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Rh incompatibility and alloimmunisation in pregnancy
An Rh-negative woman exposed to Rh-positive fetal blood makes antibodies that attack the red cells of the fetus in her next pregnancy; anti-D at the right time prevents the sensitisation, and once antibodies have formed anti-D is no longer given and the fetus is followed for anaemia.
Why it matters in the exam
The questions fall into two groups. The first is prevention: when anti-D is given to an Rh-negative woman with a negative antibody screen — at 28 weeks, within 72 hours of delivering an Rh-positive baby, and after any event in which fetal blood may reach the mother: bleeding, miscarriage, ectopic pregnancy, amniocentesis, external cephalic version and abdominal trauma. The favourite question is what to do when the bleed is large: a Kleihauer-Betke test is done and the dose adjusted.
The second group is a woman who already has antibodies. Here anti-D will not help. The antibody titre is followed, and above the critical threshold the fetal middle cerebral artery (MCA) Doppler is checked — a high peak velocity indicates anaemia. Severe anaemia is treated with intrauterine transfusion through the umbilical cord, and untreated it progresses to fetal hydrops. Kell antibodies are a special case: they suppress red cell production, so the titre predicts the severity of anaemia less well.
Key points
- Anti-D at 28 weeks, within 72 hours of delivery, and after every sensitising event.
- Kleihauer-Betke adjusts the dose after a large bleed.
- Once antibodies exist — titre and MCA Doppler surveillance.
- MCA Doppler above 1.5 MoM = fetal anaemia.
- Kell: suppressed red cell production, titre less reliable.
Summary table
| Situation | Finding | What to do | Exam notes |
|---|---|---|---|
| Rh-negative without antibodies | Negative antibody screen at the first visit | Anti-D 300 µg at 28 weeks | May be omitted if the partner or the fetus (cell-free DNA) is Rh-negative |
| After delivery | Rh-positive baby | Anti-D within 72 hours; Kleihauer-Betke to calculate the dose | 300 µg covers 30 mL of fetal blood |
| Sensitising event in pregnancy | Bleeding, miscarriage, ectopic, amniocentesis, external version, trauma | Anti-D within 72 hours | Before 12 weeks recommendations vary between guidelines |
| Woman with antibodies | Titre below the threshold | Repeat the titre about every 4 weeks | Common critical titre: 1:16 |
| Titre at or above the critical level | MCA Doppler above 1.5 MoM | Fetal blood sampling and intrauterine transfusion | Untreated anaemia → hydrops |
| Kell antibodies | Suppressed red cell production | MCA Doppler surveillance even at a low titre | The titre is less reliable |
A free exam question
בת 25 בשבוע 30 להריון פונה לביקורת היריון שגרתית. בבדיקת אולטרה - סאונד מודגם MVP (maximum vertical pocket) של 11 ס"מ. הערכת משקל עובר באחוזון 40 ? לגיל הריון. מה יהיה המשך ניהול המקרה (the question as examined, in Hebrew)
- א. בדיקות תנגודת לזרימה ב MCA- פעם בשבוע
- ב. סקירת מערכות מכוונת לסיבות לריבוי מי שפיר
- ג. הפחתת כמות מי שפיר באמצעות דיקורים חוזרים
- ד. טיפול ב Indomethacin
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Sources
- ACOG Practice Bulletin No. 181, 2017 — Prevention of Rh D Alloimmunization
- ACOG Practice Bulletin No. 192, 2018 — Management of Alloimmunization During Pregnancy
- SMFM Clinical Guideline No. 8, 2015 — The fetus at risk for anemia: diagnosis and management
- Williams Obstetrics, 26th edition (2022) — Fetal Disorders
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.