Vitamin D: A Key Player in Your Future Baby's Health

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Vitamin D is an essential nutrient in our bodies and it is especially important to fertility, pregnancy, and early childhood health. Unfortunately, vitamin D isn’t getting the attention it deserves in many prenatal healthcare settings. Most women are not being routinely tested for adequate vitamin D levels during preconception or pregnancy and the current research suggests that this policy could use an update. Read on to learn about why vitamin D is so important for you and your baby and what you can do to optimize your levels before and during pregnancy! 

We also want to give a huge shout-out to Lily Nichols RDN, CDE for tracking down and compiling many of the research articles cited in this post! Lily is a real-food dietitian who specializes in prenatal nutrition and gestational diabetes and she has an incredible commitment to providing accessible evidence-based guidance to expecting mothers. Her book Real Food for Pregnancy is quite literally the BEST book we’ve read on prenatal nutrition and we’ve learned a great deal from her work. You can find the book here.

What is Vitamin D?

Vitamin D is a fat-soluble nutrient that actually functions as a hormone. Its main role in the body is to help us maintain proper blood levels of calcium and phosphorous, both of which are essential to bone health and the development of the fetal skeleton during pregnancy. Vitamin D is also important for many other functions in the body, including:

  • Blood sugar regulation

  • Immune system health

  • Gene regulation and expression

  • Blood pressure regulation and cardiovascular health

  • Brain development and mental health

  • Regulation of cell growth and differentiation

Preconception and Pregnancy

Having low levels of vitamin D during pregnancy is associated with several pregnancy complications and has been shown to carry risks for fetal development and childhood health. There is also evidence to suggest that lower levels of vitamin D during preconception impairs fertility and may lead to early pregnancy loss.

Pregnancy Outcomes

A meta-analysis from 2013 examined 24 relevant studies and found that vitamin D deficiency was associated with preeclampsia, pre-term birth, gestational diabetes, and having a baby that is small for gestational age. A second meta-analysis of 31 studies found similar results. There are also some studies linking vitamin D deficiency to post-partum depression, bacterial vaginosis, maternal periodontal disease, and risk of C-section birth

Childhood Health

Maternal vitamin D deficiency has been shown to lead to rickets and other bone growth abnormalities in children and can even influence bone health later in life. Newer studies are beginning to investigate the impact of vitamin D on other health issues in children and have found an association between maternal deficiency and increased risk of:

Fertility

The evidence linking vitamin D deficiency and impaired fertility is still in its early stages. However, there are two studies that have investigated the impact of vitamin D levels on IVF success rates. One study found that vitamin D deficiency was associated with lower IVF success ratesThe other study found higher clinical pregnancy rates in women with higher vitamin D levels. Although both of these studies were relatively small, they indicate a need for further investigation into the role vitamin D plays in fertility.

Research has also shown a connection between poor vitamin D status and polycystic ovary syndrome (PCOS), which is a known cause of fertility problems in many women. This study showed severe vitamin D deficiency in 44% of women with PCOS (compared to 11% in controls) and also demonstrated a connection between vitamin D status and metabolic markers for PCOS (such as insulin sensitivity). Another study showed improved glucose metabolism and menstrual frequency in women with PCOS following vitamin D supplementation.

Lastly, evidence suggests that preconception vitamin D levels may also play a role in early pregnancy loss. A study of women with a previous history of early pregnancy loss showed that those with sufficient preconception vitamin D levels were more likely to achieve pregnancy and have a live birth.

Clearly, vitamin D is an important nutrient to optimize for fertility, pregnancy, and our children’s health!

How Can I Test My Vitamin D? 

Vitamin D status is most accurately measured by testing 25 (OH) D in the blood. Since the American College of Obstetricians and Gynecologists (ACOG) does not currently recommend routine screening for all women, your doctor may not think to order this test without your specific request. If you are pregnant or trying to become pregnant, you absolutely should ask to have your vitamin D levels checked so you can avoid the potential negative consequences of deficiency discussed above. The test is inexpensive and should be covered by insurance.

Optimal Level of Vitamin D

There is a bit of debate in the scientific community about what constitutes an “optimal” vitamin D level, but it is generally accepted that levels below 32 ng/ml are insufficient and levels below 20 ng/ml are deficient. Many experts believe sufficiency should be defined as 40-80 ng/ml and that 50 ng/ml is an optimal level for adults

Make sure that you read the unit of measurement properly when looking at your vitamin D level. You may see it reported either in ng/ml or in nmol/l and it makes a big difference which is used (30 ng/ml = 75 nmol/l).

Where Do We Get Vitamin D?

Sunlight

We are actually designed to get vitamin D from sunlight and we make it in our skin when we are exposed to UVB rays from the sun. However, not all people are able to make the same amount of vitamin D from sunlight. There are a number of factors that influence vitamin D production, including:

  • Skin color – darker skin allows for less vitamin D synthesis

  • Time of day – greatest capacity for vitamin D synthesis happens around mid-day

  • Time of year – less vitamin D can be made in winter months in the U.S.

  • Location – almost no vitamin D made in winter above 33 degrees latitude (majority of the U.S.)

  • Use of sunscreen/protective clothing – sunscreen blocks ~95% of vitamin D production

Diet

Vitamin D is one of the only nutrients for which food is a poor source. However, we can find small amounts of naturally occurring vitamin D in a few foods, such as:

  • Fatty fish (salmon, mackerel, herring, sardines)

  • Cod liver oil

  • Beef liver

  • Cheese

  • Egg yolk

Some foods are also fortified with vitamin D, such as dairy products and orange juice.

Supplements

We can also get vitamin D from supplements. Based on the many factors that may inhibit vitamin D synthesis from sunlight and the fact that 18-98% of women are deficient worldwide, supplementation is a very wise choice, especially during preconception and pregnancy.

There are two supplemental forms of vitamin D: ergocalciferol (D₂) and cholecalciferol (D₃). Although both have the ability to raise vitamin D levels in the blood, D₃ has been found to be more effective at maintaining vitamin D levels and is the better choice for supplementation.

How Can I Get Enough Vitamin D?

The current RDA for pregnant and breastfeeding women is 600 IU and most prenatal vitamins contain about 400 IU. However, recent research suggests that this is far too low to maintain adequate levels during pregnancy for many women and would definitely not be enough to significantly raise levels in a woman who is deficient. 

There have been multiple trials demonstrating that 4000 IU of vitamin D per day is not only safe during pregnancy, but much more effective than 400 IU at ensuring adequate levels in mother and infant at birth. One racially diverse study in the U.S. showed that only 50% of mothers and 40% of their infants achieved adequate vitamin D levels at birth when supplementing with 400 IU per day. Compare that to 82% of mothers and 79% of their infants when supplementing with 4000 IU per day! There were no adverse effects observed from taking 4000 IU per day. Keep in mind that sufficiency was defined as >32 ng/ml for mothers and >20 ng/ml for infants. As discussed above, optimal levels for women may be closer to 40 or 50 ng/ml.

The best way to ensure adequate vitamin D levels for you and your baby is to have your levels checked and work with your doctor on a supplementation plan that helps you reach at least 40 ng/ml. A daily dose of 4000 IU has been shown to be be safe and effective but if you have darker skin, are living in a northern climate (especially in winter), or are currently deficient, your ideal supplementation plan may need to include more than this. On the other hand, if you currently spend a lot of time in the sun, have lighter skin, and have tested sufficient for vitamin D, you may be able to take less than 4000 IU per day. 

You can also add in safe sun exposure to your vitamin D plan if you are living in conditions that allow for this. You can use this resource to help you determine what level of sun exposure is safe for you and get an estimate of how much vitamin D you can make in your skin (taking into account your skin type, location, and time of day/year).