Limits of viability: definition of the gray zone.
As survival and long-term morbidity of very preterm infants have improved over the past decade, the limits of infant viability, the level of maturity below which survival and/or acceptable neurodevelopmental outcome are extremely unlikely, have also decreased. In an effort to define the current limits of infant viability, the data in the literature on survival and long-term neurodevelopmental outcome in very preterm neonates have been reviewed. The gestational age and birth weight below which infants are too immature to survive, and thus provision of intensive care is unreasonable, appears to be at or =25 weeks' gestation and with a birth weight of > or =600 g are mature enough to warrant initiation of intensive care, as the majority of these patients survive, and at least 50% do so without severe long-term disabilities. Finally, for infants born between 23(0/7) and 24(6/7) weeks' gestation and with a birth weight of 500 to 599 g, survival and outcome are extremely uncertain. For these infants born in the so-called 'gray zone' of infant viability, the line between patient autonomy and medical futility is blurred, and medical decision-making becomes even more complex and needs to embrace careful consideration of several factors. These factors include appraisal of prenatal data and the information obtained during consultations with the parents before delivery; evaluation of the patient's gestational age, birth weight and clinical condition upon delivery; ongoing reassessment of the patient's response to resuscitation and intensive care and continued involvement of the parents in the decision-making process after delivery. Based on these findings an algorithm is offered for consideration for neonatologists managing infants born in the 'gray zone' of infant viability. However, caution must be exercised when one considers incorporating this guideline into clinical practice because the algorithm is based on the analysis of the findings in the literature and the authors' experience rather than direct evidence.