Antibodies and Rh factor during pregnancy
Your hemoglobin will be tested between 26 and 30 weeks to ensure that you have not developed anemia. If your hemoglobin is less than 11 g/dL, you will probably be advised to take an iron supplement. It is important to build up your red blood cell count now, since your hemoglobin is likely to drop further toward the end of pregnancy, thanks to the increased fluid content in your bloodstream. However, gastrointestinal upsets, constipation problems, and sometimes diarrhea are common side effects of iron supplements so if you have problems, try a different brand. Liquid preparations available in your local pharmacies.
The antibody screen uses a portion of the same blood sample to check your blood group again and make sure that you have not developed any red cell antibodies. This is particularly important if you are Rh negative. Although the time of greatest risk is at delivery, when a Rh negative mother may become sensitized to blood from her Rh positive baby, these women are now offered an injection of Rhogam at 28 weeks and after delivery, even if this is their first pregnancy.
If the father is also Rh negative, there is no risk that the baby's blood will be incompatible. Rh negative women who have amniocentesis, chorionic villus sampling, or those who have vaginal bleeding or abdominal trauma during pregnancy are given an injection within 72 hours.