Preterm Labor

Preterm labor is defined as uterine contractions occurring before 37 weeks of gestation that cause cervical change. Cervical change can be diagnosed if an initial examination reveals a cervix that is at least 2 cm dilated or 80% effaced, or if interval cervical examinations document progression of effacement or dilation. Preterm contractions without cervical advancement can also occur; these do not require intervention. The distinction may be difficult, in particular at the onset of contractions.

Certain women are at higher risk of developing preterm labor (Box 21-6). These women should be assessed more frequently in the prenatal period and instructed on signs and symptoms of premature labor. While many tests to identify women at risk of preterm labor have been proposed and evaluated, only ultrasound for cervical length and fetal fibro-nectin have been shown to be effective (ACOG, 2001; Iams et al., 1996). A short cervix by transvaginal ultrasound and positive cervicovaginal fetal fibronectin test is predictive of an increased risk of preterm delivery in the index pregnancy.

Assessment of patients thought to be in premature labor involves monitoring for premature contractions as well as fetal heart rate while they are in the right or left lateral recumbent position. Complete history and physical exam should be performed to find any treatable causes

Box 21-6 Risk Factors for Preterm Labor

Low socioeconomic status Uterine anomalies African American ethnic group Uterine leiomyoma Poor nutritional state Bacterial vaginosis

Low maternal weight (less than 50 Kg)

Multiple gestation

Poor pregnancy weight gain

Bacteriuria or urinary tract infection

Prior preterm labor

Placenta previa or abruptio

Cocaine use

Polyhydramnios

Nicotine use

Poor prenatal care of premature labor as well any contraindications to tocolytic therapy. Urinalysis, as well as culture, is obtained and antibiotic therapy instituted if the urinalysis is suspicious. If there is a possibility of rupture of membranes, a sterile speculum examination of the cervix should be performed and vaginal fluid for nitrazine and ferning obtained. Cultures for GBS, Chlamydia, and possibly herpesvirus and Neisseria gonorrhoeae are often performed in this setting. If there is no historical or physical evidence of rupture of membranes, digital examination of the cervix with careful assessment of consistency as well as dilation and effacement is performed. Chorioamnionitis should be ruled out by assessing degree of uterine tenderness, leukocytosis, maternal fever, and fetal well-being.

In a woman at 22 to 35 weeks' gestation, before digital examination, assessment of the presence of fetal fibronectin can be determined. Fetal fibronectin is released from the interface of the chorion and decidua in women likely to deliver preterm. Fetal fibronectin can be determined in 1 hour by most hospital laboratories and is used predominantly for its high negative predictive value. Thus, if a fetal fibronec-tin is negative, the woman will likely not deliver for at least 7 to 10 days; if positive, closer surveillance or treatment is warranted.

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