Researchers studied over 8,000 women to understand how the number of pregnancies affects vitamin D levels in the body. They found that each pregnancy was linked to slightly lower vitamin D, a nutrient important for bone health and immunity. The study also created a computer tool that can predict who might have low vitamin D by looking at age, weight, and blood test results. This discovery could help doctors identify women who need vitamin D supplements or more sun exposure to stay healthy.

The Quick Take

  • What they studied: Whether having more pregnancies is connected to lower vitamin D levels in women’s blood
  • Who participated: 8,003 women who had been pregnant before, using health data collected by the U.S. government between 2011 and 2018
  • Key finding: Each additional pregnancy was linked to a small drop in vitamin D levels (0.6 nanomoles per liter). This relationship was statistically significant, meaning it’s unlikely to be due to chance.
  • What it means for you: Women with multiple pregnancies may want to talk to their doctor about checking their vitamin D levels and possibly taking supplements. However, this study shows a connection, not that pregnancies directly cause low vitamin D—other factors like sun exposure and diet matter too.

The Research Details

This was a cross-sectional study, which means researchers looked at a large group of women at one point in time and compared their vitamin D levels to how many pregnancies they’d had. The researchers used data from the National Health and Nutrition Examination Survey (NHANES), a trusted government health study that tracks Americans’ health information.

The team analyzed the data in three different ways to make sure their findings were solid. First, they looked at the raw connection between pregnancies and vitamin D. Then they adjusted for age and race, since these factors can affect vitamin D. Finally, they adjusted for many other factors that could influence vitamin D levels, like weight, kidney function, blood sugar control, and diabetes status.

For the second part of the study, researchers used advanced computer programs (called machine learning algorithms) to create a tool that could predict who might have low vitamin D. They tested six different computer programs to see which one worked best, using 70% of the data to teach the computer and 30% to test it.

Understanding how pregnancies affect vitamin D is important because vitamin D helps bones stay strong and supports the immune system. If doctors know that multiple pregnancies might lower vitamin D, they can check levels more carefully in these women and help prevent problems. The computer prediction tool could help doctors quickly identify women at risk without waiting for symptoms to appear.

This study used a large, nationally representative sample of real women, which makes the findings more trustworthy than a small study. The researchers carefully controlled for many factors that could affect vitamin D levels, making it less likely that other things (like weight or sun exposure) explain the results. However, because this was a snapshot in time rather than following women over years, we can’t be completely sure that pregnancies cause lower vitamin D—only that they’re connected.

What the Results Show

The main finding was clear: each additional pregnancy a woman had was linked to a 0.6 nanomole per liter decrease in vitamin D levels. While this might sound small, when added up across multiple pregnancies, it could become meaningful. For example, a woman with four pregnancies might have vitamin D levels about 2.4 nanomoles lower than a woman with no pregnancies.

This relationship held true even after the researchers accounted for age, race, weight, kidney function, blood sugar levels, and diabetes. This suggests that the pregnancy itself—or something about the pregnancy experience—may be independently connected to vitamin D levels, separate from other health factors.

The researchers also created a computer prediction tool using an advanced algorithm called XGBoost. This tool was quite good at identifying women with low vitamin D (below 50 nanomoles per liter), correctly identifying them about 73% of the time. This was better than simpler statistical methods or other computer programs tested.

The computer analysis revealed that the most important factors for predicting low vitamin D were age, body weight (BMI), and kidney function (blood urea nitrogen). Interestingly, while pregnancy number was connected to vitamin D, it wasn’t the strongest predictor—age and weight mattered more. This suggests that multiple factors work together to affect vitamin D levels, not just pregnancy alone.

Most previous research on vitamin D in pregnant women focused on pregnancy itself or the immediate postpartum period. This study is unique because it looks at the long-term cumulative effect of multiple pregnancies on vitamin D status in women years after pregnancy. The finding that each pregnancy is associated with lower vitamin D is new and suggests that healthcare providers should pay special attention to vitamin D in women with multiple pregnancies.

This study has several important limitations. First, it’s a snapshot—researchers looked at women at one point in time, so they can’t prove that pregnancies directly cause lower vitamin D. Second, the study didn’t measure how much sun exposure women got, how much vitamin D they consumed, or other lifestyle factors that strongly affect vitamin D levels. Third, the computer prediction tool worked reasonably well (73% accuracy) but wasn’t perfect, so it shouldn’t be used alone to diagnose vitamin D deficiency. Finally, the study included mostly data from the U.S., so results might not apply to women in other countries with different sun exposure or dietary patterns.

The Bottom Line

Women with multiple pregnancies should consider asking their doctor to check their vitamin D levels, especially if they have risk factors like limited sun exposure or dietary restrictions. If levels are low, vitamin D supplements (typically 1,000-2,000 IU daily) or increased sun exposure may help. These recommendations are supported by the study’s findings but should be personalized based on individual health needs. Confidence level: Moderate—the study shows a connection, but more research is needed to understand the full picture.

This finding is most relevant to women who have had multiple pregnancies and want to optimize their health. It’s also important for doctors and midwives who care for women of childbearing age. Women planning pregnancies might also benefit from knowing this, as maintaining good vitamin D levels before and during pregnancy is important for both mother and baby. This doesn’t mean women should avoid pregnancy—rather, it’s a reminder to monitor vitamin D status.

If a woman starts taking vitamin D supplements after learning her levels are low, it typically takes 2-3 months to see meaningful improvements in blood levels. Benefits like improved bone health and immune function develop over months to years of maintaining adequate vitamin D.

Want to Apply This Research?

  • Track vitamin D supplement intake (if recommended by doctor) and note any sun exposure time daily. Users can log: supplement dose and time taken, minutes of midday sun exposure, and any symptoms like fatigue or muscle weakness. Aim to record this 4-5 times per week.
  • If the app identifies risk factors for low vitamin D (multiple pregnancies + limited sun exposure), users can set a goal to spend 15-20 minutes in midday sun 3-4 times per week, or take a daily vitamin D supplement as recommended by their doctor. The app could send reminders and track consistency.
  • Users should log vitamin D supplement adherence monthly and note any doctor visits where vitamin D levels were checked. The app could prompt users to schedule annual vitamin D blood tests and track results over time to see if interventions are working. This creates a long-term picture of vitamin D status trends.

This research shows an association between number of pregnancies and vitamin D levels but does not prove that pregnancies cause low vitamin D. Individual vitamin D needs vary based on age, skin tone, sun exposure, diet, and other health conditions. Before starting vitamin D supplements or making changes based on this research, consult with your healthcare provider who can assess your individual risk factors and recommend appropriate testing and treatment. This information is for educational purposes and should not replace professional medical advice. Women who are pregnant or breastfeeding should discuss vitamin D needs with their obstetrician or midwife.