Vitamin D in Pregnancy and Early Childhood: The Evidence

A baby born in Fort Kent in January arrives into a part of the world where the sun, for several months, is essentially useless as a source of vitamin D. That is not a figure of speech. At the latitudes that cover Maine and New Hampshire, winter sunlight does not carry enough UVB radiation for skin to make the vitamin at all. So the question of where a pregnant woman and her newborn get vitamin D is not academic up here. It is seasonal arithmetic.

Why Northern New England Families Watch This Closely

The classic work comes from Webb, Kline, and Holick, published in 1988. They exposed skin samples to cloudless Boston sunlight and found that from November through February, the skin produced no previtamin D3. Ten degrees farther north, in Edmonton, the dead zone stretched from October through March. Maine runs from about 43 degrees north in Portland up to 47 degrees in Fort Kent, farther north than Boston. The vitamin D winter here is long, and you can’t sunbathe your way out of it. For the wider regional picture, see our guide to vitamin D in northern New England.

What the Pregnancy Research Shows

Deficiency during pregnancy is common at high latitudes. A 2014 longitudinal study of pregnant women in Northern Ireland (54 to 55 degrees north) found that 96 percent were below the insufficiency threshold at both 12 and 20 weeks, and more than a third were outright deficient. Levels dropped in the winter months, when the skin makes little.

Does supplementing help? A 2024 systematic review and meta-analysis in BMC Pregnancy and Childbirth pooled 33 randomized trials covering 10,613 women. It linked vitamin D supplementation to roughly a 45 percent lower risk of preeclampsia and about a 30 percent lower risk of preterm labor. Notably, newborn outcomes such as low birth weight and Apgar scores showed no clear improvement.

Where the Evidence Gets Murky

Here is the honest part. The most cautious read of the data is far less rosy. The 2024 Cochrane review (Palacios and colleagues) applied stricter standards, threw out 20 previously included studies, and was left with 10 trials and 2,313 women. Its verdict on preeclampsia, gestational diabetes, and preterm birth: very uncertain. The effect on gestational diabetes rested on a single study of 165 women, with a confidence interval wide enough to drive a truck through. The reviewers called for larger, higher-quality trials before recommending routine supplementation.

That is why guidance bodies have stayed measured. ACOG’s Committee Opinion No. 495 found insufficient evidence to screen every pregnant woman, and instead suggested testing be considered for those at higher risk: people with limited sun exposure, those living far north, and women with darker skin, among others.

The One Settled Recommendation: Breastfed Babies

For infants, the picture is clearer. The CDC and the American Academy of Pediatrics both advise that breastfed and partially breastfed babies get 400 IU of vitamin D a day, starting in the first few days of life, because breast milk alone doesn’t supply enough. Formula-fed infants generally don’t need a supplement once they’re taking in about 32 ounces of fortified formula daily. Past infancy, AAP guidance rises to 600 IU a day for toddlers, older kids, and teens.

Why bother? Vitamin D lets the body absorb calcium and phosphorus, the raw material for bone. A serious shortfall in a growing child can lead to rickets, a softening of the bones. And deficiency is not rare. AAP-affiliated figures put roughly 42 percent of Americans below adequate, including an estimated 15 percent of children ages 1 to 11, 17 percent of adolescents, and 32 percent of young adults.

What It Means for Maine and New Hampshire Families

None of this is a prescription. It’s a map of where the science is firm and where it’s still arguing with itself. The infant recommendation is solid. The pregnancy story is more of a work in progress, with promising signals and plenty of caveats. For families in a state where the sun checks out for a third of the year, the takeaway is to raise it with a provider rather than assume a summer’s worth of sunlight will carry through to spring.

Sources

• Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. The Journal of Clinical Endocrinology & Metabolism (PubMed). 1988. https://pubmed.ncbi.nlm.nih.gov/2839537/
• Vitamin D deficiency and insufficiency in pregnant women: a longitudinal study. British Journal of Nutrition (Cambridge Core). 2014. https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/vitamin-d-deficiency-and-insufficiency-in-pregnant-women-a-longitudinal-study/026F51DF7FEA5368E6039D980635B884
• Efficacy of vitamin D supplementation on the incidence of preeclampsia: a systematic review and meta-analysis. BMC Pregnancy and Childbirth (PMC). 2024. https://pmc.ncbi.nlm.nih.gov/articles/PMC11668112/
• Palacios C, et al. Vitamin D supplementation for women during pregnancy (2024 update). Cochrane Database of Systematic Reviews. 2024. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD008873.pub5/full
• Vitamin D and Breastfeeding (Breastfeeding Special Circumstances). U.S. Centers for Disease Control and Prevention (CDC). 2024. https://www.cdc.gov/breastfeeding-special-circumstances/hcp/diet-micronutrients/vitamin-d.html
• Vitamin D for Babies, Children & Adolescents. HealthyChildren.org (American Academy of Pediatrics). 2024. https://www.healthychildren.org/English/healthy-living/nutrition/Pages/vitamin-d-on-the-double.aspx
• ACOG Committee Opinion No. 495: Vitamin D: Screening and supplementation during pregnancy. Obstetrics & Gynecology (PubMed). 2011. https://pubmed.ncbi.nlm.nih.gov/21691184/

This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider before making any health decisions.