About one in seven people of European ancestry is Rh-negative: their red cells lack a protein called RhD. That's harmless until they carry an Rh-positive baby. Fetal cells cross into the mother, her immune system reads the protein as foreign, and it makes antibodies it never forgets. Those antibodies then cross into the next Rh-positive pregnancy and destroy that baby's red cells, worse with each pregnancy after.

A dose of donor anti-RhD antibodies clears the fetal cells before your immune system learns them, so the memory never forms. Before this, roughly one in seven at-risk women got sensitized after delivery. The postpartum shot cut that to one or two percent; adding a shot at 28 weeks drove it to one or two per thousand ^{1 2 3}.

The work is the clinic's; your job is to confirm each shot lands — most of all if your prenatal care started late or you've switched providers.

You won't feel any of this. The shot changes nothing you can sense; the disease it prevents would only surface in a later pregnancy, when it's too late to fix. And it isn't history everywhere: where coverage has gaps, Rh disease still causes about a hundred and sixty thousand perinatal deaths and a hundred thousand disabled children a year, nearly all preventable ^{6 7}.

Already sensitized? Some women arrive at a pregnancy already carrying anti-RhD, from a shot missed in the past. The 28-week shot can't undo that, but the pregnancy isn't doomed. At a maternal-fetal medicine center, an ultrasound of blood flow in the baby's brain catches anemia early, and a transfusion into the umbilical cord treats it. Around ninety percent of affected babies survive, most without lasting problems ^{8 9}.