In pregnancy, term refers to the gestational period from 37+0 to 41+6 weeks. Preterm births occur between 24+0 and 36+6 weeks. Although births earlier than this are referred to as miscarriages, occasional survivors are seen after delivery at 23 weeks, which has become the “gray zone” for viability.
Early births occur either because delivery is believed to be in the best interests of the mother or baby (indicated deliveries) or because the mother develops spontaneous contractions or membrane rupture earlier than normal (spontaneous deliveries). The latter group has two subdivisions: spontaneous preterm labor (PTL) and preterm prelabor rupture of membranes (PPROM). Indicated deliveries, PTL, and PPROM each account for approximately one-third of early births.
From 2005 to 2009, 647,088 women delivered between 24 and 42 weeks of gestation in Ontario (BORN Ontario, Niday Perinatal Database). A total of 7.5 percent of these births were preterm, occurring before 37 weeks, but the percentage of very early births is much smaller (Fig. 30-1). Although almost identical to 2005 data from the United Kingdom, significantly higher rates of preterm birth of up to 12 percent are reported from the United States. Conversely, many Nordic countries with very reliable data collection quote rates around 5 percent. This must reflect, at least in part, differing socioeconomic and cultural factors. There is no evidence that the incidence of preterm birth is declining. In fact, the rate appears to be slowly increasing, partly because of an increasing incidence of multiple pregnancy. The 2008 Canadian Perinatal Health Report by the Public Health Agency of Canada reports that the preterm birth rate has increased from 6.4 percent in 1985 to 8.2 percent in 2004. Although just under 1 percent of singletons delivered before 32 weeks, this complicated 8.3 percent of twins and nearly 21 percent of higher multiples.
Ontario preterm live births, 2005–2009. (Data supplied by: BORN Ontario, Niday Perinatal Database.)
Preterm births contribute significantly to perinatal mortality, half of which results from babies born before 32 weeks. The survival to discharge for very preterm infants admitted to Canadian neonatal intensive care units (NICUs) in 2010 is shown in Figure 30-2. Predicted survival can be modified if accurate information concerning fetal sex, weight, and well-being is available. Parents are particularly anxious about the risks of later disability and handicap. These risks are especially significant before 26 weeks' gestation. When assessed at 6 years of age, nearly half the survivors at 23 to 25 weeks' gestation have a moderate or severe disability. Furthermore, many of these disabilities only become apparent after 2 to 3 years of age. Survival with no disability is only seen in 1, 3, and 8 percent of live births at less than 24, 24, and 25 weeks, respectively. There ...