Iron deficiency remains a very common problem during pregnancy, even for women who are not vegetarian or vegan. A recent study conducted in Belgium found that approximately 35% of pregnant women in their first trimester were iron deficient. Your OB or midwife will be testing for this to make sure you reach adequate levels. Synthetic iron supplements have one big downside - Constipation. However, food based iron seems to have a gentler affect on the stomach while still increasing healthy iron reserves.