Survival Rates and Life Expectancy for Trisomy 18 at 31 Weeks Gestation
For a fetus with confirmed trisomy 18 at 31 weeks gestation, approximately 59% will survive to term, and if born alive, the median survival is 14 days, with 20% surviving to 3 months and 8% surviving to 1 year.
Fetal Loss Risk from 31 Weeks to Term
At 31 weeks gestation, the fetus has already passed the highest risk period for intrauterine demise. Between 24 weeks and term, approximately 59% of trisomy 18 fetuses will experience fetal loss (miscarriage or stillbirth) 1. More recent data from 2017 confirms that at 39 weeks gestation, 67% of viable fetuses at that point will result in a live birth 2.
The mean gestational age at fetal demise is approximately 32-35 weeks 3, 4, meaning your patient is currently in a critical window. However, losses are uniformly distributed throughout gestation after 24 weeks with no specific clustering pattern 3.
Survival After Live Birth
If the infant is born alive, the prognosis remains guarded but not uniformly lethal as historically believed 5:
Short-term Survival (Full Trisomy 18):
Long-term Survival:
- 1-year survival: 10-25% (with variation based on interventions) 5
- 8% survive to 1 year in natural history studies 6
- 5-year survival: 12.3% 5
Critical Factors Affecting Prognosis
The maternal age of 40 increases the likelihood of full trisomy (rather than mosaic forms), which carries the poorest prognosis 5. Approximately 95% of live-born infants with trisomy 18 have full trisomy 5.
Important Distinctions:
- Full trisomy 18: Median survival 14 days, 8% one-year survival
- Mosaic trisomy 18: 70% one-year survival 6
- Partial trisomy 18: More variable, generally better outcomes 5
The presence and severity of congenital heart disease (present in >90% of cases) significantly impacts survival 5, 7. Female fetuses may have slightly better survival than males 4.
Contemporary Context and Intervention Impact
The 2025 AAP guidelines emphasize that historical mortality data reflects a "self-fulfilling prophecy" where lack of intervention was justified by high mortality, but intervention can alter outcomes 5. With aggressive interventions including:
- Cardiac surgery
- Mechanical ventilation
- Tracheostomy placement
- Gastrostomy tube feeding
Some infants survive months to years, with rare cases of survival beyond 5 years 5, 8, 9. However, all survivors have significant lifelong neurocognitive problems 5.
Counseling Framework
The diagnosis alone should not categorically preclude offering life-sustaining therapies, but exclusive comfort care remains a morally acceptable choice 5. The decision should be based on:
- Individual fetal factors: Specific cardiac defects, presence of other anomalies, gestational age at delivery
- Family values: Understanding that some families find meaning in any survival time, while others prioritize quality over quantity
- Realistic expectations: Even with maximal intervention, most infants will not survive infancy, and survivors will have profound developmental delays
The key message: This is not uniformly lethal, but survival beyond infancy remains uncommon even with aggressive intervention, and all survivors have severe neurodevelopmental impairment 5.