This chapter deals primarily with APGO Educational Topic Area:
TOPIC 24 PRETERM LABOR
Students should be able to list risk factors, possible etiologies, and complications of preterm labor. They should be able to outline a basic approach to evaluation and management, including appropriate medications and their contraindications. They should be able to counsel a patient on risk reduction for preterm birth.
Clinical Case
A 34-year-old patient was admitted to your antepartum service for advanced cervical dilation at 24 weeks of gestation. She had one other preterm delivery at 33 weeks of gestation. She was placed on progesterone for prevention of preterm delivery. She has intermittently had light vaginal bleeding. Today, at 27 weeks, the patient has another episode of bleeding and begins contracting regularly. She is very worried about her baby being delivered so early. What are the next steps in her evaluation and management? What can you do to prevent the sequelae of prematurity in the neonate?
Preterm birth is delivery that occurs prior to the completion of 37 completed weeks (259 days) of gestation. Because it is the most common cause of perinatal morbidity and mortality in the United States, prevention and treatment of preterm birth is a major focus of obstetric care. The consequences of preterm birth occur with increasing severity and frequency the earlier the gestational age of the newborn. In addition to perinatal death in the very young fetus, common complications of preterm birth include respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis, sepsis, neurologic impairment, and seizures. Long-term morbidity associated with preterm delivery includes bronchopulmonary dysplasia and developmental abnormalities, including cerebral palsy. The 11% to 12% of babies born prematurely account for 75% of all perinatal mortality and 50% of long-term neurologic impairment in children in the United States.
Preterm births may be classified into two general presentations: spontaneous and indicated. Approximately 40% to 50% of preterm births result from spontaneous preterm labor with intact membranes; 25% to 40% result from preterm premature rupture of membranes (PPROM; see Chapter 17). The remaining 20% to 30% occur following deliberate intervention for a variety of maternal or obstetric complications (e.g., eclampsia).
Preterm labor is defined as the presence of regular uterine contractions that occur before 37 completed weeks of gestation and are associated with cervical changes. It is often difficult to diagnose preterm labor because of the absence of definitive measurements. The lack of diagnostic criteria presents a problem, because treatment appears to be more effective when initiated early in the course of preterm labor.
CAUSE, PREDICTION, AND PREVENTION OF PRETERM LABOR
Causes
Preterm labor may represent a final common pathway for a number of pathogenic processes. The four main processes include 1) activation of the maternal or fetal hypothalamic– pituitary–adrenal axis due to maternal or fetal stress, 2) decidual–chorioamniotic or systemic inflammation caused by infection, 3) decidual hemorrhage, and 4) pathologic uterine distention (Fig. 15.1). Numerous risk factors have been associated with preterm labor (Box 15.1). The strongest risk factors are multifetal gestation and prior preterm birth. With a prior preterm birth, the risk in a subsequent pregnancy increases and continues to increase with each subsequent preterm pregnancy. Subclinical intra-amniotic infection has also been associated with preterm labor and PPROM, especially when it occurs at earlier gestational ages. In most cases, however, no cause or risk factor for preterm labor can be identified.
Factors Improving Outcomes
Despite the lack of effective strategies to predict and prevent preterm labor, infant morbidity and mortality following preterm birth have decreased over the last several decades as the result of several factors. First, neonatal intensive care management of preterm infants has greatly improved outcomes. Therefore, maternal transport to a regional tertiary care center is indicated for women in preterm labor presenting to hospitals without sophisticated neonatal intensive care. Second, the use of corticosteroids administered to a mother at immediate risk for preterm birth (such as a woman in preterm labor) has resulted in decreased incidence of respiratory distress syndrome, intraventricular hemorrhage, and associated infant morbidity and mortality. A major goal of therapy to stop contractions in a woman in preterm labor (tocolytic therapy) is to prolong pregnancy for up to 48 hours in order to allow time to administer corticosteroids. Third, magnesium sulfate administered prior to a preterm birth has been shown to decrease the rate of cerebral palsy in infants born preterm. Finally, prophylaxis against perinatal infection with group B streptococcus (GBS) in women with preterm labor or preterm PPROM has also decreased infant morbidity and mortality rates in the United States.
FIGURE 15.1. Preterm labor: final common pathway. The four main processes include activation of the maternal or fetal hypothalamic–pituitary–adrenal (HPA) axis, infection, decidual hemorrhage, and pathologic uterine distention. CRH, corticotropin-releasing hormone; CSF, colony-stimulating factor; E1, estrone; E3, estriol; FasL, FAS ligand; IL, interleukin; OT, oxytocin receptor; PG, prostaglandin; PTD, preterm delivery; TNF, tumor necrosis factor.
BOX 15.1 Factors Associated with Preterm Labor
Prior history of preterm birth
Preterm uterine contractions
Premature rupture of membranes
Behavorial risk factors:
• Low maternal prepregnancy weight
• Smoking
• Substance abuse
• Short interpregnancy interval
Current pregnancy factors:
• Vaginal bleeding
• Urinary tract infections
• Genital tract infection
• Periodontal disease
Prediction of Preterm Labor
Patient and physician education has focused on recognition of the signs and symptoms that suggest preterm labor (Box 15.2). Patients with symptoms are counseled to seek prompt medical attention. fFN and BV paragraphs—totally agree with elimination—am glad ACOG finally dumped these as I never agreed in the first place. Might add a sentence here to address since medical students may still hear/experience this discussion. “Other screening modalities in asymptomtic women, such as fetal fibronectin, bacterial vaginosis screening, and home uterine contraction monitoring, have been advocated in past; however, interventions based on results of these tests have not yielded improved perinatal outcome and are therefore, not recommended as screening tests for preterm labor.”
BOX 15.2 Symptoms and Signs of Preterm Labor
Menstrual-like cramps
Low, dull backache
Abdominal pressure
Pelvic pressure
Abdominal cramping (with or without diarrhea)
Increase or change in vaginal discharge (mucous, watery, light bloody discharge)
Uterine contractions, often painless
Cervical Changes
Cervical length can be used as a diagnostic factor. As cervical length decreases in midpregnancy, the risk of preterm birth has been shown to increase in a continuous fashion.Transvaginal ultrasoundexamination of the cervix is a reliable and reproducible method to assess cervical length. This test may be most helpful when evaluating women at high risk for recurrent preterm birth, those with uterine anomalies, and those who have had prior cervical cone biopsy or multiple dilation and curettage/evacuation procedures.
Early asymptomatic dilation and effacement of the cervix (cervical insufficiency) may be associated with an increased likelihood of preterm labor and delivery. Interventions such as prophylactic cervical cerclage on sonographic recognition of a shortened cervical length (often defined as less than 2.5 cm) in low-risk women have not improved outcomes; however, placement of a cerclage in high-risk women (for example, history of prior preterm birth) with a shortened cervix may be beneficial.
Other screening modalities in asymptomatic women, such as fetal fibronectin, bacterial vaginosis screening, and home uterine contraction monitoring, have been advocated in the past; however, interventions based on results of these tests have not yielded improved perinatal outcomes and are, therefore, not recommended as screening tests for preterm labor.
Prevention
There are currently no uniformly effective interventions to prevent preterm labor, regardless of risk factors. Prophylactic therapy—including tocolytic drugs, bed rest, hydration, and sedation in asymptomatic women at high risk for preterm labor—has not been shown to be effective. However, in a select group of women at very high risk who have a documented history of preterm birth, the use of weekly intramuscular injections of progesterone (17-α-hydroxyprogesterone caproate) starting at 16 to 20 weeks of gestation and continuing until 36 weeks of gestation appears to reduce spontaneous preterm birth. Vaginal progesterone supplementation in women with an ultrasonically determined shortened cervical length has also shown some benefit.
EVALUATION OF A PATIENT IN SUSPECTED PRETERM LABOR
Prompt evaluation is critical in the patient who describes symptoms and signs suggestive of preterm labor. Use of an external electronic fetal monitor (tocodynamometer) may help to quantify the frequency and duration of contractions. The status of the cervix should be determined, either by visualization with a speculum or by gentle digital examination. Because digital examination may increase the risk of infection in the setting of PROM, speculum evaluation to assess cervical dilation and effacement should be performed first if there is suspicion of rupture of fetal membranes. Changes in cervical effacement and dilation on subsequent examinations are important in the evaluation of both the diagnosis of preterm labor and the effectiveness of management. Subtle changes are often of great clinical importance, so serial examinations by the same examiner are optimal, when this is possible.
Laboratory Tests
Because urinary infections can predispose a patient to uterine contractions, a urinalysis and urine culture should be obtained. A vaginal/rectal culture should be obtained for GBS. Women with GBS bacteriuria are candidates for intrapartum antibiotic prophylaxis. When indicated by history or physical examination findings, cultures for Chlamydia trachomatis and Neisseria gonorrhoeae should be obtained.
Ultrasound
Ultrasound examination is useful in assessing the gestational age of the fetus, estimation of the amniotic fluid volume (spontaneous rupture of membranes with fluid loss may precede preterm labor and may be unrecognized by the patient), fetal presentation, and placental location, as well as the existence of fetal congenital anomalies. Patients should also be monitored for bleeding, insofar as placental abruption and placenta previa may be associated with preterm labor (see Chapter 16).
Information concerning the length of the cervix can be obtained through ultrasound examination, although results are not particularly helpful unless the gestational age is less than 26 weeks.
Amniocentesis
Amniocentesis may be performed to assess for intra-amniotic infection. Either clinical or subclinical infection of the amniotic cavity (chorioamnionitis) is thought to be associated with preterm labor. Amniotic fluid can be evaluated for the presence of bacteria, white blood cells (WBCs), lactate dehydrogenase, and glucose. Evidence of WBCs in the amniotic fluid, decreased glucose, or elevated lactate dehydrogenase may indicate infection. The presence of bacteria in amniotic fluid is correlated not only with preterm labor but also with the subsequent development of infection. A high suspicion of intrauterine infection should prompt delivery regardless of the gestational age. Tocolysis is not appropriate in the setting of intrauterine infection. At the time of amniocentesis, additional amniotic fluid may be obtained for fetal pulmonary maturity studies, which could influence subsequent management.
MANAGEMENT OF PRETERM LABOR
The purpose in treating preterm labor is to delay delivery, if possible, until fetal maturity is attained. Management involves two broad goals: 1) the detection and treatment of disorders associated with preterm labor and 2) therapy for the preterm labor itself. Fortunately, more than 50% of patients with preterm contractions have spontaneous resolution of abnormal uterine activity. However, this complicates the evaluation of effectiveness of specific treatments, because it is unclear if the contractions would have resolved spontaneously or if their cessation was due to effective treatments.
Tocolytics
Various tocolytic therapies have been used in the management of preterm labor (Table 15.1). Different treatment regimens address specific mechanisms involved in the maintenance of uterine contractions, and each, therefore, may be best suited for certain patients.
Typically, patients with a diagnosis of preterm labor receive one form of tocolytic therapy, with the addition or substitution of other medications if the initial treatment is considered unsuccessful.
The use of nifedipine as a tocolytic is increasing. In the past, magnesium sulfate has been used as a tocolytic agent; however, accumulated data show that it is ineffective when used for this purpose. Evidence is building that magnesium sulfate, administered antenatally to women with preterm labor, has a neuroprotective effect on the fetus, appearing to lower the risk of developing cerebral palsy. Evidence as to the efficacy of tocolytics beyond several days is weak, but, often, intervention with medication allows enough time for administration of corticosteroid therapy to accelerate fetal lung maturation. Adverse side effects, at times serious and even life-threatening to the mother, can occur. The gestational age of the fetus is always a consideration in deciding how vigorously to pursue therapy. For example, maternal risks may be more acceptable when treating a 26-week fetus as compared with a 32-week fetus.
Contraindications
Contraindications to tocolysis include conditions in which the adverse effects of tocolysis may be significant, such as advanced labor, a mature fetus, a severely anomalous fetus (from lethal congenital or chromosomal abnormalities), intrauterine infection, significant vaginal bleeding, and severe preeclampsia. In addition, a variety of obstetric complications, such as placental abruption, advanced cervical dilation, or evidence of fetal compromise or placental insufficiency, may contraindicate delay in delivery.
Corticosteroids
From 24 to 34 weeks of gestation, management generally includes administration of corticosteroids (betamethasone or dexamethasone) to enhance fetal pulmonary maturity. A single course of corticosteroids should be given to pregnant women between 24 and 34 weeks of gestation who are at risk for preterm delivery within 7 days. Both the incidence and severity of fetal respiratory distress syndrome are reduced with such therapy. In addition, other sequelae of prematurity, such as interventricular hemorrhage and necrotizing enterocolitis, occur less frequently in infants whose mothers received corticosteroid therapy. Maximal benefit to the fetus occurs if the therapy is administered within 7 days of delivery. Routine weekly courses of therapy are not recommended.
Clinical Follow-Up
The patient’s vital signs and hematocrit are stable. The fetal heart tone is reassuring. The bleeding has decreased; however, you are still concerned for possible abruption and decide not to tocolyse the patient. You do administer a course of antenatal steroids, antibiotic for group B streptococcus prophylaxis, and magnesium for neuroprotection of the fetus.
thePoint Visit http://thepoint.lww.com/activate for an interactive USMLE-style question bank and more!