Preterm delivery is defined as delivery before 37 weeks gestation. Approximately 50% of all preterm deliveries are the result of preterm labour, 25% are due to preterm premature rupture of membranes, and 25% are due to intervention for medical or obstetric risk.
Preterm delivery, which occurs before the infant's organs have fully developed, is one of the greates causes of neonatal death, with over one million fatalities yearly (Lawn et al, 2010). Chances of survival depend heavily on where the infant is born. In many developing countries, most infants born under 32 weeks gestational age will die, while most born in developed countries will live (Beck et al, 2010).
For infants that survive, prematurity causes much morbidity, including developmental delay, visual and hearing impairment, chronic lung disease, and cerebral palsy. Preterm birth may also be a marker for other problems that produce disease, such as systemic infection or inflammation.
There has been an increase in preterm births over the past 20 years. Of the 12.9 million preterm births globally, over 85% occurred in Africa and Asia (Beck et al, 2010). Rates vary around the world, such as among Africa (11.9%) North America (10.6%), and Europe (6.2%) (Beck et al, 2010).
Events leading to preterm birth are still not completely understood, although the etiology is thought to be multifactorial.
Emma G. is a 17 year old nulligravid woman who presents to labour and delivery at 32 weeks gestation. She states she is having mild contractions which she thinks are fairly regular, although she hasn’t timed them. She has noticed some bloody vaginal discharge this morning. What is your differential diagnosis? How will you manage Emma?
Other maternal conditions:
Several processes may contribute to preterm labour, which is a manifestation of a complex network of causal pathways. It is unclear whether preterm birth results from the interaction of several pathways or the independent effect of each pathway:
Gestational age (LMP, US)
history of present illness
past obstetrical history
past medical history
Identification of the symptoms of preterm labor will help ensure that the patient can be evaluated, diagnosed and treated appropriately.
Vitals of mother and baby
Leopolds maneuvers for presentation
Contractions should be palpated or by tocometer.
If ultrasound has demonstrated the absence of placenta previa, speculum and digital exams may be done.
digital vaginal exam: only if ruptured membranes are ruled out; assess the cervix for length and dilation.
Urine culture should be taken.
Swabs for GBS and other pathogens (as warranted) may be taken.
Fetal fibronectin (fFN) in cervicovaginal fluid is associated with preterm delivery, and is performed if cervical length by ultrasound is inconclusive. fFN is normally not present at 24-34 weeks, and has a high NPV but low PPV.
Perform a sterile speculum exam, and place the swab in the posterior fornix for 10 seconds. Ensure this is done before a vaginal exam, as ANYTHING disrupting the vagina (intercourse, digital exam, etc) within 24 hours will cause a positive result. Other contraindications include:
Premature rupture of membranes can be assessed with nitrazine swabs or ferning, as described here.
Cervical length (effacement) may be measured by trans-vaginal ultrasound to confirm diagnosis:
Labour can be confused with:
Prevention of preterm birth is very difficult. We need to educate about symptoms.
Bedrest, fluid boluses, sedation, and magnesium sulfate have no evidence for prevention.
Cerclage is the suturing of the cervix to maintain its integrity, and can be placed vaginally or abdominally. Extensive patient counselling is necessary when deciding upon cerclage, especially with urgent or rescue procedures.
Elective cerclage is planned for 12-15 weeks in people with a history consistent of incompetent cervix.
Urgent cerclage has controversial evidence. There are risks of initiation of labour and complications for the fetus. The upper limit is 24 weeks, as babies begin to be viable at this point.
Rescue cerclage occurs <24 weeks if there is an incompetent cervix with hourglassing membranes.
Sutures need to be removed if contractions begin or if an infection is present.
Women diagnosed with preterm labour earlier than 34 weeks gestation are hospitalized and managed as follows:
Transfer to a site able to care for the premature infant, ie with a newborn intensive care unit. Before transfer, be sure to assess cervical dilation. Transport is contraindicated if the mother or fetus appear unstable.
Prophylactic corticosteroids such as betamethasone or dexamethasone are frequently used between 24-34 weeks. These are primarily used to encourage lung maturity, but there is also benefit in preventing necrotizing enterocolitis.
Delivery is indicated if FHR is non-reassuring or there is evidence of chorioamnionitis. Vaginal birth appears to be as safe as Caesarean section, as long as other contraindications do not exist.
Tocolysis, or inhibition of uterine contractions, may be used for up to 48 hours to delay delivery so that glucocorticoids can be administered (see below). Prior to initiation of tocolysis, the fetus should be assessed by FHR (fetal well-being) and ultrasound (fetal growth and presentation). Tocolytics include magnesium sulfate, beta-mimetics, nifedipine, and indomethacin. There is no clear 1st line agent.
Magnesium sulfate acts as a competitive inhibitor of calcium. Side effects include nausea, vomiting, and muscle weakness. Severe complications include pulmonary edema and cardiac arrest.
Beta-mimetics (terbutaline) stimulate beta-2 receptors. Side effects include palpitations, hypotension, tachycardia, shortness of breath, hyperglycemia, and hypokalemia. Beta-mimetics may also cause pulmonary edema. ñ Nifedipine blocks calcium channels. Side effects include hypotension and tachycardia.
Indomethacin inhibits prostaglandin synthesis. Side effects include nausea, vomiting, and GI bleeding. Indomethacin is contraindicated after 32 weeks gestation as it may cause constriction of the fetal ductus arteriosus leading to right heart failure. It may also cause neonatal pulmonary hypertension, oligohydramnios, and necrotizing enterocolitis.
Contraindications to tocolysis include non-reassuring fetal status, pre-eclampsia, antepartum hemorrhage, maternal cardiac disease, or chorioamnionitis.
Antibiotics (7-day course) are indicated for GBS prophylaxis. If GBS culture returns negative, antibiotics may be discontinued. Antibiotics are also indicated for positive tests for gonorrhea or chlamydia, or positive urine culture.
After inhibition of acute preterm labour by tocolysis:
Stable patients may be discharged home. Patients should avoid prolonged standing, heavy lifting, and sexual intercourse. Weekly follow-up visits are required to continually assess cervical change.
Unstable patients include women with advanced cervical dilation, recurrent vaginal bleeding, or non- reassuring fetal status. These patients may benefit from inpatient care.
During labour, if > 7 days have elapsed since GBS antibiotic prophylaxis and the patient is GBS positive, antibiotics should be administered again.
The consequences of premature delivery are discussed in the topic on prematurity. In general, viability is very uncertain with a gestational age <24 weeks. If >24 weeks, risks depend on gestational age, but include respiratory distress syndrome, intraventricular hemorrhage, and necrotizing enterocolitis.
The risk of recurrence in fufure pregnancy is 15-20%, though depends on history and circumstances.
Although the diagnosis and treatment of preterm labour are fraught with controversy, there are areas of consensus. Preconception counseling and early prenatal care that identifies and treats risk conditions can optimize pregnancy outcome. The physician must try to accurately date a patient's pregnancy, attempt to diagnose preterm labour at an early stage and make the appropriate management decision for the patient. This decision may include transfer to a tertiary site or management with appropriate consultation. Fetal fibronectin, a biochemical marker, may be a useful diagnostic tool in the future, but there are insufficient outcome data to justify its use at present. Documented infections such as sexually transmitted diseases, urinary tract infections and vaginitis should be treated. Tocolytic therapy should be used to delay delivery in order to administer corticosteroids.
Corticosteroid therapy, group B streptococcus prophylaxis, and expert rescusitation and care improve outcomes when delivery is warranted or inevitable.