When Maternal Antibodies May Cause Anaemia in the Baby

The test commonly called the Indirect Coombs Test, or ICT, is more formally known as an indirect antiglobulin test.

It detects antibodies in the mother’s blood that may react with red blood cells.

A positive ICT does not answer all the important questions.

It does not tell us, by itself:

The laboratory must identify the antibody and assess its strength by titre or, for selected antibodies, by quantitative concentration.

An RhD-negative woman does not have an illness.

The pregnancy becomes clinically important when she develops immune anti-D and the baby is RhD-positive.

This is medically called RhD alloimmunisation.

The maternal anti-D antibody can cross the placenta, attach to the baby’s RhD-positive red blood cells and cause their destruction.

The resulting condition is called:

Haemolytic Disease of the Fetus and Newborn, or HDFN.

The severity can range from no fetal effect to anaemia, hydrops, neonatal jaundice or, in severe untreated disease, fetal loss.^[1–6]

Anti-D detected in maternal blood may be:

Passive and immune anti-D cannot be reliably distinguished simply by looking at whether the ICT reaction appears weak or strong.

The interpretation requires:

Passive anti-D normally falls with time.

Immune anti-D may persist or rise after further exposure.

When the distinction remains uncertain, the pregnancy should be monitored cautiously, and anti-D prophylaxis should generally continue until immune sensitisation has been confirmed.^[1]

A recent anti-D injection must be documented on every laboratory request form.

“High ICT” is commonly used as shorthand.

It is not a complete diagnosis.

The exact antibody determines the fetal risk.

Anti-D remains an important cause of severe fetal and neonatal haemolytic disease.

The risk depends on:

Anti-c can also cause severe fetal anaemia and HDFN.

A woman may have anti-c whether she is RhD-negative or RhD-positive.

The pregnancy should not be managed as a routine “Rh-negative pregnancy” merely because anti-D is absent.

Anti-K, commonly called anti-Kell, behaves differently.

It can cause anaemia through both red-cell destruction and suppression of fetal red-cell production.

Severe disease may occur at titres lower than those used for many other antibodies.

Anti-K therefore requires early specialist review when identified, particularly when the baby is Kell-positive or the fetal antigen status is unknown.

Other antibodies capable of causing fetal or neonatal disease include selected examples of:

The risk and monitoring pathway depend on the specific antibody, its characteristics and the previous history.

Anti-D, anti-c and anti-K are the antibodies most often associated with disease severe enough to require antenatal fetal intervention.^[1]

Two women may both have an ICT titre of 1:32.

Their pregnancies may carry very different levels of risk.

The first question is whether the detected anti-D is passive or immune.

The anti-D injection itself can make the antibody screen positive.

This result should not automatically be labelled sensitisation.

Equally, genuine immune anti-D should not be dismissed as “just the injection” without appropriate assessment.

Serial antibody measurements may help identify whether the pregnancy has reached the threshold for fetal surveillance.

The baby’s antigen status is central.

An antigen-negative baby is not at risk from that specific antibody.

The previous outcome may be more important than the present titre.

Maternal antibody titres become less reliable as predictors when a previous fetus or newborn was already affected.

The next affected pregnancy can develop anaemia earlier.

Specialist surveillance should therefore begin early without waiting for the titre to reach the same level seen in the previous pregnancy.

A history of fetal transfusion identifies a pregnancy at particularly high risk.

The fetal-therapy team should be contacted early in the next pregnancy.

The gestational age at which the previous baby first required transfusion is essential for planning.

Anti-K should not be managed by applying the usual anti-D titre thresholds.

Modern guidance supports beginning MCA Doppler surveillance when the anti-K titre reaches 1:4, or earlier according to the previous history and specialist assessment.^[5,8]

If reliable fetal testing shows that the baby does not carry the antigen targeted by the maternal antibody, that antibody cannot cause fetal anaemia in the current pregnancy.

The maternal antibody remains important for:

The first consultation should not begin and end with the latest ICT number.

It should reconstruct how the antibody appeared and whether a previous baby was affected.

For every pregnancy, I review:

The review also includes:

Please bring every available:

A chronological one-page summary is particularly useful.

Missing records should not prevent a consultation.

The history can still be reconstructed from the information available.

The purpose of investigation is to determine whether the baby is genuinely at risk and whether fetal surveillance or treatment is required.

The maternal ABO and RhD group should be confirmed on a correctly identified sample.

Weak or variant RhD results may require specialist transfusion-laboratory interpretation.

A positive antibody screen should be followed by formal antibody identification.

The report should state:

The antibody should not be reported only as “ICT positive”.

The laboratory and clinician should review:

A strong reaction cannot, by itself, prove immune anti-D.

A weak reaction cannot safely exclude it.

Depending on the antibody and laboratory system, the result may be reported as:

Anti-D and anti-c may be quantified in IU/mL in specialist systems.

Many other antibodies are followed by titre.

The baby must carry the corresponding red-cell antigen for the maternal antibody to cause fetal disease.

This can be assessed through:

A previous affected pregnancy changes the pathway.

The titre may no longer be the appropriate gatekeeper for beginning MCA Doppler surveillance.

Once the pregnancy is considered at risk, the central surveillance tool is:

Middle Cerebral Artery Peak Systolic Velocity, or MCA-PSV Doppler.

The aim is to identify fetal anaemia before hydrops develops.^[1–7]

An antibody titre is created by repeatedly diluting the maternal blood sample.

The reported titre is the highest dilution at which the antibody remains detectable.

For example:

The denominator becomes larger as the detectable antibody strength increases.

Titre testing is technique-dependent.

Results may differ between laboratories and methods.

Wherever possible:

A rise of more than one dilution—for example, from 1:4 to 1:16—may be considered significant.

A single change from 1:8 to 1:16 may partly reflect laboratory variation and should be interpreted with the complete history.^[1]

Do not compare titres from two different laboratories as though they were automatically equivalent.

A critical titre is the level at which the risk is considered sufficient to begin direct fetal surveillance.

Current expert guidance commonly uses:

These thresholds are not universal.

The laboratory method, antibody type, local protocol and previous pregnancy history must be considered.

A previous affected fetus or newborn can justify MCA surveillance regardless of the current titre.^[5]

Where validated quantitative testing is available, anti-D may be reported as:

A rising level or a previous affected pregnancy may require referral even before these thresholds are reached.^[1]

For anti-c:

A titre of 1:16 cannot be converted casually into a concentration in IU/mL.

They are different laboratory measurement systems.

The report must be interpreted according to the method used by that laboratory.

The mother’s antibody can affect the baby only when the baby carries the matching antigen.

This is one of the most important risk-separating steps.

A maternal blood sample contains small fragments of placental DNA that usually reflect the fetal genetic pattern.

Specialist cell-free fetal DNA testing can determine fetal red-cell antigen status for antibodies including:

Recent guidance supports testing from approximately 10 weeks when a validated diagnostic assay is available.

The exact timing, need for repeat testing and interpretation of an inconclusive result depend on the antigen and laboratory platform.^[1,3,5]

Routine NIPT for chromosome conditions does not automatically report the baby’s RhD, Rhc or Kell antigen status.

A specific fetal red-cell antigen test must be requested.

The baby is not at risk of anaemia from that specific maternal antibody.

Unnecessary serial MCA Doppler surveillance may be avoided.

The pregnancy requires risk-based surveillance according to:

Options may include:

If the biological father lacks the antigen, the baby cannot inherit it from him.

If he is homozygous for the antigen, every biological child will be expected to carry it.

If he is heterozygous, each pregnancy has a possibility of inheriting or not inheriting the antigen.

Direct fetal antigen testing is often more clinically useful because it answers the question for the current baby.

MCA Doppler measures the speed of blood flowing through one of the baby’s brain arteries.

When fetal haemoglobin falls:

This rise in velocity can identify moderate or severe fetal anaemia before hydrops appears.

MCA-PSV is the primary non-invasive method used to screen an at-risk fetus for anaemia.^[4,6,7]

The timing depends on the history.

For an antigen-positive or antigen-unknown baby with a critical antibody titre, modern guidance supports beginning weekly surveillance by approximately 16 weeks.

When a previous fetus or newborn was affected, surveillance may begin by 15–16 weeks, or according to the gestational age of previous disease.

A woman with a previous intrauterine transfusion should be referred early to an experienced fetal-therapy service.

MCA Doppler is commonly performed weekly once the fetus is at meaningful risk.

The interval may be adjusted according to:

The measured blood-flow velocity is compared with the expected median for that gestational age.

MoM means multiples of the median.

An MCA-PSV consistently above 1.5 MoM indicates a significant risk of moderate or severe fetal anaemia.

This is not a diagnosis from one number alone.

The result should be confirmed using correct technique and interpreted by a fetal-medicine specialist.

When the threshold is reached—or hydrops is present—fetal blood sampling should be undertaken in a centre prepared to proceed directly to intrauterine transfusion if anaemia is confirmed.^[4,6]

Accurate MCA Doppler requires:

An incorrectly performed test can create false reassurance or unnecessary intervention.

Hydrops may include:

Hydrops is a late sign of severe disease.

The purpose of MCA surveillance is to detect anaemia before this stage.

The false-positive rate of MCA Doppler increases after approximately 35 weeks.

Late-pregnancy decisions therefore combine:

Treatment can be divided into three groups.

Early referral is required when:

Anti-D immunoglobulin helps prevent an RhD-negative, non-sensitised woman from developing immune anti-D.

It does not remove immune anti-D once sensitisation has occurred.

If anti-D is confirmed to be immune, additional anti-D immunoglobulin does not treat the established antibody.

When anti-D could still be passive and sensitisation has not been confirmed, indicated prophylaxis should continue until the situation is clarified.^[1,10]

When fetal blood sampling confirms clinically significant anaemia, compatible red blood cells can be transfused directly to the baby.

This is the established treatment for severe fetal anaemia caused by red-cell alloimmunisation.

The neonatal and blood-bank teams should be informed before delivery.

Appropriate blood should be available for the mother and baby when required.

A 2025 expert guideline recommends considering antenatal intravenous immunoglobulin, or IVIG, when:

The same guideline suggests that IVIG may be considered for exceptionally high initial titres, including anti-D of 1:512 or above or anti-K of 1:64 or above, even without a previous affected pregnancy.

The certainty of evidence is very low.

IVIG does not guarantee that intrauterine transfusion will be avoided.

Its proposed aim is to delay the onset of severe anaemia and the first fetal transfusion.

This treatment should be discussed only in an experienced fetal-therapy centre after counselling about benefits, limitations, adverse effects and cost.^[5]

Plasma exchange has been used in exceptional early, severe disease.

Evidence is limited and protocols vary.

It should not be offered routinely for every high ICT titre.

The timing of the final transfusion and delivery requires individual planning.

Recent expert guidance supports continuing necessary intrauterine transfusions through the end of the 35th week when technically feasible.

Delivery is then planned according to fetal condition, the interval from the last transfusion and neonatal capability.^[5]

The following should not be used merely because the ICT titre is high:

An intrauterine transfusion, or IUT, delivers compatible donor red blood cells directly to the fetal circulation under continuous ultrasound guidance.

The procedure is performed in a specialist fetal-therapy centre.

The most common approach is an ultrasound-guided transfusion into the umbilical vein.

Before the procedure, the team plans:

A small fetal blood sample is obtained to measure:

The fetal-therapy team should be prepared to proceed directly to transfusion rather than performing fetal blood sampling as an isolated diagnostic procedure.

The blood must be specially selected and prepared.

It should be:

Maternal antibodies remain in the circulation.

The fetus may therefore require repeat transfusions until delivery becomes safer than another procedure.

After an IUT, the timing of the next procedure is based on specialist assessment, expected haemoglobin decline and Doppler findings.

IUT is a life-saving procedure but is not risk-free.

Potential complications include:

The balance changes with gestational age.

A procedure that is necessary at 23 weeks may no longer be preferable to delivery at a much later gestation.

At experienced fetal-therapy centres, perinatal survival after treatment can exceed 95%.

Outcome depends on:

^[4]

The delivery plan should involve:

The team should know:

For an at-risk pregnancy that has not required IUT and remains stable, recent guidance supports delivery between:

37 weeks 0 days and 38 weeks 6 days.

Earlier delivery may be required because of:

Delivery after IUT is individualised according to the final transfusion, fetal condition and neonatal resources.^[4–6,9]

Alloimmunisation alone does not automatically require a caesarean birth.

The mode of delivery depends on:

For a baby born to a mother with a clinically significant antibody, early testing may include:

A positive DAT alone does not prove clinically significant HDFN.

The baby’s haemoglobin, bilirubin and clinical condition determine treatment.^[1]

The baby may require:

The need for treatment varies widely.

Anaemia can appear or worsen after discharge.

This may result from:

Significant late anaemia can occur during the first 6–8 weeks, and some babies require monitoring for longer.

Parents should receive a clear follow-up plan for haemoglobin, bilirubin and clinical review.

Donor blood may temporarily influence the baby’s apparent blood-group result.

A baby transfused with RhD-negative blood before birth may temporarily test RhD-negative even when genetically RhD-positive.

The neonatal team must know that an IUT occurred.

Early discharge without an appropriate neonatal follow-up plan is not advisable in a significantly affected pregnancy.^[1,9]

A future pregnancy should be planned before conception wherever possible.

Maternal immune antibodies may remain detectable for many years.

A low initial titre does not erase a previous history of severe fetal disease.

The plan may include:

The woman should contact the treating team early.

The plan may include:

The next pregnancy should not wait for the antibody to reach an arbitrary critical titre.

When the baby is antigen positive, surveillance is guided by the previous disease pattern.

The next affected fetus may require monitoring and treatment earlier.

The mother’s antibody also matters for her own medical care.

If she requires transfusion, the blood bank must provide units that are compatible and negative for the relevant antigen.

Antibody information should never be removed from the transfusion record merely because a later blood test becomes negative.

A pregnancy complicated by maternal antibodies can become dominated by numbers:

The woman still needs a clinical team that explains what each number can—and cannot—predict.

Supportive care includes:

The prognosis depends on:

The baby will not develop fetal anaemia from that particular maternal antibody.

The pregnancy may avoid disease-specific surveillance.

The pregnancy remains under surveillance.

A high titre does not mean that fetal treatment is inevitable.

IUT can correct fetal anaemia and allow the pregnancy to continue.

Experienced centres report perinatal survival exceeding 95%, although outcomes are less favourable when severe anaemia or hydrops develops very early.^[4]

Phototherapy, exchange transfusion or later red-cell transfusion may still be required even after successful antenatal treatment.

A successful delivery is therefore not the end of surveillance.

It is the transition from fetal treatment to neonatal follow-up.