A birth is considered preterm when the baby arrives before 37 weeks of pregnancy. The final weeks in the womb are dedicated to the maturation of organs, especially the brain and lungs. The ability of a fetus to survive outside the uterus is called viability. Viability is not a single moment in time; it is a dynamic range that shifts with advances in medical care and technology. The concept of viability is heavily dependent on the resources available at the time of delivery, as a baby’s chances are directly linked to the immediate availability of highly specialized care.
Defining the Viability Threshold
The medical community defines the threshold of viability as the gestational age at which a baby has a reasonable chance of survival outside the womb. This range currently falls between 22 and 24 weeks of gestation in high-income countries with advanced medical capabilities. A baby born at 24 weeks is often the point at which hospitals mandate aggressive life-saving interventions, given the significantly improved odds of survival at that age.
While survival at 22 weeks is still extremely rare, modern neonatal intensive care has made it possible for a small number of infants to survive this early birth. The 24-week mark is frequently cited as the point where the probability of survival begins to exceed 50 percent, serving as a common benchmark for viability. This period between 22 and 26 weeks is sometimes referred to as the periviable period.
Gestational Age and Survival Statistics
A baby’s likelihood of survival dramatically increases with each day and week spent developing in the womb. At 23 weeks gestation, the chance of survival with active treatment is approximately 23 to 55 percent, according to various large-scale studies. The wide range reflects variations in medical protocols, the baby’s individual health, and the quality of the hospital where the birth occurs.
By 24 weeks, the survival rate climbs to a range between 42 and 70 percent, marking a significant milestone in fetal development. This increase is largely due to the continued maturation of the lungs and circulatory system. Moving forward to 25 weeks, survival rates are commonly reported between 67 and 80 percent.
The outcome is much more favorable for babies who reach 28 weeks of gestation, where survival rates soar to 80 to 90 percent. At this stage, the baby is still considered very preterm, but their organs are substantially more developed. Beyond simple survival, there is also the distinction between mortality (death) and morbidity (long-term health problems) to consider.
While survival rates improve steadily, the risk of long-term impairment, such as developmental or neurological issues, decreases significantly with each week of delayed birth. About 40 percent of babies born before 24 weeks may experience long-term health complications, compared to about 10 percent of those born at 28 weeks. This shift illustrates that the goal of care is not just to keep the baby alive, but to ensure survival without severe disability.
Immediate Medical Support and Key Determinants of Outcome
The survival of an extremely premature baby is heavily dependent on immediate access to a high-level Neonatal Intensive Care Unit (NICU). These specialized units provide the artificial environment necessary to replicate the conditions of the womb. Key technologies include mechanical ventilators to support underdeveloped lungs and incubators to maintain a stable body temperature, which premature infants struggle to regulate.
A primary barrier to survival is the maturity of the baby’s lungs, which often lack sufficient surfactant, a substance that prevents the tiny air sacs from collapsing. To address this, a common and effective intervention is the administration of antenatal steroids, such as betamethasone or dexamethasone, to the mother before delivery. These corticosteroids cross the placenta and accelerate the production of surfactant in the fetal lungs, significantly reducing the risk of respiratory distress syndrome and brain bleeds.
The effectiveness of this steroid treatment is greatest when administered between 24 hours and one week before birth. Beyond gestational age, a baby’s birth weight is a separate, significant factor in prognosis. Even within the same week of gestation, a higher birth weight tends to correlate with a better chance of survival, as it suggests a stronger overall development. The presence of other factors, such as infections or congenital anomalies, can also significantly impact the overall outcome.