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Rh alloimmunisation and anti-D

Obstetrics

An Rh-negative mother, an Rh-positive fetus, and antibodies that cross the placenta. Prevention is routine; a sensitised pregnancy is followed by titres and the middle cerebral artery.

Maternal

Blood group and antibody screen at booking and 28 weeks
Blood group and antibody screen at booking and 28 weeks
Anti-D at 28 weeks and within 72 hours of any sensitising event
Anti-D at 28 weeks and within 72 hours of any sensitising event
Kleihauer after a large bleed or trauma
Kleihauer after a large bleed or trauma
Titres if sensitised
the critical titre changes the plan

Fetal

Paternal genotype and cell-free fetal DNA decide whether the fetus is at risk
Paternal genotype and cell-free fetal DNA decide whether the fetus is at risk
Middle cerebral artery peak systolic velocity for anaemia
Middle cerebral artery peak systolic velocity for anaemia
Hydrops on ultrasound is late
Hydrops on ultrasound is late
Intrauterine transfusion and timed delivery by the fetal medicine unit
Intrauterine transfusion and timed delivery by the fetal medicine unit

Prevention

  • Blood group and antibody screen at the first visit and again at 28 weeks.
  • Anti-D immunoglobulin 300 micrograms at 28 weeks for every unsensitised Rh-negative patient, and within 72 hours of birth if the infant is Rh positive (dose guided by a Kleihauer-Betke test of fetomaternal haemorrhage).
  • Also after any sensitising event: miscarriage, ectopic pregnancy, termination, amniocentesis, CVS, external cephalic version, abdominal trauma, antepartum haemorrhage.
  • Cell-free fetal DNA can determine fetal Rh status from maternal blood and spare unnecessary anti-D where available.

The sensitised pregnancy

  • Antibody titres: a critical titre (often 1:16 for anti-D; any level for anti-Kell, which suppresses erythropoiesis) triggers surveillance rather than repeat titres.
  • Middle cerebral artery peak systolic velocity over 1.5 multiples of the median predicts fetal anaemia non-invasively and has replaced amniocentesis for bilirubin.
  • Intrauterine transfusion through the umbilical vein for severe anaemia before 34 to 35 weeks; delivery afterwards.
  • Hydrops (fluid in two or more compartments) is the late sign; a sinusoidal fetal heart rate is a late sign of anaemia.

Fetal hydrops (uploader's description: 19-week fetus with hydrops)

Transabdominal, transverse trunk, 19 weeks. Fluid within the fetal abdomen (ascites).

Open in the Ultrasound Lab
Reduced movements, a smooth undulating line. A sinusoidal pattern: smooth, sine-wave-like undulations at 3 to 5 cycles a minute, amplitude 5 to 15 bpm, persisting for 20 minutes or more, without accelerations or normal variability. Category III by definition.Read it in the lab

On the modelPlacentaUmbilical cord

Frameworks and sources

  • Cunningham et al.: Williams Obstetrics, 26th edition (maternal physiology, labour mechanics, placental anatomy) (2022).

Content reviewed 2026-09. Teaching material for students, not clinical decision support; check the current edition of the guideline your unit uses.