Preterm labor: Clinical sciences

Preterm labor is defined as regular uterine contractions accompanied by a change in cervical dilation, effacement, or both, that occurs between 20 and 0/7 weeks of gestation and 36 and 6/7 weeks of gestation. It’s diagnosed when a patient presents with regular uterine contractions and a cervical dilation of at least 3 cm. Preterm labor can be either spontaneous or preceded by premature rupture of membranes; or it can be iatrogenic due to maternal or fetal causes necessitating early delivery. The majority of patients who initially present with evidence of preterm labor do not deliver prematurely; however, when labor progresses to preterm birth, there’s an increased risk of neonatal morbidity and mortality.

When a patient presents with a chief concern suggesting preterm labor, your first step is to perform a focused history and physical exam. To meet criteria for preterm labor your patient’s pregnancy must be at least 20 weeks and less than 37 weeks of gestation. They may report uterine contractions, abdominal cramps, low back pain, pelvic pressure, spotting, or vaginal bleeding.

Additionally, history might reveal some important risk factors for preterm birth, including a prior preterm birth; short interval pregnancy, defined as less than 18 months between deliveries; a known short cervix, meaning less than 25 mm in the current pregnancy; past procedures on the cervix; infections during the current pregnancy, such as urinary or genital tract infections; maternal prepregnancy body mass index less than 18.5; extremes of maternal age, specifically those younger than 17 or older than 35 years; and tobacco or substance use during pregnancy. Also, certain social determinants of health are associated with preterm birth, specifically lower levels of education, living in an disadvantaged area, and lack of access to prenatal care. An additional important consideration is that preterm birth is higher for Non-Hispanic Black and Indigenous individuals.

Here’s a clinical pearl! The risk factors discussed refer to singleton pregnancies; however, multifetal gestations are at an increased risk of preterm labor and birth as well.

On physical exam, you’ll find regular uterine contractions, intact amniotic membranes, and possible vaginal bleeding. If your patient has ruptured membranes they are diagnosed with preterm, premature rupture of membranes, or PPROM, and not preterm labor.

Okay, if your patient is at least 20 weeks but less than 34 weeks of gestation, their labor is considered early preterm. Your next step is to obtain a fetal fibronectin, or fFN, specimen, by performing a sterile speculum exam and collecting a sample of cervicovaginal secretions from the posterior vaginal fornix.

Fetal fibronectin is a protein that acts like a “glue” between the amniotic membranes and the uterus. Normally, fFN is found at low levels in cervicovaginal secretions; however, when found at higher levels, its presence is associated with an increased risk of spontaneous preterm birth. That being said, the positive predictive value of fFN is quite low, meaning that when used alone it is not a reliable indicator of which patients will go into preterm labor. Rather, the utility of the fFN test is in its high negative predictive value, so when the fFN is negative in a patient with preterm labor symptoms, you can be assured that your patient is not experiencing preterm labor.

Moreover, several factors can interfere with the accuracy of the fFN test. These include the use of lubricants, soaps, and vaginal progesterone; moderate or gross vaginal bleeding; and the presence of semen or sperm, most commonly found if the patient was sexually active within 24 hours. If these are present, the specimen should not be collected.

Next, assess cervical dilation. If your patient is at least 3 centimeters dilated, diagnose preterm labor and admit them to labor and delivery.

Now it's time to begin interventions that can help decrease neonatal morbidity and mortality. First administer antenatal steroids, which promote fetal lung maturation, and decrease the risk of respiratory distress syndrome, intracranial hemorrhage, necrotizing enterocolitis, and death. There are two steroids available which are administered intramuscularly: dexamethasone which is given in 4 doses every 12 hours, and betamethasone which is given in 2 doses 24 hours apart. A full course has the greatest benefit and a patient is considered to be “steroid complete” 48 hours after the first dose of medication. Even in instances where delivery is imminent and the patient will not receive the full course, there is still a benefit to giving a partial course of steroids.

Next, obtain a GBS culture and begin antibiotics for GBS prophylaxis, generally either IV penicillin or ampicillin. Continue the antibiotics until delivery or if cultures return negative. Additionally, if your patient is between 24 and 32 weeks of gestation, administer IV magnesium sulfate for fetal neuroprotection, as it reduces the risk and severity of cerebral palsy.

Finally, begin tocolysis by administering medications to inhibit uterine contractions. The goal of tocolysis is to allow for a complete course of antenatal steroids, administration of magnesium sulfate, and transport to a tertiary facility if indicated. Accordingly, tocolysis should be continued for up to 48 hours, when possible.

Here’s another clinical pearl! There are some situations where tocolysis is contraindicated. These include intrauterine fetal demise; lethal fetal anomaly; non-reassuring fetal status; intraamniotic infection; PPROM; maternal bleeding with hemodynamic instability such as with placental abruption; severe preeclampsia or eclampsia; and if a maternal contraindication to a specific medication used for tocolysis is present.

Tocolytic options include calcium channel blockers, or CCBs; non-steroidal anti-inflammatory drugs, or NSAIDs; beta-adrenergic receptor agonists; and magnesium sulfate. CCBs, like nifedipine, may cause dizziness, flushing, hypotension, and decreased maternal heart rate, so CCBs are contraindicated in patients with hypotension or preload-dependent cardiac lesions, like aortic insufficiency.

NSAIDs, like indomethacin, are safe for patients less than 32 weeks of gestation but are associated with in-utero constriction of the fetal ductus arteriosus and oligohydramnios, as well as necrotizing enterocolitis and patent ductous arteriosus in newborns. Contraindications to NSAIDs include platelet, renal, or hepatic dysfunction; a history of gastrointestinal ulcers; and asthma.