How is the risk for preterm labor assessed during pregnancy?

Updated: May 04, 2021
  • Author: Michael G Ross, MD, MPH; Chief Editor: Carl V Smith, MD  more...
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Assessment during the first prenatal exam should include the patient's obstetric history, infection risk, and the presence of cervical or uterine abnormalities. If an evaluation for antiphospholipid syndrome is included, it should include anticardiolipin (eg, anticardiolipin IgG and IgM and anti-beta2 microglobulin) and lupus anticoagulant antibodies.

Previous preterm deliveries, including autopsy reports and medical records, if appropriate and available, should be reviewed. Social stressors (including housing and food availability), social support in the family, financial stability, domestic violence, drug abuse involving the patient or her family, and death or serious illness in a close family member should be assessed.

The integrity of the cervix and the extent of any prior injury to the cervix may be assessed by speculum and digital examination. The presence of asymptomatic bacteriuria, STD, and symptomatic BV may be investigated.

In some patients, formal cervical length assessment may be of use in risk assessment.

Cervical length during prenatal care, particularly at 24-28 weeks’ gestation, has been demonstrated to be the most sensitive prenatal predictor of preterm birth between both high- and low-risk women. In a mixed high- and low-risk population of singleton pregnancies, transvaginal ultrasound-measured cervical length at 24 weeks was highly correlated with the risk of spontaneous preterm delivery before 35 weeks. [1] The relative risk of preterm delivery among women with a cervix 25 mm or shorter at 24 weeks was 6.2. Furthermore, at 28 weeks, a short cervix (≤25 mm) was associated with a 9.6 relative risk of preterm delivery. Cervical length 25 mm or shorter at 28 weeks had a 49% sensitivity for prediction of preterm delivery at less than 35 weeks, a value markedly greater than that of cervical funneling.

Among high-risk women with a history of one or more spontaneous preterm births (excluding those with multiple gestation, uterine anomalies, and prior cervical surgeries), 20% of patients demonstrated a cervical length shorter than 25 mm by transvaginal ultrasonography at 22-25 weeks. [10] Among these patients with a short cervix and one previous preterm birth, 37.5% delivered at less than 35 weeks. In contrast, patients with a cervical length longer than 25 mm had a preterm rate (< 35 wk) of only 10.6%. Cervical length has similarly been demonstrated as the optimal predictor of preterm delivery in low-risk women. In an assessment of low-risk women, short cervical length at 24-28 weeks was detected in 8.5% of women. [11] These patients demonstrated a relative risk of 6.9 for preterm delivery at less than 35 weeks. As compared with fetal fibronectin or Bishop score, cervical length demonstrated the greatest sensitivity (39%), with a specificity of 92.5% and a negative predictive value of 98%.

Whereas cervical length assessment by digital exam is a semi subjective measurement, a recent study has demonstrated the value of an objective cervico-portio length measurement using Cerivlenz, an intravaginal measuring device. [12] These manually obtained cervical length measurements appear to be reproducible, accurate, and predictive of a short cervical length by transvaginal ultrasonography. Therefore, Cerivlenz may represent a low-cost, objective screening tool to identify at-risk patients for preterm delivery.

In addition to the 24-28 week assessment, evidence shows the value of early midtrimester cervical length measurement. Studies of Owen et al from the Maternal Fetal Medicine Units Network [13] demonstrate the value of cervical length measurements between 16 weeks and 23 weeks and 6 days. Serial transvaginal ultrasonographic cervical length measurements in a high-risk population demonstrated that a cervix shorter than 25 mm resulted in a relative risk of 4.5 for spontaneous preterm birth at less than 35 weeks, with a 69% sensitivity, 80% specificity, 55% positive predictive value, and 88% negative predictive value. As the NIH Maternal Fetal Medicine Units Network is initiating a study of progesterone treatment for patients with a short cervix in the early midtrimester, a program of routine cervical length screening may soon be justified.

Cervical pessary devices, specifically the Arabin pessary, have received increasing interest for the prevention of preterm birth in women with a short cervix. However, there is inconclusive evidence that cervical pessary use decreases the rate of preterm birth or improves fetal outcomes for women at high risk. 

Among patients with a short cervix, education should be provided concerning the signs and symptoms of preterm labor, especially as the pregnancy approaches potential viability. Prenatal visits/contacts may be scheduled at more frequent intervals to increase patient interaction with the care provider, especially between 20 and 34 weeks’ gestation, which may decrease the rate of extreme preterm birth. [5]

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