18
OBSTETRICS
Preterm labour and preterm premature rupture of the membranes (PPROM) Preterm labour Preterm labour is defined as labour occurring before 37 completed weeks. It affects 5-10% of all pregnancies but it accounts for approximately 75% of perinatal mortality. Diagnosis Diagnosis is made with difficulty as uterine activity is not always associated with cervical dilatation and may settle down with no untoward effect on the pregnancy, hence the apparent spontaneous cessation of the labour in 50% of cases. Causes of preterm labour include: « • « « « « • •
preterm rupture of the membranes polyhydramnios multiple pregnancy cervical incompetence uterine abnormalities antepartum haemorrhage fetal death maternal pyrexia, particularly associated with urinary infection idiopathic - the majority of cases.
Management The benefits of in utero existence are weighed against the risks of threatened preterm delivery and in each case a decision is reached about the best treatment options. Maternal infection should be sought and treated appropriately - mid-stream urine sample (MSU), full blood count (FBC) and high vaginal swab (HVS) should be obtained on admission, as should a clean-catch liquor sample in cases with ruptured membranes. A cardiotocograph (CTG) will determine the status of the fetus but interpretation of the CTG in the extremely preterm infant (24-26 weeks) is complicated by lack of knowledge about normal parameters (see p. 50). Assessment of cervical dilatation over the first few hours after admission will show if there is progressive cervical dilatation and the need for uterine suppression. Uterine suppression (tocolysis) Various medications are used to try to suppress uterine contractions including intravenous betamimetic
drugs, calcium channel blockers, oxytocin receptor antagonists and antiprostaglandins. Side effects which limit use of the betamimetics are palpitations, tremor, headache, restlessness, nausea and vomiting, and hypotension. If chest discomfort or breathlessness develops this may indicate pulmonary congestion - one of the more serious side effects of therapy. There are no studies which show any decrease in perinatal mortality with the use of betamimetics, though there is a reduction in the proportion of deliveries occurring within the next 24-48 hours. This allows time to administer steroid therapy and transfer the patient to a centre with neonatal intensive care facilities. Intravenous ritodrine has been studied extensively but salbutamol and fenoterol are also used. All will have an effect on carbohydrate metabolism and should be used with caution in the diabetic patient. Maintaining uterine suppression after the acute event by use of oral therapy has not been shown to reduce the incidence of preterm delivery. As there is good evidence that prostaglandins are involved in the initiation of labour, suppressing prostaglandin synthesis is logical. Indomethacin, p.r. or orally, has been shown to suppress uterine contractility, reducing delivery within 48 hours and reducing preterm birth. It too has side effects - gastrointestinal tract irritation even amounting to peptic ulceration, nausea and vomiting, diarrhoea and headache. For the fetus, the theoretical adverse effects include impaired renal function and prolonged bleeding time but the major worry is constriction of the ductus arteriosus which may result in persistent pulmonary hypertension in the newborn. Nifedipine (a calcium channel blocker) and glyceryl trinitrate have also been used, with possible success. Magnesium sulphate is the preferred treatment in the US. As infection may be an aetiological feature, there may be a role for empirical treatment with broad-spectrum antibiotics, particularly following preterm premature rupture of the membranes (PPROM).
When should tocolysis be used? « Where prolongation of the pregnancy will have beneficial effects for the fetus, to allow time to administer steroids to ensure fetal lung maturation; tocolysis works only in early labour (less than 4 cm cervical dilatation) « Not in the presence of an antepartum haemorrhage as the vasodilatation caused may potentiate the bleed • With caution in the diabetic patient as betamimetics cause gluconeogenesis and may precipitate diabetic ketoacidosis « Not with evidence of chorioamnionitis - maternal pyrexia, uterine tenderness, raised white blood count (WBC) (steroids used for fetal lung maturation cause a rise in WBC, so use of C-reactive protein may be more accurate), fetal tachycardia • Not with evidence of fetal compromise when conditions ex utero may be more favourable. Cervical cerclage There are two main ways this technique is employed: 1. In the acute situation when the cervix is found to be dilated on admission - usually in a patient with suspected preterm labour. If the cervix does not continue to dilate whilst the patient rests in bed then a suture may be placed (rescue cerclage) to prevent further passive dilatation. This may be unsuccessful with membrane rupture during suture placement. The suture may cut through the thinned cervical tissue or intrauterine infection may follow. 2. In patients with a history of previous cervical incompetence, or history of gynaecological procedures which may leave the cervix incompetent, cerclage may be considered. The suture is placed circumferentially at the level of the internal os taking four large bites into the substance of the cervix. A large, multi-centre study assessing cervical cerclage failed to show benefit in prolonging pregnancy. Practice is to assess cervical length ultrasonically in the high-risk patient and use cerclage if there is evidence of shortening of the
Preterm labour and preterm premature rupture of the membranes (PPROM) The cerclage suture is usually removed at around 37 weeks and onset of spontaneous labour is awaited. This may occur some days later.
Benefits and risks of in utero existence The survival rates for infants between 24 and 28 weeks' gestation vary from 25% early to 80% later and determine whether intervention will offer benefits over the in utero state. From 28 weeks onwards the survival rates climb gradually from 80% to 98% and give greater confidence in delivering a preterm infant. Extremely preterm infants have better survival prospects if delivered in a neonatal intensive care unit and should be transferred in utero if possible.
Delivery If labour ensues, a controlled delivery with intact membranes and a short second stage offers the best outcome for the infant. The preterm breech presentation risks delivery of the small trunk through an incompletely dilated cervix resulting in fetal head entrapment. In these circumstances it may be best to deliver by caesarean section, between 26 and 34 weeks' gestation - though the evidence for this is limited. The lower uterine segment will be poorly formed in these circumstances, so a longitudinal incision in the lower uterine segment may be needed (de Lee incision).
PPROM Premature rupture of the membranes (PROM) is when the membranes rupture before the onset of labour. In 80% of patients labour ensues within 24 hours. Once the membranes are ruptured the barrier to ascending infection is gone and if labour does not follow within 24-48 hours, induction of labour to prevent chorioamnionitis in the mother and systemic neonatal infection is usual. Preterm PROM (PPROM) is when the membrane rupture occurs before 37 weeks and induction of labour may not be the optimal management. It occurs in 2-3% of pregnancies and accounts for about one-third of preterm deliveries. A more conservative approach may be used dependent on the gestation (see Fig. 1). In uncomplicated cases: < 34 weeks - benefits of in utero development outweigh the risks of ascending infection and a conservative approach is appropriate. Pulmonary hypoplasia and skeletal deformities may be seen due to oligohydramnios following spontaneous rupture of the membranes (SRM) in extreme prematurity. Pulmonary hypoplasia after SRM occurs in 50% of cases less than 20 weeks but in only 3% over 24 weeks. Two doses of corticosteroid given 12 hours apart are associated with increased fetal surfactant production so long as there are 24 hours after the completion of the dose before delivery. The use of antibiotics prophylactically is of unproven benefit for the fetus. 34-37 weeks - no suppression of uterine activity and if no evidence of infection conservative management. The risk of respiratory distress syndrome (RDS) in the infant is about 5% and this dictates conservative management. Antibiotic therapy may be given to reduce maternal infection but it may be preferable to treat infection if detected rather than subject all patients to therapy. Induction of labour at 36 weeks avoids the continued risk of ascending infection, whilst the chance of RDS is small. > 37 weeks - if no labour ensues within 24-48 hours of membrane rupture then induction of labour avoids the development of infection with the associated morbidity.
Fig. 1 Management plan.
Complications include: • infection • antepartum haemorrhage • fetal compromise. The presence of complications makes a more active approach to delivery appropriate. If there are no complications it is acceptable to wait up to 96 hours for labour.
Making the diagnosis After palpation of the abdomen to confirm the fetal lie, presentation and size, a sterile speculum examination is performed to observe the cervix for amniotic fluid leakage unless there is obvious liquor at the vulva or on a pad. Amniotic fluid has a characteristic odour and presence of vernix caseosa is diagnostic. A high vaginal swab should be taken to check for infection or amniotic fluid aspirated and sent for microscopy and culture. If doubt exists the patient may be asked to wear a pad whilst ambulant and check the pad for presence of liquor. If there is still doubt, then an ultrasound scan to measure the amniotic fluid index and a check for the presence of fluid below the presenting part will refute the diagnosis.
Management of chorioamnionitis Labour should be induced with Syntocinon and a continuous CTG is needed. Caesarean section is only performed if clinically indicated as there will be an increased risk of postoperative pelvic sepsis and subsequent tubal blockage. Intravenous antibiotics should be broad spectrum.
Preterm labour and PPROM • Preterm labour accounts for 75% of perinatal mortality. • Most preterm labour is due to unknown reasons. m Rupture of the membranes is associated with ascending infection.
19