Fetal Viability

Fetal viability is the state at which a fetus is considered capable of surviving outside the womb if born before term. The critical factor is the development of the lungs, specifically the ability of lung tissue to produce a substance called surfactant, which prevents the air sacs of the lungs from collapsing.

Viability is not a fixed threshold that cannot change. It depends on gestational age, birth weight, sex, maternal health, and—most importantly—the level of medical care available. Hospitals with neonatal intensive care (NICU) and ventilation equipment can support extremely premature infants, whereas facilities lacking these resources have much poorer outcomes.

Gestational age and survival prospects

In general, survival prospects are very low before 22 weeks gestation and rise steadily from 23 to 26 weeks. By 28 weeks and beyond, most infants survive when appropriate care is provided. The lower threshold commonly used in medical practice falls around 22 to 24 weeks, but standards vary from country to country and hospital to hospital.

Medical care advances

Advances that have improved survival of extremely premature infants include: maternal steroid treatment before delivery to accelerate fetal lung maturation, administration of synthetic surfactant, assisted ventilation (CPAP or mechanical ventilation), thermal care, and nutritional support via umbilical catheters. These measures have lowered the threshold of viability from the mid-20th century to the present.

Terminology and distinctions

Legal and ethical implications

Many countries use viability as a legal threshold for defining the time limit for abortion. Because viability depends on medical technology, this legal standard has prompted ongoing debate: is the threshold based on a medical criterion that changes with technology, or a legal one that is fixed? Medical practice also faces ethical decisions about when to initiate or withdraw life-sustaining care when survival prospects are very low, with parents participating in these decisions.

Long-term outcomes

Survival alone does not mean full health. Infants born extremely prematurely can suffer complications including chronic lung disease, brain injury, vision loss (retinopathy of prematurity), and developmental and learning disabilities. Current measures therefore combine survival rate with measures of quality of life afterward.