The most common side effects of prenatal multivitamins include gastrointestinal issues such as constipation, diarrhea, green or dark stools, cramping, and vomiting, notes Drugs.com. If these don't go away within a few weeks or if they interfere with quality of life, it's time to talk to a doctor.
While the common side effects are not life threatening, some of the less common (but more severe) side effects are. These include intense allergic reactions such as itching, hives, rash, breathing difficulties, or swelling of the tongue, lips, face or mouth. More severe gastrointestinal side effects include bloody, tarry or black stools; ongoing and severe stomach pain and nausea; or vomit resembling coffee grounds or blood, states Drugs.com.
Despite these side effects, prenatal multivitamins deliver worthwhile benefits to women trying to conceive or who are already pregnant. The primary nutritional shortfall that women face during this time is in the area of iron and folic acid, but other shortfalls develop due to the increased nutritional needs that women have during pregnancy. Folic acid is crucial to early fetal development, which is why supplementation should begin before a woman becomes pregnant. As of May 2015, women should get no fewer than 400 micrograms per day while they are fertile, reports WebMD.