Vitamin D

Pregnancy

MotherToBaby lists 600 IU (15 mcg) per day as the pregnancy RDA and 4,000 IU (100 mcg) as the adult upper limit from all sources. The UK COT says the same adult limit applies during pregnancy and lactation. ACOG's vitamin-D-specific opinion is dated July 2011. It discusses 1,000 to 2,000 IU per day when deficiency is identified and says routine extra supplementation beyond a prenatal vitamin should await more trial evidence. ACOG's current pregnancy FAQ also lists 600 IU per day and says an ob-gyn may suggest a separate supplement when a need is identified. The Endocrine Society's 2024 guideline suggests empiric vitamin D intake during pregnancy, which may come from fortified foods, prenatal vitamins, or a separate supplement. This recommendation is conditional and its technical remarks list trial doses from 600 to 5,000 IU per day. In a 2011 randomized trial, 494 women enrolled and 350 continued to delivery; participants received 400, 2,000, or 4,000 IU per day from 12 to 16 weeks until delivery. The trial found no group differences in measured adverse-effect outcomes, but its primary outcome was maternal and newborn vitamin D status, not rare birth outcomes. The 2024 Cochrane review included 10 studies after moving 20 older studies to awaiting assessment; it rated much of the evidence low or very low certainty. A 2025 review of 66 trials and 17,276 participants reported high variation between trials, and 25 trials had at least one high-risk-of-bias domain. A 2026 pregnancy trial in women with low vitamin D status found that 20,000 IU per week raised maternal and newborn blood levels more than 10,000 IU every two weeks, but newborn bone measures did not differ significantly; only 67 infants had bone scans. A 2026 post-hoc follow-up of 498 children from a randomized trial found higher verbal and visual memory scores at age 10 after 2,800 IU per day, compared with 400 IU per day, from pregnancy week 24 to one week after birth. The set-shifting result did not remain after correction for multiple tests, so these findings need confirmation. MotherToBaby reports mixed observational findings between low blood vitamin D levels and miscarriage. One prospective cohort of 1,683 pregnancies reported 58 miscarriages and an association between levels below 50 nmol/L and first-trimester loss; this measured vitamin D status, not supplement use, and does not show that supplements prevent miscarriage. MotherToBaby also cites two small studies of maternal vitamin D levels and neural tube defects with 192 women total. One study of 60 women did not find a statistically significant difference in mean vitamin D levels, while a study of 132 women found lower levels in neural tube defect cases. These studies measured blood levels, not supplement exposure, and cannot show that recommended intake causes or prevents birth defects.

Breastfeeding

MotherToBaby lists 600 IU (15 mcg) per day as the breastfeeding RDA. LactMed says maternal doses from 400 to 2,000 IU per day generally do not provide enough vitamin D through milk alone to meet an exclusively breastfed infant's daily need. AAP and CDC recommend 400 IU per day directly for breastfed infants. In a 2015 randomized trial, 334 mother-infant pairs entered the 400 IU and 6,400 IU groups; the 2,400 IU group was stopped because infant deficiency was more common. Infant vitamin D levels in the 6,400 IU maternal-dose group did not differ from infants given 400 IU directly, but this trial does not make high maternal doses equivalent to usual intake. A 2025 placebo-controlled trial in 126 mothers in rural Ethiopia tested 15,000 IU weekly for 12 months. Maternal and infant vitamin D levels rose, while infant growth and motor development did not differ significantly between groups. A 2025 secondary analysis of a randomized trial in Tanzania included 1,312 one-year-old infants of women living with HIV; 3,000 IU per day during pregnancy and lactation did not change the measured infant development scores. These studies tested higher doses and specific groups, not ordinary maternal intake.

What was tested

Pregnancy trials have compared daily doses from 400 to 4,000 IU, including one trial that began at 12 to 16 weeks and continued until delivery. The 2026 Preg-D trial compared 20,000 IU weekly with 10,000 IU every two weeks in pregnant women with low vitamin D status; it measured maternal and newborn blood levels and newborn bone measures. A separate post-hoc study compared 2,800 with 400 IU per day from week 24 of pregnancy to one week after birth, then assessed children's cognition at age 10. Breastfeeding studies tested 6,400 IU per day or 15,000 IU weekly for maternal intake, and measured milk or blood vitamin D levels and infant outcomes. These high-dose study plans do not represent routine maternal intake.