Researchers studied pregnant women in Alaska to understand if vitamin D levels affect how much women bleed after giving birth. They looked at 1,343 pregnancies and found that women with very low vitamin D in their first trimester (the first three months) were more than twice as likely to have heavy bleeding after delivery. About 29% of women lost a significant amount of blood after birth, and those with low vitamin D early on had higher rates. While this finding is interesting, doctors say we need more research to know if taking extra vitamin D during pregnancy could help prevent heavy bleeding after delivery.

The Quick Take

  • What they studied: Whether the amount of vitamin D in a pregnant woman’s blood affects how much she bleeds after giving birth
  • Who participated: 1,343 pregnant Alaska Native and American Indian women who had vaginal births between 2017 and 2019, with at least one vitamin D measurement recorded during pregnancy
  • Key finding: Women with very low vitamin D (below 12 ng/mL) in their first trimester were 2.2 times more likely to lose 500 mL or more blood after delivery compared to women with adequate vitamin D levels
  • What it means for you: If you’re planning pregnancy or are pregnant, maintaining healthy vitamin D levels may be important for reducing bleeding risk after birth. However, this is preliminary evidence—talk to your doctor about vitamin D screening and supplementation rather than self-treating.

The Research Details

This was a retrospective cohort study, which means researchers looked backward at medical records of women who had already given birth. They examined electronic health records from a rural Alaska healthcare system for pregnancies between 2017 and 2019. The researchers identified all women who had at least one vitamin D blood test during pregnancy and who delivered vaginally (gave birth without surgery). They then compared vitamin D levels measured in each trimester (three-month period) of pregnancy to how much blood women lost after delivery. The study included 1,343 pregnancies total, with 1,258 having vaginal births that could be analyzed.

The researchers measured vitamin D levels at different points during pregnancy—first trimester, second trimester, and third trimester—to see if timing mattered. They defined ‘deficient’ vitamin D as levels below 12 ng/mL (nanograms per milliliter), which is a standard medical cutoff. They then used statistical analysis to determine if low vitamin D was connected to heavier bleeding after birth, while also accounting for other known risk factors like previous heavy bleeding, large babies, and labor complications.

This research approach is important because it looks at real-world medical data from an underserved population (rural Alaska) where vitamin D deficiency may be more common due to limited sunlight and geographic isolation. By examining records from actual pregnancies and births, rather than conducting a controlled experiment, researchers could study this question in a practical way. Looking at vitamin D levels across all three trimesters helps show whether timing of deficiency matters. Understanding these connections could eventually lead to simple, inexpensive interventions (like vitamin D supplements) that might reduce serious pregnancy complications.

This study has several strengths: it’s a large sample size (1,343 pregnancies), uses actual medical records rather than relying on people’s memory, and examines a specific population where the question is particularly relevant. However, there are limitations: it’s observational rather than experimental, so we can’t prove vitamin D deficiency causes heavy bleeding—only that they’re associated. The study couldn’t account for all possible factors that might influence bleeding risk. Additionally, the findings are specific to Alaska Native and American Indian populations, so results may not apply equally to all ethnic groups. The study was also conducted in a rural setting, which may differ from urban healthcare.

What the Results Show

Among the 1,258 women who had vaginal births, 370 (29%) experienced blood loss of 500 mL or more, and 127 (10%) lost 1,000 mL or more. These are considered significant amounts of blood loss after delivery. The key finding was that women with very low vitamin D (below 12 ng/mL) specifically in their first trimester were 2.2 times more likely to have blood loss of 500 mL or more. This means if a woman without vitamin D deficiency had a certain risk of heavy bleeding, a woman with first-trimester vitamin D deficiency would have roughly double that risk.

Interestingly, vitamin D deficiency in the second and third trimesters did not show the same association with heavy bleeding. This suggests that vitamin D status early in pregnancy may be particularly important. The researchers also identified other risk factors for heavy bleeding, including previous episodes of heavy bleeding after birth, suspected large babies, twin pregnancies, use of magnesium sulfate (a medication for high blood pressure in pregnancy), labor induction lasting more than 36 hours, oxytocin use (a medication to strengthen contractions) for more than 12 hours, and a third stage of labor (delivery of the placenta) lasting more than 20 minutes.

The study found that vitamin D deficiency was relatively uncommon in this population overall: only 10.5% of women had deficient levels in the first trimester, dropping to 4.8% in the second trimester and 5.3% in the third trimester. This suggests that vitamin D deficiency improves naturally as pregnancy progresses, possibly due to increased sun exposure in summer months or increased supplementation. The identification of multiple other risk factors for heavy bleeding is also important, as it shows that postpartum hemorrhage is a complex condition with many contributing factors, not just vitamin D.

Previous research has suggested that vitamin D plays a role in immune function, inflammation, and blood clotting—all processes that could theoretically affect bleeding after birth. Some studies have linked vitamin D deficiency to pregnancy complications, though research specifically on postpartum hemorrhage has been limited. This study adds to growing evidence that vitamin D status during pregnancy may influence birth outcomes, though the specific mechanism isn’t yet clear. The finding that first-trimester vitamin D is most relevant aligns with the biological principle that early pregnancy is a critical period for many developmental processes.

This study cannot prove that low vitamin D causes heavy bleeding—only that they’re associated. Other unmeasured factors could explain the connection. The study was conducted in a specific geographic region with a specific population (Alaska Native and American Indian women), so findings may not apply equally to other groups or locations. The researchers relied on medical records, which may be incomplete or contain errors. Additionally, vitamin D levels were measured at different times for different women, and the study couldn’t account for vitamin D supplementation that women may have taken on their own. The study also couldn’t determine whether women with low vitamin D had other health conditions that might increase bleeding risk independently.

The Bottom Line

Based on this research, pregnant women should discuss vitamin D screening and supplementation with their healthcare provider, particularly in early pregnancy. Current prenatal care guidelines already recommend vitamin D supplementation for many pregnant women, and this study provides additional support for that practice. However, this single study is not definitive—more research is needed before making major changes to pregnancy care. Women should not self-treat with high-dose vitamin D without medical guidance. If you’re planning pregnancy or are pregnant, ask your doctor about your vitamin D status and whether supplementation is appropriate for you. Confidence level: Moderate—this is promising evidence but not yet conclusive.

This research is most relevant to pregnant women, women planning pregnancy, and their healthcare providers. It’s particularly relevant for women in northern climates or those with limited sun exposure, as they’re at higher risk for vitamin D deficiency. Women with previous episodes of heavy bleeding after birth should especially discuss vitamin D status with their doctor. This research is less immediately relevant to non-pregnant individuals, though maintaining good vitamin D status is important for overall health. Healthcare providers in rural or underserved areas should pay particular attention to this research.

If vitamin D supplementation is recommended, it would need to be started before pregnancy or very early in the first trimester to potentially reduce bleeding risk, based on this study’s findings. Benefits wouldn’t be immediate—the goal is prevention of complications at delivery. If you’re already pregnant and discover low vitamin D, starting supplementation may still be beneficial, though the evidence is strongest for early pregnancy supplementation. Realistic expectations: vitamin D supplementation is one of many factors affecting postpartum bleeding risk, so it’s not a guaranteed prevention strategy.

Want to Apply This Research?

  • Track vitamin D supplementation intake (dose and frequency) throughout pregnancy, and log any vitamin D blood test results when available. Record dates of supplementation starts and any changes in dosage.
  • If your healthcare provider recommends vitamin D supplementation, set up a daily reminder in the app to take your supplement at the same time each day (such as with breakfast). Log when you take it to ensure consistency and to share records with your healthcare provider.
  • Use the app to track vitamin D supplementation adherence throughout pregnancy, particularly in the first trimester. If you have vitamin D blood tests, log the results and dates. Share this tracking data with your healthcare provider at prenatal visits to ensure you’re maintaining adequate levels. After delivery, note any postpartum bleeding concerns to discuss with your provider.

This research describes an association between low vitamin D in early pregnancy and increased postpartum bleeding risk, but does not prove causation. This information is for educational purposes and should not replace professional medical advice. Pregnant women or those planning pregnancy should consult with their healthcare provider about vitamin D screening and supplementation, as individual needs vary based on health status, location, diet, and other factors. Do not start, stop, or change any supplements without medical guidance. If you experience heavy bleeding after delivery, seek immediate medical attention. This study was conducted in a specific population and may not apply equally to all groups.