Rh factor
The plus or minus in your blood type. It barely matters day to day, but it matters a lot for transfusions and pregnancy.
Rh positive vs Rh negative
The Rh system (named after the rhesus monkeys used in Landsteiner and Wiener’s 1940 experiments) contains dozens of antigens, but when people say “Rh” they mean one: the D antigen. Have it and you’re Rh positive; lack it and you’re Rh negative.
Around 85% of people are Rh positive. Unlike ABO, Rh-negative people don’t carry anti-D from birth. They make it only after exposure to Rh-positive red cells, which is why the first exposure is the one to prevent.
How Rh is inherited
The D allele is dominant. Rh-positive people are DD or Dd; Rh-negative people are dd. Two Rh-negative parents always have Rh-negative children; two Rh-positive parents can have an Rh-negative child if both are Dd. Check the odds with the blood type calculator.
Rh in transfusion
Rh-negative patients should receive Rh-negative red cells. Rh-positive patients can receive either. In a massive-bleeding emergency with no Rh-negative stock, Rh-positive blood may be given to Rh-negative men or older women, since the risk is sensitisation rather than an immediate reaction.
Step 1 of 4
First pregnancy
An Rh-negative mother carries an Rh-positive baby. The two circulations are separate, and the baby’s blood type comes partly from an Rh-positive father.
Rh negative pregnancy: what to expect
- Booking blood test: your ABO and Rh type and an antibody screen are checked early in pregnancy.
- Partner or fetal testing: if the father is also Rh negative, the baby will be too. Many services also offer a cell-free fetal DNA blood test from the mother to find the baby’s Rh type.
- Anti-D around 28 weeks: routine antenatal anti-D is offered if the baby is or may be Rh positive (a single dose at 28 weeks in the US; some UK services use two doses at 28 and 34 weeks).
- After “sensitising events”: bleeding, miscarriage, ectopic pregnancy, abdominal trauma, amniocentesis or turning a breech baby.
- After birth: the baby’s blood type is checked; if Rh positive, another dose is given within 72 hours.
More detail on the injection itself: RhoGAM, when and why it’s given. Guidance here follows ACOG and NICE; your own care team decides what applies to you.
Frequently asked questions
What does Rh negative mean?
Your red cells do not carry the Rh D antigen. About 15% of people of European ancestry are Rh negative, fewer among people of African ancestry (around 3–8%) and under 1% in much of East Asia.
Is being Rh negative a health problem?
No. It only matters for transfusion (you should receive Rh-negative red cells) and in pregnancy, if the baby could be Rh positive.
What happens if an Rh-negative mother has an Rh-positive baby?
If the baby’s blood enters her circulation she can make anti-D antibodies, which can harm a later Rh-positive baby. An anti-D injection (RhoGAM) prevents this in almost all cases.
Does Rh incompatibility affect the first pregnancy?
Rarely. Sensitisation usually happens at the first birth, so it mainly threatens later pregnancies. It can occur earlier after bleeding, miscarriage, trauma or procedures, which is why anti-D is also given after those events.
Can two Rh-positive parents have an Rh-negative baby?
Yes, if both carry the recessive Rh-negative allele (Dd). Each child then has a 25% chance of being Rh negative.