The limit of viability is defined as the stage of fetal maturity that ensures a reasonable chance of extrauterine survival. Determining the limit of viability is desirable so that interventions that are costly and painful can be avoided in the infant who does not have a chance of survival . However, deciding upon a threshold of viability is challenging because it remains uncertain which extremely preterm infants have a reasonable chance of survival [2-4].
Survival and morbidity of extremely premature infants born below 26 weeks gestation will be reviewed here. In addition, a management approach for infants born at the limit of viability based upon prognosis will be presented.
Factors that affect survival rates in extremely premature infants (gestational age <26 weeks) include gestational age, birth weight, gender, plurality, and the use of antenatal corticosteroid therapy [3,5,6].
Gestational age — The major factor in determining viability is gestational age. Survival rates reported in several studies [2,3,7-16] during the initial neonatal intensive care unit (NICU) admission for infants 22 to 25 completed weeks gestation (220/7 to 256/7 weeks gestation) are displayed in the table (table 1). These reports span time periods ranging from six months to six years between 1994 and 2010, and represent several large geographic areas in Western Europe, Japan, and a network of tertiary level academic centers in the United States. These data clearly demonstrated the rise of survival rate as gestational age increased from 22 to 25 weeks. In addition, they also showed improved survival, as survival rates increased significantly for infants born at 23 to 25 weeks gestation. However, it remains uncertain whether the survival rates for infants born at 22 weeks gestation have improved over time.
These data emphasize the importance of basing discussions about delivery room management for infants at borderline viability upon the latest available data. In addition, although these results highlight the impact of gestational age on viability, it is important to recognize that ascertaining an accurate gestational age is challenging. Thus, reliance on estimated gestational age may not truly reflect the prognosis. (See 'Interpreting the data' below and "Postnatal assessment of gestational age" and "Prenatal assessment of gestational age and estimated date of delivery".)