Williams Manual of Pregnancy Complications, 23 ed.

CHAPTER 33. Prediction of Preterm Birth

Obstetrical approaches to preterm birth have traditionally focused primarily on treatment interventions rather than prevention of preterm labor. The first step necessary for prevention is prediction of women at risk for preterm birth.


A history of prior preterm delivery strongly correlates with subsequent preterm labor. Table 33-1 gives the incidence of recurrent spontaneous preterm birth in nearly 16,000 women delivered at Parkland Hospital.

TABLE 33-1. Recurrent Spontaneous Preterm Births According to Prior Outcome in 15,863 Women Delivering Their First and Subsequent Pregnancies at Parkland Hospital



In addition to painful or painless uterine contractions, symptoms such as pelvic pressure, menstrual-like cramps, watery or bloody vaginal discharge, and pain in the low back have been empirically associated with impending preterm birth. Such symptoms are thought by some to be common in normal pregnancy, and are therefore often dismissed by patients, physicians, and nurses. The importance of these signs and symptoms has been emphasized by some investigators. Conversely, others have not found these to be meaningful in the prediction of preterm birth. It has been shown that these signs and symptoms signaling pre-term labor, including uterine contractions, only appear within 24 hours of preterm labor. Thus, these are late warning signs.


The diagnosis of preterm labor, before it is irreversibly established, is a goal of management. To this end, uterine activity monitoring, using tocodynamometry, has received considerable interest. Subsequent widespread clinical application of home uterine contraction monitoring for the purpose of preventing preterm birth has provoked considerable controversy in the United States. The American College of Obstetricians and Gynecologists (Preterm labor. Technical Bulletin No. 206, June 1995) continues to take the following position: “It is not clearly demonstrated that this expensive and burdensome system can be used to actually affect the rate of preterm delivery.”


Asymptomatic cervical dilatation after midpregnancy has gained attention as a risk factor for preterm delivery. At Parkland Hospital, approximately one-fourth of women whose cervices were dilated 2 or 3 cm between 26 and 30 weeks deliver before 34 weeks. Many of these women had experienced the same complication in earlier pregnancies. Although it seems clear that pregnant women with cervical dilatation and effacement diagnosed early in the third trimester are at increased risk for preterm birth, it has not been established that detection appreciably improves pregnancy outcome.


Transvaginal sonography can be used to measure the length of the cervix but requires special expertise. Some authors caution those who perform these examinations to be wary of falsely reassuring findings due to potential anatomical and technical pitfalls. It seems that the use of ultrasonographic cervical measurements can increase the ability to predict spontaneous birth prior to 35 weeks in high-risk women. The use of cerclage and progesterone supplementation for women with shortened cervical length is gaining popularity.


Fibronectin is a glycoprotein produced in 20 different molecular forms by a variety of cell types, including hepatocytes, malignant cells, fibroblasts, endothelial cells, and fetal amnion. It is present in high concentrations in maternal blood and in amnionic fluid, and is thought to have a role in intercellular adhesion in relation to implantation as well as in the maintenance of adhesion of the placenta to the decidua. Fetal fibronectin (FFN) can be detected in cervicovaginal secretions in normal pregnancies with intact membranes at term, and appears to reflect stromal remodeling of the cervix prior to labor. Detection of FFN in cervicovaginal secretions prior to membrane rupture may be a marker for impending preterm labor, and this has stimulated considerable interest in the use of fibronectin assays for the prediction of preterm birth. Fetal fibronectin is measured using an enzyme-linked immunosorbent assay and values exceeding 50 ng/mL are considered a positive result. Contamination of the sample by amnionic fluid and maternal blood should be avoided. Current recommendations by the American College of Obstetricians and Gynecologists (Assessment of risk factors for preterm birth. Practice Bulletin No. 31, October 2001) for FFN testing are shown in Table 33-2.

TABLE 33-2. Current Recommendations for Use of Fetal Fibronectin Testing of Cervical-Vaginal Secretions for Prediction of Preterm Birth


The association of either a sonographically short cervix (i.e., less than 25 mm) at less than 35 weeks of gestation plus a positive fetal fibronectin test is strongly associated with preterm birth, especially in women who have a history of pre-term birth (Table 33-3). Both the use of transvaginal cervical ultrasound and fetal fibronectin may lie in their negative predictive values given the lack of proven treatment options to prevent preterm birth.

TABLE 33-3. Recurrence Risk of Spontaneous Preterm Birth at <35 Weeks of Gestation According to Cervical Length and Fetal Fibronectin in Women with a Prior Preterm Birth



Bacterial vaginosis (BV) is a condition in which the normal hydrogen peroxide-producing, lactobacillus-predominant vaginal flora is replaced with anaerobic bacteria, Gardnerella vaginalis, Mobiluncusspecies, and Mycoplasma hominis. Clinical diagnostic features include the following:

1. Vaginal pH greater than 4.5.

2. An amine odor when vaginal secretions are mixed with potassium hydroxide.

3. Vaginal epithelial cells heavily coated with bacilli—“clue cells.”

4. A homogeneous vaginal discharge.

Bacterial vaginosis can also be diagnosed with Gram staining of vaginal secretions. Typically a Gram stain of vaginal secretion in women with BV shows few white cells along with a mixed flora as compared with the normal predominance of lactobacilli.

Bacterial vaginosis has been associated with spontaneous preterm birth, pre-term ruptured membranes, infection of the chorion and amnion, as well as amnionic fluid infection.


Prophylactic treatment of at-risk women (prior preterm birth) with weekly intramuscular injections of 250 mg of 17-alpha-hydroxyprogesterone caproate has recently been associated with a reduction of preterm delivery and perinatal mortality. Similar reductions in at-risk women have been reported with the use of 100-mg natural progesterone suppositories. The American College of Obstetrics and Gynecology recommends offering progesterone for pregnancy prolongation to women with a documented history of a previous spontaneous birth at less than 37 weeks of gestation.

For further reading in Williams Obstetrics, 23rd ed.,

see Chapter 36, “Preterm Birth.”