Why do I need extra iron in pregnancy?

You and baby. Most women are on the verge of being iron deficient and will need to make more blood and have fetus make blood iron is need in all tissues and is required for good health and devlopment.
Baby. The developing baby needs a lot of iron. It gets this iron from the mother's iron stores. As a result a pregnant woman can become iron depleted. An adult multivitamin has an iron supplement in it.