Fetal Complications and Fetal Therapy
Core concepts
This domain tests understanding of fetal pathophysiology, surveillance, and the indications and limits of intervention. Know fetal growth restriction definitions (estimated fetal weight below the 10th centile) and the distinction between constitutionally small and pathologically growth-restricted fetuses using umbilical artery Doppler, cerebroplacental ratio, and growth velocity. Understand red cell alloimmunization: the role of middle cerebral artery peak systolic velocity for detecting fetal anemia and the indications for intrauterine transfusion. Know twin-twin transfusion syndrome staging and laser therapy, fetal hydrops as a final common pathway, and the principles of fetoscopic and open fetal surgery (myelomeningocele repair, congenital diaphragmatic hernia, lower urinary tract obstruction).
Fetal surveillance modalities - biophysical profile, modified BPP, contraction stress test, and umbilical artery Doppler - each have specific indications and false-positive profiles. Understand fetal arrhythmia evaluation and the limited role of transplacental antiarrhythmics.
Traps
- Using MCA-PSV without accounting for gestational-age-specific thresholds.
- Treating oligohydramnios as equivalent to growth restriction without Doppler confirmation.
- Missing that TTTS is a monochorionic-diamniotic complication and does not occur in dichorionic twins.
- Assuming all hydrops is immune; most is nonimmune.
- Overlooking that open fetal surgery carries preterm birth risk and maternal morbidity.
Rule of thumb
Before any fetal intervention, confirm the diagnosis, exclude lethal or untreatable anomalies, verify a center with appropriate expertise, and document maternal counseling. Fetal therapy is a maternal-fetal dyad decision.
Worked example: Monochorionic-diamniotic twins at 18 weeks with a recipient polyhydramnios and donor oligohydramnios (Quintero stage I–II). Refer for evaluation for selective fetoscopic laser photocoagulation, which addresses the placental anastomoses rather than just draining fluid.