When should rh tested in pregnancy?

At the beginning: In prenatal care visit in early pregnancy, a mom's blood type is determined. If she is rh-, then she should be treated with Rhogam at 28 weeks and within 3 days of delivery. Failure to do so can result in severe complications to baby, but most importantly to any future pregnancies she may have.

As soon as possible: It is necessary to know at latest by 28 weeks in order to arrange treatment that is typically given at that time if needed. However it is important to know whether or not you are rh negative if you were to have any vaginal bleeding earlier than that.