Premature Labour

Synonym: Pre-term labour

Premature labour may be defined as the presence of contractions of sufficient strength and frequency to effect progressive effacement and dilation of the cervix between 20 and 37 weeks' gestation.¹ Infants born as a result of premature labour suffer significant morbidity as a result of immaturity. Accurate diagnosis of pre-term labour can allow for the prevention or delay of pre-term birth where possible, and where this is not possible, earlier provision can be made to provide optimal support for the immature infant.

The UK Office of National Statistics collects much information about childbirth, but strangely nothing about premature labour. USA statistics quote an incidence of 6-8% of all pregnancies.¹ Statistics from the North Thames database demonstrate significant ethnic variation in pre-term birth rates, with higher rates among black women. This is associated with an accelerated rate of maturity in the black fetus and neonate, with correspondingly lower gestation-specific neonatal mortality rates below 38 weeks, and higher at 38 weeks of gestation and beyond.²

Risk factors^2,3

Worldwide, the greatest risk factor is infection, mainly from malaria, HIV and mycoplasma. In developed countries, iatrogenic factors account for almost half of the births between 28 and 35 weeks. Hypertension and pre-eclampsia are the main pathologies.
Other factors include:

• Multiple pregnancy (increasing with the increase in IVF)
• Intrauterine growth restriction
• Maternal stress
• Heavy physical work
• Previous premature labour
• Prolonged bleeding in early pregnancy
• Maternal cardiac disease
• Maternal age <15
• Cigarette smoking during pregnancy
• Low BMI
• Cervical incompetence
• Low social class
• Unsupported and/or unmarried mothers

Pregnant women may present with a history of painful contractions and assume that they are in a premature labour. Many of these women are experiencing Braxton Hicks contractions and over 60% will not have delivered within 48 hours of presentation, many going on to full term. Additional indications of pre-term labour may be gained through history and examination:-

History

• Length of time that the contractions have been experienced
• Interval between contractions
• Bleeding or amniotic fluid loss
• Previous obstetric history - any previous pre-term deliveries
• History of current pregnancy - infections, bleeding, pain, single or multiple fetuses
• Smoking history

Examination

• Speculum examination may reveal dilatation of the cervix and/or amniotic fluid leak through the cervix.
• Digital examination should not be performed if it is thought that the membranes have ruptured, as this will increase the risk of infection to the fetus. If the membranes have not ruptured, however, vaginal examination should be performed as it is the best way of assessing the onset of premature labour.

• Nitrazine sticks may be useful in detecting the presence of amniotic fluid, signifying rupture of the membranes.⁴ However, a false positive result may be given by any substance which has changed the pH of the vagina, such as urine.
• Vaginal swab - this should be taken in all women who are being examined with possible premature labour as this will allow appropriate antibiotic therapy to be given if an infection develops at a later stage.

Once a diagnosis of premature labour has been made, the priority should be to ensure that the pregnant mother is transported to the safest available facility for delivery of a pre-term infant. If the patient is at home and the labour appears well established, or if the fetus is visualised on speculum examination, a midwife and an ambulance should be summoned. If the patient is in hospital, the emergency paediatric team should be alerted.

• Predicting Premature Labour All women should have a risk assessment for pre-term labour in early pregnancy (see Risk Factors, above). Several risk scoring systems based on epidemiology and previous obstetric history.⁵ These are less helpful in primigravid patients. Those identified as being high risk may also benefit from:⁶
  • Ultrasound assessment of cervical length (this may be more accurate in multiple pregnancies).
  • Fibronectin assessment- this is a glycoprotein produced by the fetus which acts as a 'biological glue' to attach the fetal sac to the uterine lining. Its presence in cervico-vaginal secretion between weeks 22-3 of pregnancy suggest a risk of premature labour.⁶
  • Salivary oestriol assessment - is another marker which can be used to predict premature labour.
• Tocolytic drugs help to promote tocolysis (the delay or inhibition of contractions during the birthing process).
  • Typical agents are ritodrine, terbutaline and salbutamol. These have beta2-agonist properties which promote the relaxation of uterine smooth muscle.
  • Magnesium sulfate and non-steroidal anti-inflammatories such as indomethacin have are also used on occasion.⁷ The precise mode of action of magnesium sulfate is not understood but it is thought to affect the uptake, binding and distribution of calcium ions in uterine smooth muscle cells. Non-steroidals on the other hand inhibit the production of prostaglandins which are known to promote uterine smooth muscle contraction as well as the process which stimulates thinning of the cervix.
  The evidence for the use of tocolytics is poor, and they rarely effective in reducing the strength and frequency of contractions for more than 48 hours.¹ Their main use therefore lies primarily in delaying delivery for a sufficient time to allow corticosteroids to be given to the mother and thereby reduce the risk of respiratory distress syndrome in the infant.⁸ This delay in delivery may also allow for the transportation of the mother to a centre with facilities available for supporting a premature infant.
  • Tocolytics have a significant list of contraindications, of which cardiac, pulmonary and maternal infection (e.g. pyelonephritis) top the list. ¹
  • Current research is focussing on drugs such as atosiban and nifedipine which may help to maintain tocolysis for a longer period of time, as they have fewer adverse effects than ritodrine.⁹ Atosiban acts by inhibiting oxytocin, and nifedipine is thought to act by a combination of vasodilatation and a direct effect on uterine smooth muscles.
  • Tocolytic drugs should always be given in conjunction with a broad spectrum antibiotic to cover the possibility that infection may have been the cause of the premature labour.
• Antibiotics - Recent systematic review has found that by giving macrolides or clindamycin in the second trimester to women at risk of pre-term labour the risk can be lowered. The study also found that giving metronidazole in the second trimester can increase the risk. The authors concluded that as many as 50% of pre-term births were associated with infection, the commonest organism being Mycoplasma.¹⁰
• Delivery If the presentation of the baby is cephalic, then most pre-term babies will be delivered safely vaginally with only a few requiring delivery by caesarean section.
  Breech presentations below 32 weeks gestation will be more safely delivered by caesarian section.
• Post-Delivery Hypoglycaemia and cold stress (hypothermia) are the two most significant factors which require monitoring immediately after birth.¹¹

• Babies delivered with optimal care after 30 weeks most often survive without any lasting abnormality .
• Babies delivered before 26 weeks have an increased risk of long term handicap.
• Babies born at 23-24 weeks gestation have a 50% risk of lasting handicap.¹²

• Addressing social factors such as poverty and lifestyle issues such as smoking would help to promote a significant reduction in the incidence of premature labour, but the success of various interventions have so far been limited.⁶
• The insertion of a cervical suture (cervical cerclage) in early pregnancy has been shown to reduce pre-term delivery in women identified as being at high risk (i.e. women with three or more second trimester miscarriages or pre-term deliveries).¹³
• The detection and treatment of asymptomatic infections such as bacterial vaginosis and asymptomatic bacteriuria has been shown to be effective in preventing pre-term labour.⁶