Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Blood group, antibodies and rhesus prophylaxis
Obstetrics and gynaecology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending
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Fetal anaemia can deteriorate rapidly
A pregnancy with a clinically significant maternal antibody, rising risk markers, abnormal middle cerebral artery Doppler, hydrops or reduced fetal movement may have severe haemolytic disease and needs urgent fetal-medicine assessment.
Action: Contact fetal medicine and transfusion laboratory promptly, confirm gestation and antibody history, assess fetal wellbeing, and arrange specialist Doppler surveillance or intrauterine transfusion planning rather than giving prophylactic anti-D as treatment.
Purpose and principles
Interpret maternal blood group and antibody results, prevent avoidable RhD alloimmunisation and escalate established red-cell alloimmunisation before fetal anaemia develops.
Maternal red-cell alloimmunisation occurs when fetal erythrocyte antigens absent from the pregnant person's cells enter the maternal circulation and provoke IgG antibodies. In a later exposure, those antibodies can cross the placenta and destroy antigen-positive fetal red cells or, with anti-K, suppress erythropoiesis. The resulting haemolytic disease of the fetus and newborn ranges from neonatal jaundice to severe fetal anaemia, hydrops and death. Prevention and surveillance are therefore separate tasks: anti-D prevents one antibody, while laboratory and fetal-medicine pathways manage antibodies already present.
At booking obtain ABO group, RhD type and an antibody screen. Repeat the antibody screen at 28 weeks for all pregnancies, drawing it before routine anti-D where feasible so passive anti-D does not complicate interpretation. The laboratory identifies any antibody and advises its clinical significance, quantification or titre, compatibility implications and recommended repeat interval. Anti-D, anti-c and anti-K account for much severe HDFN, but other antibodies can matter for the fetus or maternal transfusion, so an unrecognised result should never be dismissed as simply positive.
Indications, selection and cautions
Non-sensitised RhD negative
RhD-negative typing with no immune anti-D identifies a pregnancy in which anti-D prophylaxis can prevent primary sensitisation.
Passive anti-D pattern
A low anti-D signal after a documented prophylactic dose may be passive, but only the transfusion laboratory can reconcile timing and level safely.
Clinically significant antibody
Anti-D, anti-c, anti-K or another laboratory-designated antibody can affect the fetus, newborn, maternal transfusion or more than one of these.
Susceptible fetus
Paternal or cell-free fetal genotyping shows the relevant antigen, so maternal IgG can bind fetal cells and disease surveillance is required.
Possible severe HDFN
A previously affected pregnancy, hydrops, abnormal Doppler or impaired fetal wellbeing points to anaemia needing specialist intervention rather than titre review alone.
Red flags requiring action
Hydrops, abnormal middle cerebral artery peak systolic velocity, fetal tachycardia, reduced fetal movement or ultrasound evidence of anaemia requires urgent fetal-medicine review.
Previous intrauterine transfusion, neonatal exchange transfusion, severe jaundice, stillbirth or an earlier pregnancy affected by HDFN can confer high risk even when the current titre seems modest.
Assessment before treatment
01ABO and RhD typingFirst step
Why
Define transfusion group and identify RhD-negative pregnancies that may benefit from prophylaxis.
Interpretation and limitations
RhD-negative status alone does not prove eligibility: immune anti-D status and, where used, fetal RHD genotype determine the plan.
02Red-cell antibody screen and identification
Why
Detect maternal alloantibodies at booking and again at 28 weeks and classify their fetal and transfusion relevance.
Interpretation and limitations
A positive result requires named antibody, level or titre and prior-history review; the word antibody positive is not sufficient for risk stratification.
03Cell-free fetal antigen genotyping
Why
Establish whether the fetus carries the antigen targeted by a clinically significant maternal antibody.
Interpretation and limitations
A reliable antigen-negative result can avoid unnecessary fetal surveillance or anti-D; an antigen-positive or inconclusive result retains the risk pathway.
04Fetomaternal haemorrhage quantification
Why
Measure fetal red cells in maternal blood after sensitising events from 20 weeks and after birth.
Interpretation and limitations
The measured volume determines supplementary anti-D and follow-up sampling; it is not a test for established maternal sensitisation.
Treatment approach
01Routine preventionPrevent anti-D sensitisationFirst stepThe pregnant person is RhD negative and no immune anti-D has been identified.+
1Confirm the booking and 28-week antibody results and consider validated fetal RHD testing where the commissioned pathway offers it.
2Administer the licensed local routine regimen at 28 to 30 weeks or at 28 and 34 weeks, documenting product, dose, batch, route and consent.
3Continue to assess each sensitising event separately and arrange infant RhD and fetomaternal-haemorrhage testing after birth.
02Sensitising eventDeliver event-related prophylaxis on timeBleeding, trauma, invasive procedure or another recognised event could expose an RhD-negative, non-sensitised patient to fetal D-positive cells.+
1Confirm gestation, RhD and immune anti-D status, identify the event and contact the transfusion service if circumstances or previous doses are uncertain.
Regimens, contraindications and interactions
Prevents new maternal immune anti-D arising from otherwise unrecognised fetomaternal haemorrhage in late pregnancy.
Routine antenatal anti-D immunoglobulin
Give intramuscular anti-D using the licensed local regimen: one 1500 IU dose at 28 to 30 weeks or 500 IU at both 28 and 34 weeks.
Use only for a non-sensitised RhD-negative patient whose fetus is not known RhD negative; follow the specific product route and SmPC.
Covers fetal D-positive cells entering the maternal circulation after bleeding, trauma, procedures or other qualifying events.
Anti-D after a sensitising event
Give the indicated dose within 72 hours; from 20 weeks use at least 500 IU and add further doses according to fetomaternal-haemorrhage testing.
Routine antenatal prophylaxis does not cover a later event, and immune anti-D is not treated by giving more immunoglobulin.
Reduces sensitisation from fetal cells transferred during delivery and protects future antigen-positive pregnancies.
Postnatal anti-D immunoglobulin
Give at least 500 IU within 72 hours after an RhD-positive birth, with supplementary dosing determined by quantified fetomaternal haemorrhage.
Confirm the infant is RhD positive and the mother is not already sensitised; record the laboratory calculation and any follow-up sample.
Complications, monitoring and follow-up
At every dose, record anti-D product, amount, route, batch, time, indication and whether a fetomaternal-haemorrhage sample is pending.
Maintain a visible distinction between passive prophylactic anti-D and immune anti-D using the transfusion laboratory interpretation and administration history.
Trend clinically relevant antibodies at the laboratory-recommended interval and escalate on the antibody-specific rule or adverse obstetric history.
For a susceptible fetus under surveillance, track MCA Doppler, growth, fluid, hydrops signs and fetal movements within the fetal-medicine plan.
Before delivery, confirm compatible maternal blood availability and neonatal plans for cord testing, bilirubin, haemoglobin, phototherapy or transfusion.
After a large fetomaternal haemorrhage, verify that the calculated additional anti-D was administered and that clearance testing was completed.
Special situations
Anti-D has one preventive target
It prevents maternal immunity to the D antigen; it neither prevents anti-K or anti-c nor reverses any established antibody response.
Twenty-eight weeks has two jobs
Repeat antibody screening can detect new alloimmunisation, while routine anti-D then prevents D sensitisation during the remaining pregnancy.
Anti-K behaves differently
Kell antibodies can cause profound anaemia through erythroid suppression, so fetal risk is not safely inferred from titre alone.
Genotyping can remove risk
If validated testing shows that the fetus lacks the relevant paternal antigen, maternal antibody cannot target fetal red cells for that antigen.
Visible bleeding underestimates transfer
The volume seen vaginally or at delivery does not measure fetal cells in maternal circulation; laboratory quantification determines additional cover.
Common pitfalls
01
Giving prophylactic anti-D to treat established immune anti-D instead of arranging fetal-risk assessment.
02
Assuming the routine 28-week dose removes the need for prophylaxis after subsequent trauma or bleeding.
03
Applying obsolete blanket early-miscarriage rules instead of the 2026 NICE gestational recommendations.
04
Failing to draw a 28-week antibody screen because the patient is RhD positive or received anti-D previously.
05
Using the apparent amount of vaginal bleeding to decide that fetomaternal-haemorrhage quantification is unnecessary after 20 weeks.
06
Waiting for a high titre before referring anti-K or a pregnancy with a history of severe HDFN.
Practice
Two practice questions
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Question 1 of 2
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Obstetrics and gynaecologyOriginal SBA
Choosing routine rhesus prophylaxis
A 29-week pregnant patient is RhD negative, has no immune anti-D and cell-free fetal RHD status is unavailable. No anti-D has yet been given. What is appropriate now?
Sources and review status5 sources · checked 27 Aug 2026 · clinical review pending+
Sources
Sources and review status
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
An RhD-negative person without immune anti-D is eligible for routine antenatal anti-D prophylaxis unless validated fetal RHD genotyping shows an RhD-negative fetus or prophylaxis is declined after counselling. NICE supports either a single-dose regimen at 28 to 30 weeks or a two-dose regimen at 28 and 34 weeks, depending on the licensed product and local service. Routine prophylaxis addresses silent fetomaternal bleeding in late pregnancy; it does not cover a later sensitising event automatically and does not replace postnatal assessment.
Sensitising events include invasive prenatal procedures, abdominal trauma, antepartum haemorrhage, external cephalic version, intrauterine death and other events capable of fetomaternal haemorrhage. Give indicated anti-D promptly and within 72 hours. At 20 weeks or later, a minimum 500 IU dose is used and maternal blood is tested to estimate the fetal-cell volume; the transfusion laboratory calculates additional anti-D and any repeat testing. Before 20 weeks, the event, gestation and current national or transfusion guidance determine the minimum dose, with 500 IU often being the smallest stocked UK preparation.
Early-pregnancy anti-D rules changed in NICE NG126 in 2026. Do not offer anti-D for threatened miscarriage, miscarriage or ectopic pregnancy up to 11+6 weeks. From 12+0 to 12+6 weeks, offer at least 250 IU after medical or surgical management of ectopic pregnancy or miscarriage, and consider at least 250 IU for threatened miscarriage only when bleeding is heavy or recurrent. Other sensitising events and later gestations continue to follow transfusion guidance. This gestation-specific approach replaces the older blanket distinction between medical and surgical miscarriage before 12 weeks.
After birth, test cord or newborn blood for RhD status according to the maternity transfusion pathway. A non-sensitised RhD-negative mother who delivers an RhD-positive infant receives at least 500 IU anti-D within 72 hours and has fetomaternal haemorrhage quantified so the dose is adequate. No postnatal anti-D is needed when the baby is confirmed RhD negative. A large bleed may require extra doses and a follow-up sample to confirm fetal cells are cleared; the laboratory, not visual assessment of blood loss, directs this calculation.
When an antibody is detected, first decide whether anti-D is passive from recent prophylaxis or immune; laboratory timing and concentration, previous results and administered doses are essential. Determine the paternal or fetal antigen where appropriate, often using cell-free fetal genotyping for D, c, C, E, e or K through specialist services. If the fetus lacks the antigen, it is not at risk from that antibody. If antigen positive or status is unknown, maternal testing and prior outcome define referral and surveillance; anti-K and a severely affected history deserve particular caution.
Fetal anaemia surveillance uses specialist ultrasound, especially middle cerebral artery peak systolic velocity, after the fetus is shown or presumed susceptible and maternal risk is sufficient. A rising velocity reflects reduced blood viscosity and increased cardiac output but is a screening measure interpreted with gestation and trend. Significant concern prompts invasive fetal blood sampling only in a centre able to proceed to intrauterine transfusion. Delivery, neonatal cord tests, bilirubin and haemoglobin plans must be coordinated before birth, and maternal antibodies remain relevant for future transfusion and pregnancies.
Key points
Determine ABO group, RhD status and red-cell antibodies at booking and repeat antibody screening at 28 weeks, regardless of RhD group.
Routine antenatal anti-D prophylaxis is for non-sensitised RhD-negative pregnancies: use the licensed local regimen of one dose at 28 to 30 weeks or two doses at 28 and 34 weeks.
Routine prophylaxis does not replace event-related prophylaxis after a qualifying sensitising event or postnatal anti-D when the newborn is RhD positive.
Give indicated anti-D as soon as possible and within 72 hours; if the deadline is missed, obtain transfusion advice because some benefit may remain rather than abandoning prophylaxis.
After 20 weeks, give at least 500 IU for a sensitising event and quantify fetomaternal haemorrhage so additional anti-D can cover a larger bleed.
Current NICE NG126 says no anti-D for ectopic pregnancy, miscarriage or threatened miscarriage up to 11+6 weeks; at 12+0 to 12+6 weeks, offer at least 250 IU after medical or surgical ectopic pregnancy or miscarriage and consider it for heavy or recurrent threatened-miscarriage bleeding.
Prophylactic anti-D prevents new immune anti-D; it cannot remove established maternal anti-D or treat a fetus already affected by haemolysis.
When non-invasive fetal RHD testing shows an RhD-negative fetus, routine and event-related anti-D may be withheld under the validated local pathway because no D antigen mismatch exists.
Immune anti-D, anti-c and anti-K are important causes of severe fetal anaemia; anti-K may suppress erythropoiesis without a titre that mirrors disease severity.
Major abdominal trauma, heavy bleeding, fetal death, invasive procedure or another substantial fetomaternal haemorrhage after 20 weeks needs timely anti-D assessment and quantification of fetomaternal haemorrhage.
A transfusion need in a patient with red-cell antibodies requires early transfusion-laboratory involvement because compatible antigen-negative blood may take time to source.
05
Middle cerebral artery Doppler
Why
Screen a susceptible fetus for haemodynamic evidence of moderate or severe anaemia.
Interpretation and limitations
An abnormal gestation-adjusted peak systolic velocity needs prompt fetal-medicine review and may lead to fetal blood sampling and transfusion.
06Neonatal cord blood assessment
Why
Determine infant RhD and evaluate haemolysis risk when maternal antibodies are present.
Interpretation and limitations
RhD result directs postnatal prophylaxis, while direct antiglobulin testing, haemoglobin and bilirubin are selected according to the antibody and neonatal plan.
2
Give the gestation-appropriate anti-D as soon as possible and within 72 hours, applying the updated NG126 early-pregnancy exceptions accurately.
3From 20 weeks, send fetomaternal-haemorrhage testing, give extra anti-D when calculated and obtain any laboratory-requested clearance sample.
03AlloimmunisationAssess antigen and fetal anaemia riskA clinically significant maternal red-cell antibody is identified or a previous pregnancy had HDFN.+
1Refer according to antibody type, level, trend and prior outcome, and involve the transfusion laboratory early for maternal blood provision.
2Establish fetal antigen status by validated genotyping where possible and arrange specialist antibody and Doppler surveillance when the fetus is susceptible.
3EscalationEscalate abnormal fetal wellbeing or Doppler promptly for fetal blood sampling, intrauterine transfusion or timed birth within an experienced fetal-medicine service.