Friday, February 23, 2007

Perinatal mortality and gestational age

This week the CDC released the most recent comprehensive perinatal mortality statistics. They are relevant to the issue of homebirth because a large proportion of preventable neonatal deaths at homebirth are due to postdates. Furthermore, PD ("natural") childbirth advocates often criticize obstetricians for inducing women after they have passed their due date, arguing that any cut off date for induction is purely arbitrary.

Fetal and PerinatalMortality United States 2003 by MacDorman et al. shows that fetal and perinatal mortality rates have continued to decline steadily. Fetal mortality is defined as intrauterine death before delivery, and perinatal mortality includes fetal deaths plus deaths up to 7 days or 28 days after birth.

I'd like to draw your attention to Figure 8 on page 7 of the paper. This is a graph of prospective fetal mortality by gestational age. According to the paper:
This prospective fetal mortality rate is computed as the number of fetal deaths at a given gestational age (in single weeks), per 1,000 live births and fetal deaths at that gestational age or greater.Prospective fetal mortality rates are shown in Figure 8 for fetal deaths between 20 and 43 weeks of gestation. The rate was
highest (0.65) at 21 weeks of gestation, declined to a low of 0.18–0.19 at 29–31 weeks of gestation. The rate remained relatively low until about 36 weeks of gestation, and then increased rapidly to a high of 0.51–0.52 at 41–43 weeks of gestation...

The prospective fetal mortality rate was useful in identifying two distinct peaks in fetal mortality risk: early fetal mortality (less than 24 weeks), and fetal mortality at 40 weeks of gestation or more. These two peaks suggest etiological differences. Early fetal mortality may be more related to congenital infections, anomalies, utero-placental insufficiency, and underlying maternal medical conditions. Fetal mortality at 40 weeks or more may include the previously mentioned conditions, but may also be related to problems that manifest around the time of delivery, such as placental(abruptio, previa) and cord (prolapse) problems, or other problems in the labor and delivery process. However, investigations into late fetal deaths have found that a substantial number are of unknown cause.
This means that the chance of suffering a stillbirth after 36 weeks gestation is approximately 0.27/1000 and rises to 0.43/1000 after 40 weeks and 0.5/1000 after 42 weeks. Interestingly, the bulk of the increase occurs between 36-40 weeks, although it continues to rise after 40 weeks. In other words, for every 10,000 women who reach 40 weeks of pregnancy, approximately 4 will go on to have a stillbirth. For every 10,000 women who reach 41 weeks of pregnancy, approximately 4.5 will go on to have a stillbirth, and for every 10,000 women who reach 42 weeks of pregnancy, 5 will go on to have a stillbirth. Keep in mind that this does not include problems at delivery or neonatal mortality.

Therefore, every day beyond 40 weeks, the chance of stillbirth rises. Induction after term is not arbitrary. It is an attempt to avoid these stillbirths. Indeed, the steadily falling rate of stillbirths identified by MacDorman et al., may be the result of the increased induction rate.


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