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Key Clinical Updates in Preterm Labor

The recommended regimen for antimicrobial prophylaxis against group B streptococcus is penicillin G, 5 million units intravenously as a loading dose and then 2.5–3 million units intravenously every 4 hours until delivery. In penicillin-allergic patients not at high risk for anaphylaxis, 2 g of cefazolin can be given intravenously as an initial dose and then 1 g intravenously every 8 hours until delivery. In patients at high risk for anaphylaxis, vancomycin, 20 mg/kg intravenously every 8 hours until delivery, can be used. Clindamycin, 900 mg intravenously every 8 hours until delivery, can also be used after a group B streptococcal isolate has been confirmed to be susceptible to clindamycin.

American College of Obstetricians and Gynecologists. Obstet Gynecol. [PMID: 34794160]

ESSENTIALS OF DIAGNOSIS

  • Preterm regular uterine contractions approximately 5 minutes apart.

  • Cervical dilatation, effacement, or both.

GENERAL CONSIDERATIONS

Preterm birth is defined as birth between 20 0/7 and 36 6/7 weeks’ gestation, and spontaneous preterm labor with or without premature rupture of the fetal membranes causes at least two-thirds of all preterm births. Prematurity is the largest single contributor to infant mortality, and survivors are at risk for a myriad of short- and long-term complications. It also the most common reason for antepartum hospitalization. Rates of infant death and long-term neurologic impairment are inversely related to gestational age at birth. The cusp of viability in contemporary practice is 23–25 weeks’ gestation, and infants born before 23 weeks rarely survive. About two-thirds of the preterm births occur between 34 weeks and 36 weeks and 6 days (termed “late preterm birth”), and good outcomes are expected at these gestational ages. Importantly, however, even these late preterm infants are at significantly increased risk for both morbidity and mortality when compared to those infants born at term.

Major risk factors for spontaneous preterm labor include a past history of preterm birth and a short cervical length as measured by transvaginal ultrasound. Other known risk factors include Black race, multifetal pregnancies, intrauterine infection, substance abuse, smoking, periodontal disease, and socioeconomic deprivation. Numerous preterm births are preceded by ruptured membranes.

CLINICAL FINDINGS

In women with regular uterine contractions and cervical change, the diagnosis of preterm labor is straightforward. However, symptoms such as pelvic pressure, cramping, or vaginal discharge may be the first complaints in high-risk patients who later develop preterm labor. Because these complaints may be vague and irregular uterine contractions are common, distinguishing which patients merit further evaluation can be problematic. In some cases, this distinction can be facilitated by the use of fetal fibronectin measurement in cervicovaginal specimens. This test is most useful when it is negative (less than 50 ng/mL), since the negative predictive value for delivery within 7–14 days is 93–97%. A negative test, therefore, usually means the patient can be reassured and discharged home. Because of its low sensitivity, ...

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