Variations in the duration of pregnancy

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Last modified 10/03/2015

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Chapter 19 Variations in the duration of pregnancy

CHAPTER CONTENTS

Premature or preterm birth 147

Prevention of preterm birth 147
Preterm labour 148

Management of preterm labour of less than 34 completed weeks 148
Prelabour rupture of membranes (PROM) 149
Fetal maturity 150
Post-term pregnancy 150

Management 150

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

Prevention of preterm birth

Ceasing smoking is an effective way of preventing preterm birth, leading to a reduction of 16% in preterm birth and 19% reduction in low birthweight. Increased contact with health professionals to provide better social support (France), the prophylactic use of tocolytic agents such as terbutaline (Germany), uterine activity monitoring at home or in hospital (USA) or bed rest in early pregnancy have been tried and have not been shown to be effective.

Regular antenatal examinations will detect medical conditions complicating pregnancy at an early stage of their development, but it is uncertain if this will reduce the incidence of preterm birth. Cervical cerclage increases the duration of pregnancy in women who have a diagnosis of incompetent cervix, but not in other cases.

PRETERM LABOUR

The diagnosis of preterm labour must be made carefully or women who are having mild contractions, which do not cause cervical dilatation, will be included. The criteria for diagnosis are:

• The gestational period is less than 36 completed weeks.
• Uterine contractions, preferably recorded on a tocograph, occur every 5–10 minutes, last for at least 30 seconds and persist for at least 60 minutes.
• The cervix is more than 2.5 cm dilated and more than 75% effaced.

Using these criteria, two-thirds of women presenting with presumed preterm labour will not be in labour. They need reassurance, not drug treatment.

A confirmatory test is to measure fetal fibronectin, which in cases of preterm labour is released into cervical and vaginal secretions. A negative test means that it is very unlikely that the woman will deliver within 7 days (negative predictive value approaches 100%). A positive test can occur in association with coitus, vaginal infection and examination. Its specificity is low and positive predictive value is 35–50%. Its main benefit is to reduce the need to transfer women in possible preterm labour to tertiary units and/or to commence tocolytic therapy.

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