Regional neonatal intensive care: bias and benefit.

Among very low birthweight infants born to residents of Merseyside in 1979-81 those booked at or transferred in utero to the regional unit were more likely to survive than those born elsewhere in the region. A study was carried out to determine whether the increased survival rate was attributable to better care or differences in the babies treated at the unit, or both. Four subpopulations were defined by district of maternal residence to obtain fairly comparable samples. Survival to 2 years increased as proportions treated at the regional unit increased, this "dose-response" effect being most pronounced among the smallest infants. Despite less favourable social state infants from the district housing the regional unit were apparently in the best condition at birth; had been managed most actively before, at, and after birth; and were the most likely to survive. This increased survival, however, was associated with a marginally significant increase in prevalence of major impairments. These data underline the need for routine surveillance of large representative samples of low birthweight survivors.