Premature Labor

At a Glance

Premature delivery is defined as delivery before the completion of 37 weeks of gestation. The causes of preterm delivery are not completely understood but may include preterm labor, preterm spontaneous rupture of membranes, cervical incompetence, or other medical problems. Since clinical and laboratory evidence suggest multi-factorial causes of preterm delivery, diagnostic, preventive, and treatment efforts aimed at a single etiology have been unsuccessful to date.

Traditionally used methods for predicting preterm delivery, such as obstetrical history, demographic factors, or symptoms of labor, are neither sensitive nor specific. Various biochemical and biophysical approaches have been evaluated. However, even after a clear diagnosis of preterm labor, the lack of proven treatment options to prevent preterm delivery remains a challenge.

Patients with symptoms of preterm labor, such as menstrual-like cramping, constant low backache, uterine contractions at infrequent and/or irregular intervals, and increased vaginal discharge, warrant further evaluation. However, these symptoms are not specific in predicting which women will deliver preterm, since normal pregnancy is also associated with these common symptoms.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The most commonly used biochemical test is measurement of fetal fibronectin (fFN) in cervicovaginal fluid to distinguish women at high risk for preterm labor from those who are not. fFN is performed alone or combined with ultrasonography to assess cervical length.

The following criteria must be present for measuring cervicovaginal fFN:

high risk women with signs and symptoms of early preterm labor

intact amniotic membranes

minimal cervical dilation less than 3 cm

sampling performed no earlier than 24 weeks and 0 days and no later than 34 weeks and 6 days gestational age

Testing in the first half of pregnancy or after rupture of amniotic membranes is not recommended for predicting preterm delivery, as fFN is normally present in cervicovaginal discharge prior to 20-22 weeks gestation and is always present in amniotic fluid.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?

Semen or sperm from coitus within the previous 24 hours or a grossly bloody specimen can cause false-positive fFN results. False-positive or false-negative fFN results can also be caused by actions, such as manipulation of the cervix by digital or ultrasound examination or an introduction of intravaginal substances like lubricants or medications prior to obtaining the sample. If fFN and sonographic determination of cervical length are conducted together, the Dacron fFN swab must be obtained before cervical sonography.

What Lab Results Are Absolutely Confirmatory?

There is limited or inconsistent scientific evidence to help determine the optimal approach to management of women with suspected preterm labor and intact membranes. The approach subsequently described is based on the American College of Obstetrics and Gynecology (ACOG) practice bulletin and data from observational studies.

Ultrasonography to determine cervical length, fFN testing, or a combination of both may be useful in identifying women at high risk or with symptoms of preterm labor. The most beneficial clinical feature may rest primarily with the negative predictive value (i.e., the ability to pick out women least likely to deliver). Identifying women at low risk for preterm delivery avoids unnecessary interventions.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Commonly, transvaginal cervical sonography is performed to obtain cervical length. The cut-off values are variously defined. Several studies have consistently found that a length of 20 mm or less had the best positive predictive value and a length of 30 mm or more had the best negative predictive value for preterm delivery. Preterm delivery is more likely in women with cervical length between 20 and 30 mm than in women with longer cervices. However, most women in this group do not deliver preterm. Therefore, selective use of fFN analysis is suggested to support or exclude the diagnosis of preterm labor and guide therapy.

fFN testing has a high negative predictive value (i.e., among pregnant women presenting with signs and symptoms of preterm labor, 99.5% subsequently have a negative cervicovaginal fFN test and remain undelivered within 7 days). The positive predictive value is much lower. ACOG suggested that, if the fFN test is to be clinically useful, the results must be available from a laboratory within a time frame that allows for clinical decision making (ideally within 24 hours).

The combination of fFN testing and transvaginal cervical sonography may help avoid false-positive results and unnecessary interventions.

In low risk, asymptomatic women, neither fFN testing nor transvaginal cervical sonography is recommended to screen for risk of preterm delivery. Although negative test results may be useful in ruling out preterm delivery within 2 weeks, the clinical implications of positive test results have not been evaluated fully. The utility of these tests for predicting preterm delivery is low, and few potential effective treatments are available. Therefore, screening for risk of preterm delivery by means other than historic risk factors is not beneficial in the general obstetric population.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?

Using cotton swabs and glass can cause false-negative fFN results as fFN binds to these substances. Large numbers of red or white blood cells or bacteria may cause interference with the fFN test. Since fFN may bind to Candida, a vaginal yeast, the results of fFN test may be falsely negative if a patient has a vaginal yeast infection.

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