A clenched fist with the index finger overlapping the middle finger and the little finger overlapping the ring finger is the classic hand of trisomy 18 (Edwards syndrome). It is distinctive but not pathognomonic: the fingers are fixed in that position before birth, and fetal akinesia (Pena-Shokeir) sequence and distal arthrogryposis type 1 can produce a similar hand. Abnormal forearm and hand muscles and tendons, plus reduced fetal movement, help explain it. It points to trisomy 18 when it comes with growth restriction, a heart defect, rocker-bottom feet, a prominent occiput and a small jaw.
Trisomy 18 is the second most common autosomal trisomy after Down syndrome, with a live-birth prevalence of roughly 1 in 3,600 to 1 in 10,000, and liveborn babies are more often girls. About 94–97% of cases are full trisomy from nondisjunction, most often in the egg, so the risk rises with maternal age. On the quad screen, AFP, hCG and unconjugated estriol are all low; in Down syndrome, hCG is high.
About half of liveborn babies die in the first week, yet population cohorts find 8–13% alive at 1 year and 8–10% at 10 years, at or above the 5–10% one-year figure many texts quote.
Clenched fist with overlapping fingers: what it means
In trisomy 18 the hand is held in a tight fist with the index (second) finger over the middle (third) finger and the little (fifth) finger over the ring (fourth) finger; Merck describes the index finger covering both the third and fourth fingers. The fingers are often bent in fixed flexion (camptodactyly). The rest of the hand often shows small, hypoplastic nails, underdeveloped thumbs and no distal crease on the fifth finger; syndactyly occurs, and Merck even pictures a trisomy 18 fist with postaxial polydactyly.
When it appears. In one fetus examined by fetoscopy, the fingers were positioned normally at 12 weeks and overlapped by 14 weeks. Abnormal hand posture shows on first-trimester ultrasound in about 6% of affected fetuses, and more than 30% show hand abnormalities later in pregnancy.
Why it happens. Quintero and colleagues attribute it in part to abnormal forearm and hand muscles (absent thenar muscles, anomalous tendons and attachments, fused flexors) that displace the extensor tendons, pulling the second finger toward the ulnar side and the fourth and fifth toward the radial side. Reduced fetal movement probably contributes: mothers often notice the baby moving less, skeletal muscle is markedly hypoplastic, and other joints can be contracted, which is why some affected newborns are first labeled as having arthrogryposis. Any condition that impairs fetal movement for more than 3 weeks can cause congenital contractures.
How specific it is. Cereda and Carey call the overriding fingers "particularly distinctive," but StatPearls stresses that no single feature of trisomy 18 is pathognomonic. On second-trimester scans after abnormal serum screening, soft markers such as clenched hands appeared mostly alongside structural anomalies; isolated ones were mostly not linked to trisomy 18. Healthy newborns grip too, through the palmar grasp reflex, which fades between 4 and 6 months; what suggests trisomy 18 is a fixed, overlapping posture plus other anomalies.
What else causes clenched hands with overlapping fingers
| Condition | Hands | Other clues | Cause |
|---|---|---|---|
| Trisomy 18 | Index over middle, little over ring; small nails | Growth restriction, polyhydramnios, heart defect, prominent occiput, small jaw | Extra chromosome 18 |
| Fetal akinesia deformation sequence (Pena-Shokeir phenotype) | Overriding fingers among multiple contractures | Polyhydramnios, pulmonary hypoplasia; "pseudotrisomy 18" probably belongs here | Reduced fetal movement from any cause: nerve or muscle disease, or crowding (oligohydramnios, multiple gestation, uterine malformation) |
| Amyoplasia (classic arthrogryposis) | Flexed wrists and fingers | Symmetric limb contractures, hypoplastic muscles, usually normal intelligence | Nearly always sporadic |
| Distal arthrogryposis type 1 | Camptodactyly, overlapping fingers, ulnar deviation | Clubfoot; other body systems typically unaffected; often an affected parent | Autosomal dominant (TPM2, MYBPC1); about 1 in 10,000 people |
Contractures plus microcephaly, cleft palate, or heart or urinary tract anomalies suggest a chromosomal or genetic syndrome; karyotype, FISH or microarray settles trisomy 18.
Trisomy 18 feet: rocker-bottom feet and clubfeet
Rocker-bottom foot is the common name for congenital vertical talus: the navicular dislocates onto the top of the talar head, the talus points downward and the sole becomes convex. It is rigid, affects about 1 in 10,000 live births, and about half of cases occur with a neuromuscular disorder or syndrome such as arthrogryposis or myelomeningocele; it is also associated with aneuploidy of chromosomes 13, 15 and 18.
In trisomy 18, clubfeet and rocker-bottom feet are both common, and the big toe is short and often dorsiflexed; about half of babies have a positional foot deformity. Because rocker-bottom feet also occur in trisomy 13, the feet can't separate 18 from 13: the hands (overlapping fingers vs polydactyly) and face (prominent occiput and small jaw vs midline clefts and small eyes) can.
Trisomy 18 vs 13 vs 21
Advancing maternal age raises the risk of all three, and all commonly involve the heart, so neither clue separates them. These features do:
| Feature | Trisomy 18 (Edwards) | Trisomy 13 (Patau) | Trisomy 21 (Down) |
|---|---|---|---|
| Live births | About 1 in 3,600–10,000 | About 1 in 16,000 | About 1 in 700 |
| Karyotype | About 94–97% full trisomy; mosaic under 5%; partial 1–2% | About 80–90% complete trisomy; the rest translocation, mosaic or partial | About 95% nondisjunction; about 4% translocation |
| Signature | Clenched fists with overlapping fingers, prominent occiput, small jaw | Midline defects: holoprosencephaly, cleft lip and palate | Hypotonia, flat facial profile |
| Eyes | Short palpebral fissures; microphthalmia uncommon | Microphthalmia, iris coloboma, retinal dysplasia | Upslanting fissures, epicanthal folds, Brushfield spots |
| Hands | Overlapping fingers, hypoplastic nails | Polydactyly, narrow hyperconvex nails, single palmar crease | Short broad hands, single palmar crease, curved fifth finger |
| Feet | Rocker-bottom feet, clubfeet, short dorsiflexed big toe | Rocker-bottom feet can occur | Sandal gap |
| Heart | Over 90%: VSD, ASD, PDA, polyvalvular disease | Severe defects in 57–80%; dextrocardia common | About 50%: VSD, atrioventricular septal defect |
| Other | Horseshoe kidney, omphalocele, short sternum | Scalp defects, genital anomalies, apneic spells | Duodenal atresia, Hirschsprung disease, hypothyroidism, leukemia, early Alzheimer disease |
| Intellect | Marked to profound disability | Severe disability | Usually mild to moderate |
| Survival | Median 8–14 days; 8–13% at 1 year | Median 5–12.5 days; 8–20% at 1 year | Median life expectancy about 60 years |
Microphthalmia (abnormally small eyes) is a trisomy 13 clue; in trisomy 18 it is among the less frequent (5–25%) malformations. Drill the other two in the Patau syndrome Q-bank breakdown and the Down syndrome Q-bank breakdown.
Trisomy 18 features by system
| System | Findings |
|---|---|
| Growth and feeding | Prenatal growth restriction; mean birth weight 1,700–1,800 g at a mean of 37 weeks; feeding problems, often needing tube feeding |
| Head and face | Microcephaly, prominent occiput, short palpebral fissures, small mouth and jaw (a "pinched" face), low-set malformed ears |
| Chest and abdomen | Short sternum; hernias; omphalocele or esophageal atresia in 5–25% |
| Heart | Defects in over 90%: septal defects, patent ductus arteriosus, polyvalvular disease; complex lesions in about 10% |
| Kidneys | Horseshoe kidney (frequent, 25–75%) |
| Brain | Cerebellar hypoplasia, agenesis of the corpus callosum; central apnea; seizures in about 30% |
| Development | Marked to profound disability; some older children use a walker and understand words |
| Tumors | Wilms tumor (about 1% risk) and hepatoblastoma |
Review the two common lesions in the VSD cheat sheet and the PDA guide.
Chromosomal abnormalities in Edwards syndrome
Full trisomy 18 (about 94–97%). Every cell carries three copies of chromosome 18 because of nondisjunction. The extra chromosome is usually maternal, and about half or more of the maternal errors occur in meiosis II, unlike other human trisomies, where meiosis I errors predominate; the few paternal-origin cases arise after fertilization. Prevalence is flat until a maternal age of about 30, then rises steeply. Recurrence after a child with full trisomy 18 is usually quoted as about 1%.
Mosaic trisomy 18 (under 5%). A trisomic and a normal cell line coexist. The phenotype ranges from full trisomy 18 with early death to apparently healthy adults, and the percentage of trisomic cells does not predict severity. See mosaicism compared.
Partial trisomy 18 (1–2%). All or part of the long arm (18q) is present in three copies, often because a parent carries a balanced translocation or inversion. Triplication of the whole 18q can be as severe as full trisomy, while smaller segments may cause milder signs. Because a parent can carry the rearrangement, recurrence can be higher than after full trisomy.
Prenatal screening: first trimester, quad screen and cell-free DNA
| Test (timing) | Trisomy 18 | Trisomy 21 |
|---|---|---|
| First-trimester combined (10–13 weeks) | NT increased, PAPP-A low, free β-hCG low (as in trisomy 13) | NT increased, PAPP-A low, free β-hCG high |
| Quad screen (15–22 weeks) | AFP, hCG and unconjugated estriol low; inhibin A not used in Mayo Clinic Laboratories' trisomy 18 risk | AFP and estriol low; hCG and inhibin A high |
| Cell-free DNA (from 10 weeks) | Detection 97.9%, false-positive rate 0.04% (2017 meta-analysis) | Detection 99.7%, false-positive rate 0.04% |
Quad screen. Detection estimates vary: Mayo Clinic Laboratories quotes 60–80% at a 1-in-100 risk cutoff; Merck cites about 100% at a 9% false-positive rate. Merck describes all four markers as low, so the exam answer is AFP, hCG and estriol low, with inhibin A low or normal. The quad screen does not assess trisomy 13.
Cell-free DNA. The fetal fragments come from the placenta, and ACOG recommends offering cfDNA screening to all pregnant patients. Positive results are less reliable for trisomy 18 than for Down syndrome: in one large US laboratory series, 76% of high-risk trisomy 18 results were confirmed, vs 84% for trisomy 21. StatPearls quotes 61%; the figure varies with the population tested.
Ultrasound. About 90% of affected fetuses show at least one anomaly: growth restriction and polyhydramnios (each about 28–29% in the second trimester, 87% and 62% in the third), a "strawberry-shaped" skull, choroid plexus cysts (about 50%), overlapping fingers, heart defects, omphalocele and a single umbilical artery (about one-third). In reported series only about 5% of fetuses with choroid plexus cysts had trisomy 18.
Diagnosis. Confirming a positive screen requires chorionic villus sampling (CVS, 10–13 weeks) or amniocentesis (usually 15–20 weeks), tested by FISH for chromosomes 13, 18 and 21 (about 2 days, confirmed by karyotype), karyotype or chromosomal microarray, which better characterizes mosaicism. Trisomy 18 on CVS may be mosaicism confined to the placenta, which amniocentesis or postnatal testing can sort out.
Survival: what current data show
| Cohort | Live births | Early survival | 1 year | Longer term |
|---|---|---|---|---|
| Nine US states (Meyer 2016) | 1,113 (1999–2007) | Median 8 days | 13.4% | 12.3% at 5 years |
| Ontario, Canada (Nelson 2016) | 254 (1991–2012) | Median 9 days | 12.6% | 9.8% at 10 years |
| 13 European registries (Glinianaia 2023) | 602 (1995–2014) | 38% at 4 weeks | 13% | 8% at 10 years |
| 24 registries, 18 countries (Goel 2019) | 1974–2014 | 42% died in week 1 | 88% died in year 1 | — |
Older reviews quote a median survival of 3 to 14.5 days and 5–10% survival to the first birthday. Mosaic and partial cases, included in the US and European cohorts, survive longer: in England and Wales, full trisomy 18 had a median survival of 14 days and 8% 1-year survival, vs 70% for 17 mosaic cases. Babies who survive the first month do much better: 21% of 4-week survivors in the European cohort were expected to reach age 10. Shorter gestation predicts death, and girls survive longer than boys.
Most deaths come from central apnea, heart failure from the cardiac defect, or respiratory failure from hypoventilation, aspiration or upper airway obstruction, often combined.
Management and the cardiac surgery debate
For decades comfort care was standard and trisomy 18 was called "lethal," a label Cereda and Carey consider inaccurate because 1 in 10 to 1 in 20 babies reach their first birthday. A 2025 American Academy of Pediatrics (AAP) clinical report finds no ethical justification for blanket policies that treat these children differently because of the diagnosis alone: interventions should rest on the best evidence, be tailored to the child and be decided with parents through shared decision-making.
Heart surgery is the main flashpoint:
- For: among 1,020 US infants with trisomy 18 (2004–2015), 91% had a heart defect but only 7% had heart surgery, and in-hospital mortality was 64% lower in those who did. In Japan, 502 children with trisomy 18 had heart surgery in 2013–2021, mostly simpler closed-heart operations, with 8.4% in-hospital mortality.
- Cautions: in the US Society of Thoracic Surgeons database (2010–2017), operative mortality after 270 trisomy 18 operations was 15.6%, at least three times the overall rate in every risk category, with complications in 55.6%; preoperative mechanical ventilation was linked to 8.5-fold higher odds of death. These are observational series of highly selected children.
Children who survive need regular abdominal ultrasound for Wilms tumor and hepatoblastoma, plus heart, hearing, vision, feeding and spine follow-up.
The Q-bank vignette
A 37-year-old woman, gravida 2, para 1, delivers a girl at 38 weeks after a pregnancy complicated by fetal growth restriction and polyhydramnios; she had declined prenatal screening. The baby weighs 1,900 g. She has a prominent occiput, a small jaw, low-set malformed ears and a short sternum. Both hands are held in tight fists, with the index finger overlapping the third finger and the fifth finger overlapping the fourth, and the fingernails are small. The soles are convex (rocker-bottom feet). Echocardiography shows a ventricular septal defect.
Which karyotype is most likely?
- A. 45,X
- B. 46,XX,del(5p)
- C. 46,XX with a 22q11.2 deletion
- D. 47,XX,+13
- E. 47,XX,+18
- F. 47,XX,+21
Answer: E, 47,XX,+18 (Edwards syndrome)
The hands decide it: index over third, fifth over fourth, with small nails. The rest fits: growth restriction, polyhydramnios, prominent occiput, small jaw, low-set malformed ears, short sternum, rocker-bottom feet and a VSD in a girl. Confirm with a karyotype; FISH is faster.
Why every other answer choice is wrong
A. 45,X (Turner syndrome). About 30% have a webbed neck, a low posterior hairline, or lymphedema of the hands and feet, and one-third to one-half have a heart defect such as coarctation of the aorta or an aortic valve abnormality. Puffy hands, not clenched overlapping fingers. Review the Turner syndrome Q-bank breakdown.
B. 46,XX,del(5p) (cri-du-chat syndrome). A deletion at the end of the short arm of chromosome 5, not a trisomy. The giveaway is a high-pitched, cat-like cry, with microcephaly, low birth weight, hypotonia, widely set eyes, low-set ears and a small jaw. Compare it in the cri-du-chat comparison table.
C. 22q11.2 deletion (DiGeorge syndrome). Low-set ears and a small jaw occur here too, so the face alone doesn't settle it. Look for conotruncal defects (interrupted aortic arch, truncus arteriosus, tetralogy of Fallot), hypocalcemic tetany within 24–48 hours of birth, thymic hypoplasia with T-cell deficiency, and cleft palate. See the DiGeorge syndrome Q-bank breakdown.
D. 47,XX,+13 (Patau syndrome). The midline trisomy: holoprosencephaly, cleft lip and palate, microphthalmia and iris colobomas, scalp defects and polydactyly. Heart defects and even rocker-bottom feet occur in both 13 and 18, so they won't separate them.
F. 47,XX,+21 (Down syndrome). Hypotonia, flat facial profile, upslanting palpebral fissures, epicanthal folds, Brushfield spots, a single transverse palmar crease and a sandal gap, with VSD or atrioventricular septal defect, duodenal atresia and Hirschsprung disease.
Follow-up vignettes
Vignette 2: the prenatal version. A 39-year-old woman at 17 weeks has a quad screen with low AFP, low hCG and low unconjugated estriol. Ultrasound shows growth restriction, choroid plexus cysts, a VSD and clenched fetal hands with overlapping fingers. What is the most likely diagnosis, and what is the next step?
Answer: Trisomy 18; amniocentesis for FISH and karyotype or microarray.
Reasoning: The quad screen and ultrasound only estimate risk. Down syndrome would raise hCG and inhibin A. Cell-free DNA is another screen, and CVS is done at 10–13 weeks.
Vignette 3: the look-alike. A term newborn of normal weight holds both hands with bent, overlapping fingers angled toward the little finger and has bilateral clubfeet. The face, heart and kidneys are normal, and her father had similar hands at birth. Trisomy 18, fetal akinesia deformation sequence, amyoplasia or distal arthrogryposis type 1?
Answer: Distal arthrogryposis type 1, autosomal dominant (TPM2 or MYBPC1).
Reasoning: Contractures limited to the hands and feet in a well-grown baby with an affected parent fit distal arthrogryposis. Trisomy 18 adds growth restriction, heart defects and a characteristic face; fetal akinesia, widespread contractures with polyhydramnios and pulmonary hypoplasia; amyoplasia is sporadic, with symmetric limb contractures.
USMLE traps
| Trap | What's actually true |
|---|---|
| Overlapping fingers always mean trisomy 18 | Distinctive, not pathognomonic; distal arthrogryposis and fetal akinesia cause them too |
| Rocker-bottom feet mean trisomy 18 | Also trisomy 13, other aneuploidies, arthrogryposis and myelomeningocele |
| Polydactyly or microphthalmia rules out 18 | Both point to 13 but can occur in 18 |
| hCG is low in every trisomy | Quad screen: low in 18, high in 21; first-trimester free β-hCG is low in 18 and 13 |
| High-risk cfDNA is a diagnosis | It is a screen; confirm with CVS or amniocentesis |
| Trisomy 18 is incompatible with life | About 1 in 12 to 1 in 8 reach age 1; for exam prognosis questions, most die within the first year |
More newborn topics are on the Pediatrics hub.
Key takeaways
- Clenched fist, index over middle, little over ring = trisomy 18 until proven otherwise; distal arthrogryposis and fetal akinesia are the look-alikes.
- Add growth restriction, polyhydramnios, a heart defect (over 90%), prominent occiput, small jaw and rocker-bottom feet.
- About 94–97% full trisomy, mostly from maternal nondisjunction; mosaic under 5%; partial 1–2%, sometimes inherited.
- Quad screen: AFP, hCG and estriol low; Down syndrome raises hCG and inhibin A. Confirm positive screens with CVS or amniocentesis.
- About half die in the first week; 8–13% reach 1 year and 8–10% reach 10 years.
Sources
- Trisomy 18 (Edwards Syndrome) — StatPearls, NCBI Bookshelf (2026)
- Cereda A, Carey JC. The trisomy 18 syndrome — Orphanet J Rare Dis (2012)
- Trisomy 18 — Merck Manual Professional Edition (2025)
- Trisomy 18 — MedlinePlus Genetics (2021)
- Alberman E, et al. Cytological and epidemiological findings in trisomies 13, 18, and 21: England and Wales 2004–2009 — Am J Med Genet A (2012)
- Quintero RA, et al. Ontogeny of clenched-hand development in trisomy 18 fetuses — Fetal Diagn Ther (1999)
- Cho RC, et al. Second trimester prenatal ultrasound for the detection of pregnancies at increased risk of Trisomy 18 based on serum screening — Prenat Diagn (2009)
- Arthrogryposis Multiplex Congenita — Merck Manual Professional Edition (2025)
- Kalampokas E, et al. Diagnosing Arthrogryposis Multiplex Congenita: A Review — ISRN Obstet Gynecol (2012)
- Distal arthrogryposis type 1 — MedlinePlus Genetics (2017)
- Primitive Reflexes — StatPearls, NCBI Bookshelf (2026)
- Day J, et al. Congenital Vertical Talus: An Updated Review — Cureus (2023)
- Trisomy 13 — Merck Manual Professional Edition (2025)
- Trisomy 13 — MedlinePlus Genetics (2021)
- Down Syndrome (Trisomy 21) — Merck Manual Professional Edition (2025)
- Shiefa S, et al. First Trimester Maternal Serum Screening Using Biochemical Markers PAPP-A and Free β-hCG — Indian J Clin Biochem (2013)
- Quad Screen (Second Trimester), Maternal, Serum (QUAD1) — Mayo Clinic Laboratories, Duke Health test catalog (2026)
- Noninvasive Prenatal Fetal Screening Tests — Merck Manual Professional Edition (2024)
- Gil MM, et al. Analysis of cell-free DNA in maternal blood in screening for aneuploidies: updated meta-analysis — Ultrasound Obstet Gynecol (2017)
- Petersen AK, et al. Positive predictive value estimates for cell-free noninvasive prenatal screening — Am J Obstet Gynecol (2017)
- Prenatal Genetic Screening Tests — ACOG (2020, reviewed 2025)
- Prenatal Genetic Diagnostic Tests — ACOG (2022, reviewed 2026)
- Meyer RE, et al. Survival of children with trisomy 13 and trisomy 18: a multi-state population-based study — Am J Med Genet A (2016)
- Nelson KE, et al. Survival and Surgical Interventions for Children With Trisomy 13 and 18 — JAMA (2016)
- Goel N, et al. Trisomy 13 and 18: prevalence and mortality — Am J Med Genet A (2019)
- Glinianaia SV, et al. Ten-year survival of children with trisomy 13 or trisomy 18 — Arch Dis Child (2023)
- Wu J, et al. Survival of trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) in England and Wales: 2004–2011 — Am J Med Genet A (2013)
- Pyle AK, et al. Guidance for Caring for Infants and Children With Trisomy 13 and Trisomy 18: Clinical Report — Pediatrics (2025)
- Kosiv KA, et al. Congenital Heart Surgery on In-Hospital Mortality in Trisomy 13 and 18 — Pediatrics (2017)
- Cooper DS, et al. Cardiac Surgery in Patients With Trisomy 13 and 18 — J Am Heart Assoc (2019)
- Nemoto S, et al. Current Nationwide Landscape of Cardiac Surgery for Children With Trisomy 18 in Japan — Am J Med Genet A (2026)
- Turner syndrome — MedlinePlus Genetics (2023)
- Cri-du-chat syndrome — MedlinePlus Genetics (2022)
- DiGeorge Syndrome — Merck Manual Professional Edition (2026)
- 22q11.2 deletion syndrome — MedlinePlus Genetics (2019)
Frequently Asked Questions
What does a clenched fist with overlapping fingers mean?
A clenched fist with the index finger over the middle finger and the little finger over the ring finger is the classic hand of trisomy 18 (Edwards syndrome), seen in newborns and on prenatal scans. The posture is fixed before birth, partly because of abnormal forearm and hand muscles and tendons. It is distinctive but not diagnostic: look for growth restriction, a heart defect and rocker-bottom feet, and confirm with a karyotype or FISH.
What else causes clenched hands with overlapping fingers?
Anything that limits fetal movement can cause congenital contractures, including overriding fingers. Fetal akinesia deformation sequence (Pena-Shokeir phenotype) adds polyhydramnios and pulmonary hypoplasia, and arthrogryposis can look similar. Distal arthrogryposis type 1, an autosomal dominant condition that can be caused by TPM2 or MYBPC1 variants, gives bent, overlapping fingers, ulnar deviation and clubfoot while other organs are typically normal. A normal karyotype and an affected parent favor it over trisomy 18.
How do you tell trisomy 18 from trisomy 13 and trisomy 21?
Look at the hands, the face and the screening pattern. Trisomy 18 gives clenched fists with overlapping fingers, a prominent occiput, a small jaw and rocker-bottom feet. Trisomy 13 is the midline trisomy: holoprosencephaly, cleft lip and palate, microphthalmia, scalp defects and polydactyly. Trisomy 21 gives hypotonia, a flat facial profile, upslanting eyes and a single palmar crease. On the quad screen, hCG is low in trisomy 18 but high in Down syndrome.
What are rocker-bottom feet, and do they mean trisomy 18 or 13?
A rocker-bottom foot is congenital vertical talus: the talus points downward and the navicular dislocates onto the top of the talar head, so the sole becomes convex. It is common in trisomy 18, along with clubfeet and a short, dorsiflexed big toe, but it also occurs in trisomy 13, other aneuploidies, arthrogryposis and myelomeningocele. The feet alone cannot separate 18 from 13; overlapping fingers point to 18, while polydactyly and midline clefts point to 13.
What causes Edwards syndrome, and what is partial trisomy 18?
About 94 to 97% of cases are full trisomy 18 from nondisjunction, usually in the mother's egg (about half or more in meiosis II), so risk rises with maternal age. Under 5% are mosaic, and the share of trisomic cells does not predict severity. About 1 to 2% are partial trisomy 18: all or part of the long arm is present in three copies, often from a parent's balanced translocation or inversion, which can raise recurrence risk.
What is the life expectancy of a baby with trisomy 18?
About half of liveborn babies die in the first week, and median survival in large population studies is 8 to 14 days. Roughly 8 to 13% reach their first birthday, at or above the 5 to 10% older texts quote, and two cohorts found 8 to 10% alive at age 10. Most deaths result from central apnea, heart failure or respiratory failure. Care is individualized, and some children have heart surgery after shared decision-making with their families.