“…During the past 5 years, large cohort studies from developed nations in North America, 11–13,20,21 South America, 22 Europe, 14,15,23–25 Asia, 18,19,26 and Australia 27 have reported estimates of gestational age-specific survival following periviable birth. Direct comparisons of estimated survival rates among these studies are limited, however, by potential biases introduced from differences in the data sources, ascertainment of death, selection of denominators, and definitions of live birth.…”