Hi Emily,
Iron levels cannot be determined by hemoglobin level and it is quite normal for the Hb to drop during pregnancy of thalassemia minors. Some minors do need iron during pregnancy but it shouldn't be taken for granted that they do. Instead, it would be better to do a series of blood tests called iron studies to verify that you are iron deficient. Ask your hematologist to run these tests before taking iron, as iron should only be taken by those who are confirmed as having iron deficiency.
We have many post about pregnancy in thal minor. You can see them by going to
http://www.thalassemiapatientsandfriends.com/index.php?topic=3041.msg30075#msg30075