How small is too small? Considerations in evaluating the outcome of the tiny infant.

An aggressive approach to the obstetric and neonatal care of the pregnant mother and her infant in the late second trimester has led to an increase in survival. Surviving infants have a high rate of neonatal morbidity, including bronchopulmonary dysplasia and intraventricular hemorrhage. No long-term follow-up reveals handicap rates of 22 to 35 per cent, with multiple sensory and neurodevelopmental handicaps. With current methods of care, the biologic threshold of extrauterine survival is 23 to 24 weeks' gestation, or a birth weight of 500 to 600 gm. There is no simple arithmetical answer to the question of "how small is too small?" It should be apparent from the information presented in this article that the state of the art is constantly in flux. New developments have, to date, continued to decrease the threshold of viability. Application of the full weight of the available technology results in only a few infants of less than 23 to 24 weeks' gestation or with birth weights of less than 500 to 600 gm surviving. The future continues to lie in the development of the methodology to prolong the sojourn in utero, rather than in dramatic biophysical, biochemical, and technologic advances.