning to become pregnant, consult your doctor before starting any new supplements or making changes to your vitamin D intake. Individual vitamin D needs vary based on factors like skin tone, location, diet, and health conditions. Your healthcare provider can determine the right vitamin D level and supplementation strategy for your specific situation.