(1)
Department of Family Medicine, University of California, Riverside, Riverside, CA, USA
Key Points
1.
Preterm labor is uterine contractions resulting in progressive cervical change prior to 37 weeks’ gestation. Preterm delivery is delivery prior to 37 weeks’ gestation; low-birth weight infants are those that weigh less than 2500 g at delivery.
2.
Prior to 34 weeks’ gestation, most patients should be considered for tocolysis; from 34 to 37 weeks’ gestation such decisions must be made on a case-by-case basis.
3.
Complications associated with preterm delivery include increased perinatal mortality and complications of prematurity (including respiratory distress, gastrointestinal dysfunction, hemorrhage, and abnormalities of growth and development).
Background
Preterm labor is among the most common and most serious of prenatal complications. Preterm labor and its potential sequelae of preterm delivery and low-birth weight (LBW) infants remain one of the most significant challenges of current obstetrical practice. Preterm labor is defined as uterine contractions resulting in progressive cervical change prior to 37 weeks’ gestation. Preterm delivery is delivery prior to 37 weeks’ gestation. LBW infants are defined as those infants weighing less than 2500 g at delivery regardless of gestational age. LBW infants should be distinguished from small-for-gestational-age (SGA) infants who are defined as those infants below the fifth percentile for weight based on gestational age.
Preterm labor affects approximately 10 % of all pregnancies. Preterm delivery affects approximately 13 % of all live births. Preterm delivery and LBW infants represent approximately 70 % of all perinatal mortality (~25,000 deaths annually) and 50 % of all neurological morbidity.
Factors Associated with Preterm Labor
A number of factors have been associated with an increased risk of preterm labor. These are summarized in Table 7.1. These factors can be divided into pre- and postconception factors. Although the mechanisms that link these factors to the onset of preterm labor is, in most instances, poorly understood, a thorough review of the patient’s history will allow providers to more carefully outline the risk of preterm labor, preterm delivery, and LBW infants.
Table 7.1
Risk factors for preterm labor
Preconception factors |
Lower socioeconomic status |
Anatomic abnormalities (e.g., septate/bicornuate uterus, cervical incompetence) |
Prior uterine surgery |
Myomata |
Diethylstilbestrol exposure |
Past history of preterm labor |
Under 18 years old, over 40 years old |
Possible genetic predisposition |
Postconception factors |
Tobacco |
Cocaine |
Infection (e.g., group B streptococcus, N. gonorrhea, C. trachomatis, trichomonas, gardnerella, ureaplasma, mycoplasma) |
Although preterm labor alone is not associated with perinatal complications, concomitant conditions and outcomes are. Preterm labor may be complicated by preterm premature rupture of membranes (PPROM). PPROM is associated with a variety of complications discussed in Chap. 8. Preterm labor may also result in preterm delivery. Prematurity, in turn, is potentially associated with pulmonary dysfunction, gastrointestinal abnormalities, neurological complications, abnormalities of growth and development, and a significant risk of perinatal mortality. Complications of preterm delivery are the leading cause of perinatal mortality, responsible for approximately two-thirds of all deaths.
Preconception Factor
Environmental Factors
A number of environmental factors have been associated with an increased risk of preterm labor. The most significant environmental factor associated with preterm labor is lower socioeconomic status.
Patient-Related Factors
A number of pre-existing patient conditions also contribute to the risk of preterm labor. Patients may have a pre-existing genetic risk, a congenital anomaly (e.g., septate/bicornuate uterus or cervical incompetence), a pre-existing acquired obstetrical/gynecological risk (e.g., myomata, uterine surgery, diethylstilbestrol exposure), or a past history of preterm labor or second trimester spontaneous abortions. The recurrence rate of preterm labor is approximately 25 %. Additionally, the risk of preterm labor is highest among younger (<18 years old) and older (>40 years old) obstetrical patients. These conditions can be screened for early in pregnancy (or during preconception counseling). Although many of these factors are not modifiable, their presence can usefully contribute to a conversation between the provider and the patient concerning the risk for preterm labor during the current pregnancy.
Postconception Factors
Once conception occurs, a number of additional factors contribute to the risk of preterm labor. An increased risk for preterm labor is associated with tobacco and cocaine use. Infections such as group B streptococcus, gonorrhea, chlamydia, trichomonas, gardnerella, ureaplasma, and mycoplasma have all been associated with increased preterm labor risk. Such exposures should be screened for at the first prenatal history (either directly through testing or via history) and as appropriate throughout the course of pregnancy.
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