If you are Rh negative, meaning that your blood does not contain the Rh factor protein, your doctor will probably recommend that you get an Rh immune globulin shot (such as RhoGAM) within 72 hours of the start of your bleeding. With the shot you have lower odds of developing antibodies (becoming sensitized) against Rh positive blood from this pregnancy; if you have circulating antibodies against Rh factor, you can face problems in future pregnancies. If given later than 72 hours after the start of the miscarriage, the shot is not likely to make a difference.
If it has been more than three days since your miscarriage, however, don't panic. The odds that you have become sensitized against Rh factor are low. Although there's no strong evidence that the shot is totally necessary after a first-trimester pregnancy loss, the shot is low-risk, so doctors recommend it after any pregnancy bleeding because of the theoretical risk of Rh sensitization.
Still, it is a good idea to get it within the recommended time span if you can, as it makes the small risk of Rh sensitization even lower. There's much more evidence on the shot's necessity after a later pregnancy loss or after giving birth.
If you are concerned, you can ask your doctor about a blood test to check for RH sensitization so that you can set your mind at ease.
Source:
Jabara, S., "Is Rh immune globulin needed in early first-trimester abortion? A review." American Journal of Obstetrics and Gynecology Mar 2003. Accessed 1 Dec 2008.
