The latest in the seemingly endless series of studies being released involving various effects of vitamin D as it pertains to general health, immune health, cellular health and disease prevention involves the role of vitamin D relative to pregnancy, prenatal health and the effects of inadequate levels of the nutrient in the mother during pregnancy and lactation on both the mother and baby. These studies have shown that the aforementioned inner-workings of vitamin D and pregnancy are both numerous and complex. One common theme with virtually all of the studies published over the past half-decade involving vitamin D is that the fat-soluble hormone known to society as a vitamin is that this nutrient is of critical importance to both the health of mother and baby during pregnancy, birth and post-birth infancy.
One study conducted by researchers at a South Korean medical school determined that the importance of “adequate” vitamin D intake is of particular importance to pregnant women in ensuring both maternal and fetal health, that inadequate levels of vitamin D3 are unusually common among women who are pregnant, and that this rampant inadequacy can be extremely detrimental to the immune health of both mother and baby. The research showed that vitamin D deficiency in expecting mothers can lead to an increased risk of preeclampsia, gestational diabetes mellitus, and bacterial vaginosis, and an increased risk for C-section delivery.
“Adequate vitamin D intake is essential for maternal and fetal health during pregnancy, and epidemiological data indicate that many pregnant women have sub-optimal vitamin D levels. Notably, vitamin D deficiency correlates with preeclampsia, gestational diabetes mellitus, and bacterial vaginosis, and an increased risk for C-section delivery. Recent work emphasizes the importance of nonclassical roles of vitamin D in pregnancy and the placenta. The placenta produces and responds to vitamin D where vitamin D functions as a modulator of implantation, cytokine production and the immune response to infection.” 
A 2011 study by the Hannover School of Medicine in Germany found recent evidence that vitamin D status influences female reproductive and pregnancy outcomes. According to that study:
“Human and animal data suggest that low vitamin D status is associated with impaired fertility, endometriosis and polycystic ovary syndrome. Evidence from observational studies shows higher rates of preeclampsia, preterm birth, bacterial vaginosis and gestational diabetes in women with low vitamin D levels.” 
Two recent studies; one by medical scientists from New Jersey and another by the University of Calgary showed a correlation between vitamin D and birth weight:
The first one these determined that intake of Vitamin D was associated with increased infant birth weight, and that the lack thereof may lead to lower-than-ideal birth weight of the infant. 
The latter arrived at virtually the same conclusion, with the main difference being that it also found that intake of milk (which contains vitamin D3) during pregnancy are each associated with infant birth weight, independently of other risk factors. 
An study by Australian researchers showed that a relationship may exist between vitamin D deficiency and impaired fertility in women, an increased risk of pre-eclampsia and gestational diabetes mellitus. The researchers noted that insufficient evidence existed at the time of the study to definitively declare causality.
“Fertility may be impaired in mothers with low vitamin D. During pregnancy, maintaining vitamin D and calcium levels may decrease the risks of pre-eclampsia, while gestational diabetes mellitus appears to be more common in those with low vitamin D status.” 
Those of us who need not adhere to the scientific method, and are therefore free to speculate are fairly safe in assuming that based on the results of similar studies involving vitamin D deficiency and more than four dozen other illness and diseases, the chances that causality will eventually be established are highly probable.
While the complete extent of both sufficient and inadequate levels of vitamin D throughout pregnancy are not yet known and may not be for some time. There are quite a few diseases and illnesses, the likelihood of an individual being diagnosed with which relative to the individual’s vitamin D levels during pregnancy (for both mother and infant) have yet to be studied, studied again, peer reviewed and then studied one more time for good measure (standard procedure before researchers declare anything as scientific fact) are many, and the time and money needed to conduct such a massive amount of research is somewhat prohibitive as it pertains to the immediate answers most in Western society have become accustomed to and have come to expect.
What is indisputable regarding the effects of vitamin D deficiency and/or supplementation during pregnancy is that both the mother and her infant will be far less likely to experience low birth weight, pregnancy complications and the diagnosis of a number of diseases and/or illnesses during pregnancy, post-birth, infancy and possibly even into adulthood if the mother makes a point of supplementing with vitamin D3 at a level determined by her doctor and based on her individual blood-levels, diet and need than if she were to go the entire term of the pregnancy without ingesting vitamin D3 from supplements and/or other sources.
Pregnant women (and anyone else buying vitamin D supplements) should make sure to check the label very closely to make sure the bottle contains vitamin D3 (cholecalciferol) and not D2 (ergocalciferol). Studies have shown the efficacy and absorption of the former to be vastly superior to those of the latter, with some experts going so far as to condemn vitamin D2 as a nutrient unfit for use as a dietary supplement in humans. For more information regarding the differences between vitamins D2 and D3, please refer to previous articles published on this website in which the topic is covered in-depth.
The bottom-line for pregnant women is that there’s no excuse for failing to take prenatal vitamins, and if there is any doubt over whether or not the prenatal vitamin a given mom-to-be is taking has sufficient vitamin D3, be sure to check with a doctor or nutritionist and ask whether or not it is necessary to take additional vitamin D supplements in order to ensure optimal health.
- Vitamin D Effects on Pregnancy and the Placenta
Shin JS, Choi MY, Longtine MS, Nelson DM
Source: Department of Obstetrics and Gynecology, CHA University School of Medicine, Seoul, Republic of Korea.
- Vitamin D – roles in women’s reproductive health?
Grundmann M, von Versen-Höynck F.
Source: Department of Obstetrics, Gynecology and Reproductive Medicine, Hannover Medical School, Hannover, Germany.
- Vitamin D intake during pregnancy: association with maternal characteristics and infant birth weight.
Scholl TO, Chen X.
Source: Department of Obstetrics and Gynecology, University of Medicine and Dentistry of New Jersey-SOM, Stratford, New Jersey, USA.
- Association of low intake of milk and vitamin D during pregnancy with decreased birth weight.
Mannion CA, Gray-Donald K, Koski KG.
Source: Faculty of Nursing, University of Calgary, Calgary, Alta.
- Vitamin D deficiency and pregnancy: from preconception to birth.
Lewis S, Lucas RM, Halliday J, Ponsonby AL.
Source: Public Health Genetics, Murdoch Childrens Research Institute, Royal Children’s Hospital, Melbourne, Australia.