Periviable births: epidemiology and obstetrical antecedents.

Confusion in terminology and non-standardized definitions can arguably be classified as the most important characteristics that lead to disparate study findings. While such situations abound in all of perinatal epidemiology, the study of "periviable birth" is a classic case in point. The most common term used to describe periviable birth has been extremely low birthweight (ELBW), often defined as newborns with birthweight below 1000g; yet, we identified 11 other descriptors for periviable birth. Due to the multitude of definitions, the reported incidence of periviable births varies from 0.03% to 1.9% (a relative difference of 7025%). Antecedent risk factors for periviable birth, though geographically heterogeneous, include nulliparity and multiple gestations, each accounting for one-third and one-fourth of all periviable births, respectively. Spontaneous preterm labor precedes 34% of these deliveries and premature rupture of membranes in 25%. The pregnancy was complicated by hypertensive disease in 21% and bleeding and chorioamnionitis in 18% each. Over 50% of these births are cesarean deliveries. Six clinical interventions before delivery that may improve outcomes include transfer of patient to a tertiary center, administration of antenatal corticosteroids, tocolytics, or antibiotics after premature rupture of membranes, assessment of fetal well-being with electronic fetal heart rate monitoring, and willingness to perform a cesarean delivery after the limit of viability is reached. While there is an accessible predictive model for mortality and long-term morbidities for newborns delivered at 22-25 weeks with weights of 400-1000g, it is hampered by the need to know the birthweight. In addition to reaching a consensus regarding what birthweight-gestational age thresholds determine a periviable birth, the acute need to optimize neonatal outcomes remains a paramount target worthy of future research, and efforts to advance obstetrical medicine are needed to minimize the likelihood of its occurrence.