Depends on the type of iron, honest
There are three types of ferrous iron supplements: ferrous sulfate, ferrous fumarate, and ferrous gluconate.
Iron is best absorbed on an empty stomach, so take it before you eat in the morning or when you go to bed at night. Certain foods and vitamins inhibit the absorption of iron. One of these is calcium, so be careful not to take your calcium at the same time as your iron supplement. If your prenatal vitamin contains calcium (which it almost certainly does), don't take it at the same time either.
Caffeine also prevents iron absorption, so wait a while to drink tea, coffee or a caffeinated soft drink after taking your iron supplement. Better still, avoid caffeine altogether. Vitamin C, on the other hand, helps your body absorb iron, so it's a good idea to take your iron with a glass of orange juice or other juice that is high in vitamin C. Many women find it works well to take iron in the morning with juice and then take calcium or prenatal vitamins containing calcium before bed. If, however, the iron seems to be upsetting your stomach, switch these around and take the iron before bed when you can sleep off any discomfort.
Unfortunately, taking iron supplements can cause constipation, already an annoying problem for many pregnant women. If you suffer from these unwanted side effects, try increasing the amount of fiber in your diet by eating lots of fruits and vegetables and drinking plenty of fluids.
And finally, if you already have children, be sure to keep vitamins and iron supplements out of their reach. Accidental iron poisonings are all too common among children because they can mistake the brightly colored pills for candy.