If Vitamin C is taken in mega doses during pregnancy, it can theoretically have a negative effect on the pregnancy and the developing baby.
Vitamin C is perfect for keeping the immune system healthy. When pregnant, the RDA is 85 mg per day, and vitamin C supplementation is safe up to 2,000 mg. All vitamin C supplements should be approved by the doctor/midwife before adding it to your daily vitamin regime.
Some people use multivitamins that contain very high amounts of vitamin C, which are not considered safe during pregnancy. The best vitamin supplement during pregnancy is one that's specifically designed for pregnant women and contains just the right amount of vitamins.
You should also take into consideration the foods that you are consuming which contain vitamin C. Some people find levels as little as 500 mg a day can cause severe gastrointestinal discomfort, cramping, and diarrhea. If this is the case, either the diet will need to be adjusted or the prenatal vitamin will need to be changed. Diarrhea during pregnancy can lead to dehydration, which can pose a huge threat to baby and health risks to mom.
If mom gets dehydrated during pregnancy, the amniotic fluid level will decrease. The amniotic fluid is the lifeline for baby. Without this fluid, the baby will not gestate to term. Dehydration can also cause nausea which can lead to vomiting and even worse dehydration.
If mom is feeling cramps, gas, bloating and has diarrhea soon after taking vitamin C, notify your doctor and stop taking the supplement until the doctor advises otherwise.