Variations in the duration of pregnancy

Published on 10/03/2015 by admin

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Last modified 10/03/2015

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Chapter 19 Variations in the duration of pregnancy

The duration of human pregnancy averages 260 days from conception and 280 ± 14 days from the first day of the last menstrual period. In 7–13% (Australia and USA respectively) of pregnancies the duration of pregnancy is curtailed and a preterm birth (less than 36 completed weeks) occurs. In 7% of cases the pregnancy is prolonged, defined as gestation more than 41 completed weeks (i.e. 41 weeks and 6 days).

PREMATURE OR PRETERM BIRTH

There are some problems inherent in defining preterm birth as one that occurs before the 36th completed week of pregnancy. This is because the survival of the neonate depends not only on the duration of the pregnancy, but also on the baby’s birthweight.

Studies of preterm births show that premature birth may be associated with low social class, young maternal age, eating disorders leading to a low body weight (body mass index <19), fetal abnormalities, multiple pregnancy and smoking. Preterm births may also be associated with medical complications, such as a history of abortion or stillbirth, uterine bleeding in pregnancy (threatened abortion, abruptio placentae and placenta praevia), hypertensive disorders and anaemia.

Bacterial vaginosis (see p. 262) has been implicated, which has been associated with an increase in preterm birth two to three times that of women who do not have bacterial vaginosis (15–20% compared with 6%). A Cochrane review of antibiotic therapy to eradicate bacterial vaginosis showed that it was effective in reducing the incidence of preterm birth, but only in women with a previous history of spontaneous premature delivery (RR 0.37). Progesterone as a depot intramuscular injection or as pessaries reduces the recurrence of preterm birth by 35%. Treatment with metronidazole actually increases the rate of preterm birth. It should also be noted that a large number of preterm births follow a spontaneous rupture of the membranes from unknown causes (Table 19.1).

Table 19.1 Causes of curtailment of pregnancy and prematurity

Cause Percentage
No cause found (including premature rupture of the membrane) 35–45
Hypertensive disorders 18–30
Multiple pregnancy 12–18
Maternal disease 5–15
Abruptio placentae 5–7
Placenta praevia 3–4
Fetal malformations 1–2