While prenatal vitamins are a healthy regimen during pregnancy to ensure that you are getting adequate nutrients for you and your baby, there are certain vitamins that should be avoided during pregnancy.
The main vitamin that should be avoided in excess quantities is Vitamin A. In large doses as a supplement, it has been shown to have a potentially negative effect on fetal development. Carrots and vegetables, on the other hand, do not contain vitamin A itself but carotene. Carotene is provitamin A and is converted to vitamin A in your body only if your body needs it, so you cannot get too much Vitamin A by eating carotene because the body regulates conversion from carotene to Vitamin A.
Other vitamins that could cause a problem due to build-up in the body are Vitamins D, E, and K.
Vitamin D and toxicity is a heavily studied topic. The upper level of Vitamin D supplementation is set at 2,000 IUs a day. The human body can produce 20,000 IUs of Vitamin D if exposed to the sun without clothing in one shot. Herein lies the trouble with supplementation. Vitamin D toxicity does occur, but due to the overwhelming conflict in studies, Vitamin D supplementation is not recommended for pregnant women.
Vitamin E supplementation is just as tricky as Vitamin D supplementation. There are two forms of Vitamin E supplements, synthetic and natural. Natural vitamin E is more readily absorbed by the body, but most supplements contain the synthetic version. There is a link between babies with heart defects and supplemental Vitamin E. In a recent study, women who took supplemental Vitamin E (over and above their prenatal vitamin) were 70% more likely to give birth to a baby with a heart defect.
Too much Vitamin K is associated with babies having worse cases of jaundice, the yellowing of the skin due to a buildup of bilirubin in the body.