Trisomy 18 and 13 Prenatal Features
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Direct answer
Trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) are severe autosomal trisomies whose prenatal features include growth restriction, multiple malformations and characteristic ultrasound signs. Trisomy 18 classically shows clenched hands with overlapping fingers and rocker-bottom feet, while trisomy 13 shows holoprosencephaly, midline facial defects and polydactyly. Both carry a very poor prognosis, and trisomy 18 gives a classic low-level pattern on maternal serum screening.
What you must remember
- Edwards syndrome: trisomy 18 with clenched fists, rocker-bottom feet, growth restriction, choroid plexus cysts and congenital heart defects, often a ventricular septal defect.
- Patau syndrome: trisomy 13 with holoprosencephaly, cleft lip and palate, polydactyly, scalp defects and cardiac or abdominal wall anomalies such as omphalocele.
- Screening pattern: trisomy 18 gives low AFP, unconjugated oestriol and hCG on triple or quadruple testing, which points away from a neural tube defect. Trisomy 13 gives low PAPP-A and free beta-hCG in the first trimester, and AFP is variable.
- Confirmation: ultrasound and screening give probability only, so karyotype from chorionic villus sampling or amniocentesis is the confirmatory test.
- Prognosis: most affected pregnancies end in miscarriage or stillbirth, and most liveborn infants die in the first year, with a small minority surviving longer.
- Counselling: plan palliative and neonatal care, and note that termination for a substantial fetal anomaly is governed by the MTP Act, with medical board approval beyond 24 weeks.
Recognising the pattern on scan and screen
Both trisomies show multiple anomalies and severe growth problems on the scan. Recognising the pattern lets the team counsel accurately before any decision is taken.
| Feature | Trisomy 18 (Edwards) | Trisomy 13 (Patau) |
|---|---|---|
| Hands and feet | Clenched fists, overlapping fingers, rocker-bottom feet | Polydactyly is the typical limb finding |
| Brain | Choroid plexus cysts, small head | Holoprosencephaly |
| Face and lip | Micrognathia, low-set ears | Cleft lip and palate, microphthalmia |
| Heart and abdomen | Ventricular septal defect, omphalocele | Cardiac defects, omphalocele |
| Serum screen | Low AFP, uE3 and hCG | Low PAPP-A and free beta-hCG; AFP variable |
| Origin | Mostly maternal meiotic nondisjunction | Mostly maternal meiotic nondisjunction |
| Outcome | Most liveborn infants die in the first year | Very early death is common |
The pattern of malformations, not any single feature, points to the trisomy, and a clenched hand and a low screen are read differently from a cleft palate alone. Advancing maternal age raises the chance of both, which is why it enters the counselling conversation alongside the scan.
Where the exam tries to catch you
Vignettes describe a fetus with several malformations and a low screening profile, then test the most common confusions.
- Trap: using cleft lip as the hallmark of trisomy 18. Correct: cleft lip and palate are more typical of trisomy 13, whereas trisomy 18 shows clenched hands and rocker-bottom feet.
- Trap: reading a low AFP as a neural tube defect. Correct: neural tube defects raise AFP, and a low AFP with low uE3 and hCG suggests trisomy 18.
- Trap: counselling without a confirmed karyotype. Correct: chorionic villus sampling or amniocentesis confirms the diagnosis before counselling is finalised.
- Trap: presenting these conditions as routine survival cases. Correct: most affected infants die early, so palliative counselling is central.
Frequently asked questions
Which hand finding is classic for trisomy 18?
Clenched fists with overlapping fingers, with the index finger over the third and the fifth over the fourth, are classic for trisomy 18 (Edwards syndrome). Rocker-bottom feet often accompany this hand finding on the same scan.
Which trisomy is most associated with holoprosencephaly?
Trisomy 13, Patau syndrome, is the trisomy most associated with holoprosencephaly. Midline facial defects, cleft lip and palate and polydactyly that accompany it together suggest Patau syndrome on the scan.
What maternal serum pattern suggests trisomy 18?
Low maternal serum AFP, unconjugated oestriol and hCG on triple or quadruple screening suggest trisomy 18. This pattern differs from the raised AFP of neural tube defects and the raised hCG of Down syndrome. Trisomy 13 shows low PAPP-A and free beta-hCG in the first trimester.
Why must trisomy 18 or 13 be confirmed by karyotype?
Ultrasound and maternal serum screening only estimate probability, so a karyotype is needed to confirm trisomy 18 or 13. Chorionic villus sampling or amniocentesis provides the tissue for that test before counselling is finalised.