What is being tested
At your first antenatal visit you have your ABO blood group, your Rhesus D status and an antibody screen — the indirect Coombs test. The antibody screen looks for antibodies in your blood that could cross the placenta and attack your baby's red blood cells. The commonest and most important is anti-D, but antibodies to Kell, c, E and Duffy antigens also matter.
How Rh incompatibility happens
If you are Rh negative and your baby inherits Rh positive blood from the father, your immune system may encounter the baby's red cells — most often at delivery, but also after bleeding, miscarriage, abortion, ectopic pregnancy, amniocentesis, external cephalic version or abdominal trauma. Once your immune system recognises the D antigen, it produces antibodies permanently. Those antibodies rarely harm a first baby, because sensitisation usually occurs at the end. The risk is to the next pregnancy, where antibodies cross the placenta early and destroy fetal red cells, causing fetal anaemia, jaundice and, in severe cases, hydrops.
Anti-D immunoglobulin, and why timing matters
Anti-D works by clearing fetal red cells from your circulation before your immune system learns to recognise them. It only works before sensitisation, which is why it is given prophylactically rather than in response to a positive antibody screen.
- At 28 weeks — routine antenatal prophylaxis, sometimes with a second dose at 34 weeks depending on local protocol.
- Within 72 hours of delivery, if the baby is Rh positive. Cord blood is tested to determine this.
- After any sensitising event — bleeding, miscarriage, termination, ectopic pregnancy, invasive testing or abdominal injury — again within 72 hours.
Make sure your Rh status is written in your own records and that you know your anti-D dates. Missed doses are the commonest preventable cause of sensitisation.
If antibodies are already present
Anti-D cannot reverse existing sensitisation. Care shifts to monitoring the baby: antibody titres repeated through pregnancy, middle cerebral artery Doppler ultrasound to detect fetal anaemia, and in severe cases intrauterine transfusion at a specialist fetal medicine centre. Outcomes with modern monitoring are good, but the pregnancy needs specialist management from the second trimester, not routine antenatal care.
ABO incompatibility is different
If you are group O and your baby is A or B, the baby may develop jaundice in the first days of life. This is far milder than Rh disease, needs no injection during pregnancy, and is managed with monitoring and phototherapy if required.
What to do with this information
- Know your blood group and Rh status, and carry it.
- If you are Rh negative, ask at every visit whether your anti-D is due and get the date recorded.
- Report any bleeding, fall or abdominal injury immediately — the 72-hour window is strict.
- Ask for the baby's cord blood group after delivery.