Pitfalls of prenatal diagnosis associated with mosaicism

Key content Fetal placental mosaicism, of which confined mosaicism is a subtype, occurs in 2–3% pregnancies. Confined may lead to false positive result on non-invasive prenatal testing (NIPT) for common aneuploidies. The risk chorionic villus sample (CVS) following NIPT 2, 4, 22 and 59% trisomy 21, 18, 13 45, X respectively. Following the absence significant fetal structural anomaly (FSA), care required selecting optimal diagnostic invasive test. Discussion limitations implications essential referral clinical genetics be warranted. Learning objectives To understand embryological causes types mosaicism. appreciate underlying principles genomic strategies relation follow suggested management test counselling. Ethical issues Clinicians face dilemma high-risk setting normal ultrasound. Awaiting long-term culture, as opposed short-term culture CVS, or amniocentesis delays potential termination pregnancy. Sex chromosome abnormalities without an identifiable FSA cannot interpreted reliably. Hence, should not offered sex aneuploidy.