I'd also assume they're talking abut Rh factor. I don't think they usually worry about a shot in the first pregnancy, though. I believe it's only given in subsequent pregnancies (but I could be wrong).
You DO need to get a shot(s) with your 1st pregnancy. However, if the blood were to mix the first pregnancy, it wouldn't cause a problem with the first pregnancy, but with subsequent pregnancies.
This. I am RH negative as well. This is my first pregnancy. I received the Rhogam injection at 28 weeks, and will get another right after delivery. Antibodies only cause a problem (i.e. increased risk of early miscarriage, preterm delivery etc.) when your subsequent baby is NOT a negative blood type. Since positive blood-type babies are more common than negative, and most practitioners don't routinely check the blood-type of an in-utero baby, they DO administer the Rhogam injection in your first pregnancy, because like PP have mentioned, your body can create antibodies anytime there is an opportunity for a positive-blood type baby's blood to mix with yours (vaginal delivery, c-section, spotting during pregnancy etc). Your are already at 28 weeks, so I would imagine your Dr. will keep a close eye on you, and may even test to determine baby's blood type (if it's negative, there is no increased risk of the aforementioned problems). Worst case scenerio, they may need to induce/perform a c-section earlier than your due-date. However, just keep in mind that many negative blood-type mothers gave birth to mulitple, healthy, positive babies in the past, with no intervention/injections. T&Ps to you and your baby!