Step 2 CK · Obstetrics & Gynecology

Red Cell Alloimmunization & Fetal Hydrops

Name: Red Cell Alloimmunization & Fetal Hydrops
Creator: Veli Bakalov, MD

From a positive antibody screen to a transfusion inside the womb.

Obstetrics & Gynecology·Obstetric Complications·5 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Every pregnancy gets ABO/Rh typing and an antibody screen; a positive screen usually means maternal IgG against a fetal red-cell antigen.
Anti-D, Kell, c, E, Duffy, and Kidd antibodies cause fetal hemolysis; anti-Lewis is IgM and harmless (Kell kills, Lewis lives).
Anti-Kell suppresses fetal erythropoiesis, so severe anemia can occur at low titers; use a critical titer of 1:8 or go straight to MCA Doppler.
Anti-D immune globulin prevents sensitization but is useless once the mother has made anti-D.
At the critical titer (usually 1:16–1:32), establish fetal antigen status by paternal typing or cell-free fetal DNA, then follow MCA peak systolic velocity.
MCA-PSV above 1.5 MoM indicates fetal anemia and calls for cordocentesis with intrauterine transfusion, or delivery near term.
Hydrops is fluid in at least 2 fetal compartments; nonimmune causes include heart disease or arrhythmia, Turner syndrome, parvovirus B19, α-thalassemia major, and TTTS.
Mirror syndrome is maternal edema, hypertension, and proteinuria mirroring fetal hydrops, with hemodilution rather than hemoconcentration.
Kleihauer-Betke or flow cytometry quantifies fetomaternal hemorrhage; each 300 μg vial of anti-D covers about 30 mL of fetal whole blood.

Sensitization and the antibodies that matter

How it happens. Every pregnant patient has ABO/Rh typing and an antibody screen (indirect Coombs test) at the first visit. A positive screen usually means the mother has made IgG against a red-cell antigen she lacks, usually after a fetomaternal hemorrhage (delivery, miscarriage, abortion, ectopic pregnancy, procedures, trauma) or a transfusion. IgG crosses the placenta and coats antigen-positive fetal red cells, which are destroyed in the fetal spleen, producing hemolytic disease of the fetus and newborn (HDFN).

Red-cell antibodies in pregnancy

Antibody	Class	Fetal significance
Anti-D	IgG	The classic cause of severe HDFN
Anti-c, anti-E, anti-C	IgG	Can cause significant HDFN (anti-c most often)
Anti-Kell	IgG	Suppresses erythropoiesis; low titers still dangerous
Anti-Duffy (Fya), anti-Kidd (Jka)	IgG	Can cause HDFN; managed like other significant antibodies
Anti-Lewis, anti-I	IgM	Harmless: cannot cross the placenta, and Lewis antigen is weak on fetal cells

Mnemonic: Kell kills, Duffy dies, Lewis lives.

Kell is different. Anti-Kell targets erythroid progenitors, so it suppresses fetal red-cell production as well as destroying circulating cells. The result is severe anemia with relatively little bilirubin, and maternal titers underestimate risk. A lower critical titer (1:8) is used, or surveillance goes straight to MCA Doppler if the fetus may be Kell-positive. Because about 90% of people are Kell-negative, anti-Kell usually follows a prior transfusion.

Anti-D immune globulin. Given to an unsensitized D-negative woman at 28 weeks, within 72 hours of delivering a D-positive infant (if missed, it can still be given up to 28 days after delivery), and after bleeding, procedures, abortion, ectopic pregnancy, or trauma, it prevents sensitization. It cannot reverse sensitization once the mother has made her own anti-D. A low anti-D titer (usually 1:4 or less) within weeks of a dose reflects passively given antibody, not true sensitization.

Managing the sensitized pregnancy

Identify and titerName the antibody and follow titers about monthly, then every 2 weeks after ~24 weeks, in a first sensitized pregnancy.
Critical titerUsually 1:16–1:32 for anti-D (1:8 for Kell): the level at which significant fetal anemia becomes possible.
Fetal antigen statusAntigen-negative father with certain paternity → no risk. Homozygous father → fetus is antigen-positive. Heterozygous father → 50% risk, so genotype the fetus with cell-free fetal DNA in maternal plasma or by amniocentesis.
MCA DopplerAntigen-positive or unknown fetus: middle cerebral artery peak systolic velocity every 1–2 weeks.
Cordocentesis and transfusionMCA-PSV >1.5 multiples of the median → sample fetal blood and transfuse; after ~35 weeks, delivery is usually preferred.

Why MCA Doppler works. An anemic fetus has thinner blood and a higher cardiac output, so middle cerebral artery flow speeds up; this noninvasive test replaced serial amniocentesis for bilirubin (ΔOD450). Intrauterine transfusion uses type O red cells negative for the offending antigen (D-negative for anti-D, c-negative or Kell-negative for those antibodies), irradiated, CMV-negative packed cells, crossmatched to maternal serum and given into the umbilical vein.

After a previously affected pregnancy (hydrops or intrauterine transfusion), titers no longer predict risk: start MCA Doppler at about 16–18 weeks regardless of titer.

Fetal hydrops

Definition. Hydrops fetalis is abnormal fluid in at least two fetal compartments: ascites, pleural effusion, pericardial effusion, or skin edema. Polyhydramnios and a thickened placenta often accompany it.

Immune hydrops. Severe anemia causes high-output heart failure and raises venous pressure. Extramedullary hematopoiesis crowds the fetal liver, impairing albumin synthesis and causing portal hypertension. Together these drive fluid into the fetal body cavities.

Nonimmune hydrops. Since anti-D prophylaxis, most hydrops is nonimmune:

Cardiac: structural heart disease, tachyarrhythmias (SVT), complete heart block.
Chromosomal: Turner syndrome (45,X) with septated cystic hygroma, trisomy 21.
Hematologic: α-thalassemia major (Hb Bart's) in couples who both carry α-thalassemia trait, which is common in Southeast Asia; large fetomaternal hemorrhage.
Infection: parvovirus B19, which enters erythroid precursors through the P antigen and causes red-cell aplasia; also CMV, toxoplasmosis, syphilis.
Other: TTTS, chylothorax and thoracic masses, lymphatic and metabolic disorders.

Workup. Start with the maternal antibody screen to separate immune from nonimmune causes. Then obtain a detailed ultrasound with fetal echocardiography, MCA Doppler for anemia, maternal parvovirus B19 IgM and IgG (plus CMV, toxoplasmosis, and syphilis testing), a Kleihauer-Betke test, and amniocentesis for chromosomal microarray and infection PCR. Parents with microcytosis, normal iron studies, and a normal HbA2 suggest α-thalassemia trait. Treatment targets the cause: intrauterine transfusion for parvovirus anemia (one transfusion often bridges the fetus until its marrow recovers), maternal antiarrhythmics for fetal SVT, shunting of large effusions, and laser therapy for TTTS.

Mirror (Ballantyne) syndrome: the mother develops edema, hypertension, and proteinuria that mirror fetal hydrops of any cause. Unlike preeclampsia, it shows hemodilution with a falling hematocrit. It resolves with delivery or with correction of the fetal hydrops.

Fetomaternal hemorrhage and the newborn

Quantifying the bleed. After trauma, abruption, or delivery in a D-negative woman, a qualitative rosette test screens for D-positive fetal cells. The Kleihauer-Betke test quantifies them: acid elution strips adult hemoglobin, while fetal cells containing HbF resist and stain. Flow cytometry is a more precise alternative. Each 300 μg vial of anti-D immune globulin covers about 30 mL of fetal whole blood, so large hemorrhages need extra vials.

Hemolytic disease in the newborn. In utero the placenta clears bilirubin, so hemolysis declares itself after birth: anemia, rapidly rising unconjugated bilirubin with a risk of kernicterus, and a positive direct Coombs test on the infant's cells. Treatment is intensive phototherapy, with exchange transfusion for severe anemia or hyperbilirubinemia and IVIG in selected cases. Late anemia can appear weeks later from ongoing hemolysis and suppressed erythropoiesis, so hemoglobin is followed.

ABO incompatibility (a type O mother with an A or B infant) can affect a first pregnancy and cause mild neonatal jaundice, but it rarely causes significant fetal anemia or hydrops and is not prevented by anti-D immune globulin.