Rh factor and blood type compatibility calculator

The rule this tool applies. RhD haemolytic disease of the fetus and newborn requires an RhD-negative mother carrying an RhD-positive baby. If the pregnant parent is RhD positive, incompatibility is impossible and anti-D is irrelevant, whatever the father's blood type. If the pregnant parent is RhD negative, the baby may be RhD positive and anti-D prophylaxis becomes relevant.

Why the father's type does not decide it. An RhD-positive father may carry one copy of the D gene or two. About 40% carry two copies and 60% carry one; with two, every baby is RhD positive, with one, about half are — and in practice up to 40% of RhD-negative pregnant women turn out to be carrying an RhD-negative baby. ACOG puts the practical position plainly: if paternity is certain and the father is known to be RhD negative, antenatal prophylaxis is unnecessary, but since most clinicians will not blood-type the father, routine prophylaxis remains the preferred option. Two RhD-negative parents should have an RhD-negative baby — but maternity services plan prophylaxis from the mother's blood group, because a reported paternal blood type cannot be verified. The route out of unnecessary anti-D is not paternal typing but fetal RHD genotyping from the mother's own blood.

ABO groups. The tool enumerates every ABO group the baby could have from the parents' groups, treating A and B as codominant and O as recessive, so a group A parent may be AA or AO. Meaningful ABO haemolytic disease is essentially confined to a group O mother with a group A or B baby, because group O people make IgG anti-A and anti-B which crosses the placenta, whereas group A and group B people make mostly IgM, which does not.

Routine antenatal anti-D prophylaxis, by guideline body.

• UK — NICE TA156: a single dose of 1500 IU at 28 weeks (or between 28 and 30 weeks), two doses of 500 IU at 28 and 34 weeks, or two doses of 1000–1650 IU at 28 and 34 weeks. NICE HealthTech guidance HTG420, which replaced DG25 in December 2025 with recommendations unchanged, recommends high-throughput non-invasive testing of maternal blood for the fetal RHD genotype provided the test costs £24 or less, so anti-D can be withheld entirely when the fetus is RhD negative. This is offered in England only — not in Scotland or Northern Ireland, and with no national provision in Wales.
• United States — ACOG Practice Bulletin 181: anti-D immune globulin 300 micrograms (1500 IU) at around 28 weeks, and a further 300 micrograms within 72 hours of birth if the newborn is RhD positive. ACOG does not recommend cell-free DNA fetal RHD genotyping for routine use, and explicitly declined the UK two-dose schedule, finding no compelling data to change from the single-dose procedure used in the United States.
• Australia — National Blood Authority guideline v1.3 (August 2024) and RANZCOG C-Obs 6: 625 IU at approximately 28 weeks and again at approximately 34 weeks, either dose allowed within about two weeks either side. The NBA found insufficient evidence to switch to a single 1500 IU dose. Fetomaternal haemorrhage is quantified after the birth of an RhD-positive baby to set the postnatal dose, given within 72 hours; flow cytometry is the preferred method. Fetal RHD genotyping from 11+0 weeks is recommended in the guideline, but it states that as of February 2024 the test is not widely available in Australia and universal prophylaxis should be maintained until it is. In New Zealand, NZ Blood supports 28 and 32 weeks, but RANZCOG records that this has not become standard practice there.

Early pregnancy loss is where the three bodies genuinely disagree. NICE NG126, updated in June 2026, says do not offer anti-D for ectopic pregnancy, miscarriage or threatened miscarriage up to and including 11+6 weeks — medical or surgical management alike — and offer at least 250 IU (50 micrograms) at 12+0 to 12+6 weeks for medical or surgical management, with anti-D to be considered for threatened miscarriage with heavy or recurrent bleeding. RANZCOG gives 250 IU in the first trimester for miscarriage, ectopic pregnancy, molar pregnancy, chorionic villus sampling, and abortion after 10 weeks. ACOG moved the other way: a Clinical Practice Update published in December 2024 now suggests forgoing routine Rh testing and anti-D immune globulin altogether for abortion or pregnancy loss before 12 weeks 0 days, superseding Practice Bulletin 181 in that window. The Society for Maternal-Fetal Medicine reached the opposite conclusion earlier in 2024, so US units genuinely differ.

Other sensitising events, broadly agreed across all three: bleeding later in pregnancy, ectopic pregnancy, amniocentesis, chorionic villus sampling, fetal blood sampling or fetal surgery, external cephalic version whether successful or not, abdominal trauma, and intrauterine death. Anti-D works best given within 72 hours.

Prevention is not treatment. Anti-D prevents sensitisation. It does not reverse it. If the antibody screen already shows anti-D antibodies, care moves to antibody titres, middle cerebral artery Doppler monitoring for fetal anaemia, fetal medicine referral, and intrauterine transfusion if needed. Anti-D also covers only the D antigen — other red cell antibodies such as anti-c and anti-K (Kell) can cause haemolytic disease and are not prevented by it.

This tool describes what is genetically possible and what the routine pathway is. It cannot tell you your baby's blood group or whether you are already sensitised, and it does not replace your booking bloods.