These days, it seems babies are getting smaller and smaller. Because so many infants are born so small, they would not have survived before today’s modern neonatal intensive care units (NICU). Being small for gestational age (SGA) is a major public health problem and prevention is a major public health goal. Simply put, very small babies are less likely to survive than larger babies. The definition of SGA takes into account the babies born early who tend to weigh less.
In late 2012, researchers at the University of Pittsburgh, led by Dr. Alison Gernand, looked at vitamin D levels and the risk of having a SGA baby. They studied 2146 live births from a group of pregnant women from 12 medical centers across the United States during 1959 to 1965. Eighteen percent of the women had SGA births.
Gernand AD, Simhan HN, Klebanoff MA, Bodnar LM. Maternal Serum 25-Hydroxyvitamin D and Measures of Newborn and Placental Weight in a U.S. Multicenter Cohort Study. J Clin Endocrinol Metab. 2012 Nov 16
The authors found that the risk for SGA babies were twice as high in severely deficient mothers, levels less than 15 ng/ml. They also found that birth weight and head size were positively associated with vitamin D levels, with a tendency for the effect to level off above 15 ng/ml. However, the number of mothers with 25(OH)D levels above 40 and 50 ng/ml was not listed, so it is impossible to know where the effect would actually plateau.
They found that this association with SGA and 25(OH)D only held for levels obtained during the first trimester of pregnancy, not the second trimester. That is, SGA is associated with 25(OH)D levels from early pregnancy. Furthermore, they found no evidence for a U shaped curve this time, as they did in an earlier smaller study using mothers from the early 2000 period. They did not elaborate on the reasons for the different findings. The authors called for more research in the area, and for randomized controlled trials.
The Vitamin D Council recommends that women thinking of getting pregnant take 5,000 IU/day of vitamin D3. Once pregnant, the woman should increase the dose to 6,000 IU/day, 5,000 IU for the woman and 1,000 IU for the developing fetus. More specifically, take enough vitamin D or get enough UVB light to obtain natural blood levels, around 50 ng/ml. You will find the amount needed varies with genetics and with body weight. As most pregnant women cannot sunbathe every day, some women may need even up to 10,000 IU/day to maintain natural vitamin D levels throughout pregnancy.