Two recently published studies from Israel have examined vegan diets in pregnancy.
The first examined 60 vegans, 64 vegetarians, 37 fish eaters, and 112 omnivores (Avnon, 2020a). The second study examined 234 vegans, 133 vegetarians, and 1,052 omnivores (Kesary, 2020). Both studies found that:
The first study found that the average birth weight of each group was within the normal range for the local population. The second study found that the average birth weight percentile was within the normal range.
These results suggest that while babies of vegan women may be slightly smaller than babies of omnivores, their birth weights, on average, are considered to be normal.
Both studies found that significantly more infants whose mothers were vegan were classified as being small for gestational age compared to infants whose mothers were omnivores. Babies that are classified as small for gestational age are smaller than 90% of babies of the same gestational age and have a higher risk of problems like low blood sugar and difficulty maintaining body temperature after birth.
Many factors can result in having a small for gestational age infant including the parents being smaller; the mother having conditions such as diabetes, high blood pressure, or an infection; problems with the placenta; and birth defects. These factors were not controlled for in the first study; only maternal diabetes was controlled for in the second study. Another report about the same subjects as the first study found that the infants of vegan women were not significantly different from the infants of omnivores in terms of umbilical cord vitamin B12, folate, ferritin, and hemoglobin (Avnon, 2020b).
Although its not known for certain what caused the higher incidence of small for gestational age infants in these studies, it is clear that starting a pregnancy underweight and gaining less than the recommended amount in pregnancy are associated with delivering a small baby. This could be due to not eating enough calories. You can see current weight gain recommendations in pregnancy in the Center for Disease Control and Prevention’s article Weight Gain During Pregnancy and read more about vegan pregnancy in VeganHealth’s article Pregnancy, Infants, and Children.
Avnon, T., Dubinsky, E. P., Lavie, I., Bashi, T. B. M., Anbar, R., & Yogev, Y. (2020a). The impact of a vegan diet on pregnancy outcomes. Journal of Perinatology, 1-5.
Avnon, T., Anbar, R., Lavie, I., Bashi, T. B. M., Dubinsky, E. P., Shaham, S., & Yogev, Y. (2020b). Does vegan diet influence umbilical cord vitamin B12, folate, and ferritin levels?. Archives of gynecology and obstetrics, 301(6), 1417-1422.
Kesary, Y., Avital, K., & Hiersch, L. (2020). Maternal plant-based diet during gestation and pregnancy outcomes. Archives of gynecology and obstetrics, 302(4), 887-898.