Variations in the duration of pregnancy

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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.

If the criteria are met, the woman is in preterm labour. If the pregnancy is less than 35 completed weeks’ gestation and the woman is not in a facility with a neonatal intensive care unit, she should have uterine contractions suppressed and be transferred as soon as possible to such a facility. If she has access to neonatal intensive care the choice is to attempt to suppress uterine activity or to permit labour to proceed. In some third-level hospitals women who start preterm labour at or after the 34th week of gestation are permitted to proceed, as the premature infant can be competently cared for with no increase in perinatal mortality.

The greatest challenge is the management of a woman whose labour starts when the gestation period is less than 34 completed weeks.

Management of preterm labour of less than 34 completed weeks

The major risk to the fetus is pulmonary immaturity. The principal objective of management is to enhance surfactant production in the fetal lungs to prevent hyaline membrane disease. The usefulness of bed rest to prevent premature rupture of membranes is uncertain. To be effective corticosteroids, two doses of 11.4 mg betamethasone 24 hours apart, must be given to the mother more than 24 hours and less than 10 days before birth. The corticosteroids also reduce the chance of intraventricular haemorrhage in these infants. After 34 weeks’ gestation this treatment is no longer necessary, as lung maturation is adequate. The common practice of repeating the corticosteroids 7–10 days later if delivery has not occurred has been questioned, as a number of animal studies have reported effects associated with changes in the development of the central nervous system, pancreatic function and growth restriction. The most recent randomized control trial of one further course of betamethasone reported a reduction in neonatal morbidity without any apparent increased risk.

Once labour is established, tocolytic therapy can be administered (Table 19.2) to postpone delivery for at least 24 hours to allow steroid-induced lung maturation to occur. This time can also permit the mother to be transferred to a hospital with neonatal intensive care facilities.

Table 19.2 Tocolytic drugs in the management of preterm labour

Drug Dose Side-Effects
Calcium-channel blocker
Nifedipine 10 mg orally; repeated 20 min later if contractions persist (max. 40 mg in first hour). If contractions cease, 20 mg by mouth 4–6-hourly Substantially less than with beta agonists. Principal side-effect: headache; others: hypotension, nausea, palpitation
Beta agonists
Salbutamol Infused at 4 µg/min, increasing by 4 µg/min every 20 min until uterine contractions suppressed; dose maintained for 6 hours and then reduced. Oral salbutamol started 8 mg 6-hourly for 5 days Tachycardia, palpitations, apprehension, anxiety. Increase in cardiac output. Pressure in the chest
Ritodrine Infused at 50 µg/min increased by 50 µg every 10 min until the contractions cease. Maximum dose 350 µg/min. Run for 24 hours then oral ritodrine 10–20 mg 2–6-hourly Hypokalaemia. Pulmonary oedema and myocardial ischaemia in 5%. Fluid retention in all women

Note: Calcium-channel blockers and beta agonists are relatively ineffective if the cervical dilatation is >5 cm.

The current tocolytic drug of choice is the calcium- channel blocker nifedipine, which has been shown in a Cochrane analysis of 12 random controlled trials (RCTs) to be superior to the β mimetics, reducing the number of women delivering within 7 days (RR 0.76). In addition there were significantly fewer neonatal complications, including necrotizing enterocolitis, respiratory distress syndrome, intraventricular haemorrhage and jaundice. Of major importance is that few women taking nifedipine had to stop treatment because of side-effects. Other drugs such as nitric oxide donors – nitroglycerin or magnesium sulphate – have not been shown to be effective tocolytic agents. There is insufficient evidence to recommend continuation of any oral therapy after successful inhibition of uterine contractions.

Before starting tocolytic therapy, the following conditions should apply:

• The pregnancy is less than 35 weeks.
• There has been confirmation of preterm labour.
• The cervix is less than 5 cm dilated.
• There is no evidence of abruptio placentae.
• There is no evidence of chorioamnionitis.
• The fetus is alive and no potentially lethal malformations have been detected by ultrasound.

These exclusions mean that fewer than 15% of women in preterm labour should receive tocolytic drugs.

The membranes should be kept intact for as long as possible, and when they rupture a vaginal examination is carried out to exclude a prolapsed cord. If labour starts either it may be allowed to proceed, or a caesarean section may be performed. If there is a delay when the infant’s head is in the pelvic outlet, a forceps delivery may be appropriate.

The place of caesarean section in the management of preterm labour is controversial. Most obstetricians believe that if labour starts when the pregnancy is 26–31 weeks’ gestation, caesarean section should be performed if:

• The fetus does not present cephalically.
• There is associated antepartum haemorrhage.
• The labour fails to progress normally.
• Fetal distress develops.

Although there can be little disagreement about these indications, the routine use of caesarean section to deliver babies weighing less than 1500 g is debatable; current evidence shows it increases maternal morbidity, but is insufficient to determine if there is a benefit to the neonate.

PRELABOUR RUPTURE OF THE MEMBRANES (PROM)

Prelabour (premature) rupture of the membranes may lead to the onset of preterm labour, with or without other causal factors. In most cases the baby is born within 7 days of the rupture; 50% within 2–4 hours, 80–90% by 7 days. Rupture of the membranes after the 35th week is of less clinical significance, and if labour does not start quickly it may be induced by the use of Syntocinon (see p. 190), depending on the state of the cervix. A woman whose cervix is ‘not favourable’ (see p. 189), may prefer to delay induction for 48–72 hours in the anticipation that, during this time, the cervix will ripen and induction will be easier and more successful.

A multicentre study of 5000 women with prelabour rupture of the membranes, whose gestation was greater than 37 weeks, showed that there was no increase in the caesarean section rate or in infection whether or not induction, using oxytocics, or expectant treatment was chosen. The mother should be informed of the choices and her wishes accepted. Routine antibiotic therapy has been shown to reduce maternal infection, but to have no significant beneficial effects on the neonate. If the woman is a carrier of group B streptococcus she must be informed that there is an increased risk of neonatal morbidity.

The real problem occurs when the membranes rupture between the 26th and 35th weeks (preterm premature rupture of membranes PPROM), as the longer the birth is delayed the more mature the baby becomes. Against this is the risk of ascending intra-uterine infection occurring. The woman should be admitted to or transferred to a hospital with a neonatal intensive care level nursery. A high vaginal swab is also taken for bacteriological examination.

Regular assessments of maternal temperature and pulse are made to detect intra-uterine infection, and observations are made to see if the amniotic fluid continues to leak. An ultrasound examination may be carried out to determine whether oligohydramnios has occurred.

The use of prophylactic tocolytics has been suggested, but studies show them to be of little value. Prophylactic corticosteroids should be given, as the risk that they will increase the chance of intra-uterine infection is less than their beneficial effect on fetal lung maturation.

Prophylactic erythromycin should be given to the mother, as four trials have shown a significant reduction in the number of babies born within 48 hours, and a reduction in neonatal infection and the need for oxygen therapy. The use of Augmentin was associated with an increased incidence of necrotizing enterocolitis and therefore it should not be used.

If antibiotics are not given and the mother develops signs of intra-uterine infection (chorioamnionitis), such as tachycardia, fever (higher than 38 °C), leucocytosis and an offensive vaginal discharge, antibiotics should be prescribed and delivery expedited. In most cases the infection is polybacterial, with anaerobes and group B streptococci predominating. Antibiotics are chosen that take these findings into account.

If labour does not start within 48 hours the patient may ambulate, and provided her home conditions are suitable and she lives near the hospital, may go home. At home she should rest as much as possible, avoid sexual intercourse, and return to hospital if she develops a fever or an offensive vaginal discharge.

FETAL MATURITY

As more women attend for antenatal care early in pregnancy and with the increasing use of ultrasound, the degree of maturity of the fetus is generally known to within 2 weeks. In a few cases the woman does not seek medical attention until the third trimester, at which time the estimation of fetal maturity is less exact. In a mature fetus, hyaline membrane disease is unlikely to develop. Even if it does, neonatologists today have effective treatments. If there is then a need to determine the lung maturity this can be done by measuring the lecithin/sphingomyelin (L/S) ratios. As pregnancy advances the L/S ratio increases (Figs 19.1 and 19.2).

image

Fig. 19.1 Changes in the concentration of lecithin and sphingomyelin in amniotic fluid.

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Fig. 19.2 Mean lecithin and sphingomyelin ratios during normal gestation.

POST-TERM PREGNANCY

Historically in 10% of cases the pregnancy lasted for more than 42 weeks; accurate dating of pregnancy by ultrasound and a policy of offering induction after 41 weeks and 3 days in many centres has resulted in only 1–2% of pregnancies proceeding past 42 weeks’. In some cases there may be a genetic cause for postdatism, but for most the aetiology is not known.

The risk of a post-term pregnancy is that the perinatal mortality rate increases, being doubled at 42 weeks’ and 6 times at 43 weeks’ compared with 39–40 weeks’. At least one-third of the increased mortality is due to the death of a malformed fetus.

Management

If the woman is an older primigravida, has a history of infertility or has pre-eclampsia, diabetes mellitus or other pregnancy complications, the pregnancy should be terminated before it becomes post-term. In other cases three options are available and they should be discussed with the patient and her partner. These are the induction of labour, to continue with the pregnancy, or to perform an elective caesarean section. The induction of labour at 10 days post-term has been shown to be associated with lower rates of caesarean section and reduced neonatal morbidity and perinatal mortality.

If the option of continuing with the pregnancy in the expectation that labour will commence spontaneously is chosen, the welfare of the fetus must be monitored. The mother records fetal movements daily, non-stress cardiotocography is performed twice weekly, and ultrasound examinations are made to measure the volume of the amniotic fluid. An amniotic fluid index of less than 5 is considered abnormal. Umbilical blood flow may also be measured. Abnormalities in these tests (see Ch. 20) may indicate the need for induction of labour or caesarean section.

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