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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 22+0 to 23+6 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 the membranes (PPROM). Indicated deliveries, PTL, and PPROM each account for approximately one-third of early births.


From 2012 to 2020, there were 1,120,959 livebirths between 23 and 42 weeks of gestation in Ontario (BORN Ontario). A total of 7 percent of these births were preterm, occurring before 37 weeks, but the percentage of very early births is much smaller (Fig. 30-1). Although similar data is seen 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. National Perinatal Health Reports and Indicators from the Public Health Agency of Canada demonstrate that the preterm birth rate increased from 6.4 percent in 1985 to 8.2 percent in 2004 but has remained steady since then, with data up to 2014. Although just under 1 percent of singletons are delivered before 32 weeks, this rises to 8 percent for twins and nearly 40 percent for higher multiples.

FIGURE 30-1.

Ontario preterm livebirths, 2012-2020. (Data supplied by BORN Ontario.)

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 2019 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 anxious about both survival rates and also the risks of later disability and handicap. These risks are especially significant before 26 weeks’ gestation. The CNN data suggests that 60 percent of parents elect for neonatal palliative care only at 22 weeks falling to 28 percent at 23 weeks. 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 ...

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