ABC of preterm birth: Epidemiology of preterm birth

Definitions

Preterm birth is the delivery of a baby before 37 completed weeks' gestation. Most mortality and morbidity affects “very preterm” infants (those born before 32 weeks' gestation), and especially “extremely preterm” infants (those born before 28 weeks of gestation). In the past 20-30 years advances in perinatal care have improved outcomes for infants born after short gestations. The number of weeks of completed gestation that defines whether a birth is preterm rather than a fetal loss has become smaller. In 1992 the boundary that required registration as a preterm live birth in the United Kingdom was lowered from 28 completed weeks' gestation to 24 weeks' gestation. This boundary varies internationally, however, from about 20 to 24 weeks. Some classification of fetal loss, stillbirth, and early neonatal death for these very short gestations may be unreliable.

Gestational age versus birth weight

Even in developed countries, there is often uncertainty and incomplete recording of estimates of gestation. In most of the United Kingdom data on birth weight but not on gestational age are collected routinely. Although some concordance exists between the categories of birth weight and gestational age, they are not interchangeable. The categories for birth weight are:

• Low birth weight (<2500 g)

• Very low birth weight (<1500 g)

• Extremely low birth weight (<1000 g).

Only about two thirds of low birthweight infants are …