Vitamin D and pregnancy: 9 things you need to know

With all the publicity surrounding vitamin D lately, it’s no surprise that you have lots of questions. Should you test your patients for deficiency? When? What numbers should you use? And how do you treat a low vitamin D level?

In pregnancy, these issues become critical because there are not one but two patients to consider. Despite the lack of clear guidelines, there is sufficient evidence to suggest that you should at least consider monitoring the vitamin D status of your pregnant patients.

Fetal needs for vitamin D increase during the latter half of pregnancy, when bone growth and ossification are most prominent. Vitamin D travels to the fetus by passive transfer, and the fetus is entirely dependent on maternal stores.¹ Therefore, maternal status is a direct reflection of fetal nutritional status.

The vitamin D level in breast milk also correlates with the maternal serum level, and a low vitamin D level in breast milk can exert a harmful effect on a newborn.

In this article, I address nine questions regarding vitamin D and pregnancy:

• Is vitamin D really a vitamin?
• Why do the numbers vary?
• Does the vitamin D level affect pregnancy outcomes?
• Can’t people get enough vitamin D through their diet?
• What level signals deficiency?
• How many women are deficient?
• Should you test all pregnant patients?
• How should you treat vitamin D deficiency in pregnancy?
• Can a person get too much vitamin D?

1. Is vitamin D really a vitamin?

For years, vitamin D was discussed solely in relation to bone metabolism and absorption, and deficiency states were the purview of endocrinologists and gynecologists who treated menopausal patients at risk of osteoporosis. Recent studies demonstrate that vitamin D plays a role in multiple endocrine systems. Indeed, vitamin D may be more correctly considered a hormone because it is a substance produced by one organ (skin) that travels through the bloodstream to target end organs. Vitamin D receptors have been found in bone, breast, brain, colon, muscle, and pancreatic tissues. Not only does vitamin D affect bone metabolism, it also modulates immune responses and even glucose metabolism.² Vitamin D receptors have also been found in the placenta; their role in that organ remains to be elucidated.

2. Why do the numbers vary?

Some of the confusion surrounding vitamin D concerns the units used to measure and discuss it. Vitamin D can be measured in nanograms per milliliter (ng/mL) or in nanomoles per liter (nmol/L). A measurement of 1 ng/mL equals approximately 2.44 nmol/L. Therefore, deficiency in some articles is described as a vitamin D level below 20 ng/mL and in other articles as a level below 50 nmol/L. As for normal range, it may be listed as a level above 32 ng/mL or as a level above 75 nmol/L.

Compounding the confusion, vitamin D in supplement form can be written in two different measurements—using micrograms or international units. A measurement of 1 μg equals 40 IU, so a supplement of 150 μg/day is the same as one of 6,000 IU/day.

3. Does the vitamin D level affect pregnancy outcomes?

Vitamin D’s role in pregnancy outcomes has yet to be fully described, making it an exciting field to explore. Research into vitamin D and its effects on pregnancy is still in its infancy, but many intriguing associations have been noted. For example, lower levels of vitamin D have been associated with increased rates of cesarean delivery,³ bacterial vaginosis,⁴ and preeclampsia,⁵ as well as less efficient glucose metabolism.⁶

There is biological plausibility for vitamin D to play a role in pregnancy outcomes, given the presence of receptors in gestational tissues. Vitamin D receptors in uterine muscle could affect contractile strength, and vitamin D has been shown to have immunomodulatory effects, thereby potentially protecting the host from infection.

As I mentioned, placental vitamin D receptors and their role need further exploration.

4. Can’t people get enough vitamin D through their diet?

Very few foods contain a large amount of vitamin D, and the few that do (herring, cod liver oil) are not standard fare. Even fortified foods such as milk lack a substantial amount. TABLE 1 lists the amount of vitamin D in various foods.⁷

TABLE 1

In food, the vitamin D level is generally low