Several studies have investigated the incidence of infectioninduced PPROM. Bacterial cultures of amnionic luid support a role for infection in a significant proportion. One review of 18 studies and almost 1500 women with PPROM found that bacteria were isolated from amnionic fluid in a third of cases (Goncalves, 2002). Accordingly, some have given antimicrobial treatment to women in spontaneous preterm labor with intact membranes, however, results have been disappointing as discussed later on page 825 (Kenyon, 2008b).
The inflammatory response that leads to membrane weakening and mediators of this process are currently areas of research. One goal is to identiy early risk markers for PPROM.
Twins and higher-order multifetal births account for approximately 3 percent of neonates born in the United States (Martin, 2017). Preterm delivery continues to be the major cause of the excessive perinatal morbidity and mortality with multifetal pregnancies. The efects of uterine stretch discussed on page 809 are obvious. Many of these interrelationships are discussed in Chapter 45 (p. 885).
Several genetic and environmental factors afect the frequency of preterm labor. Of these, threatened abortion in early pregnancy is associated with higher rates of later adverse outcomes. Weiss (2004) reported outcomes with vaginal bleeding at 6 to 13 weeks' gestation in nearly 14,000 women. Both light and heavy bleeding were associated with subsequent preterm labor, placental abruption, and pregnancy loss before 24 weeks.
Birth defects in the fetus may also predispose to preterm birth. In a secondary analysis of data from the First-and Second-Trimester Evaluation of Risk (FASTER) Trial, birth defects were associated with preterm birth and low birthweight neonates (Dolan, 2007).
Cigarette smoking, inadequate maternal weight gain, and illicit drug use afect the incidence and outcome of low-birthweight neonates (Chap. 44, p. 849). Extremes of maternal weight-both underweight and obese mothers-have an enhanced risk of preterm birth (Cnattingius, 2013; Girsen, 2016). Other maternal factors implicated include young or advanced maternal age, poverty, short stature, and vitamin C deficiency (Casanueva, 2005; Gielchinsy, 2002; Kramer, 1995; Leveno, 2009; Meis, 1995).
As discussed on page 809, psychological factors such as depression, anxiety, and chronic stress are associated with preterm birth (Hofman, 2016; Venkatesh, 2016). In one review of more than 50 studies, Donovan and coworkers (2016) found a signiicant link between low birthweight and preterm birth in women injured by physical abuse (Chap. 47, p. 925).
Studies of work and physical activity related to preterm birth have yielded conlicting results (Goldenberg, 2008). Some evidence suggests that working long hours and hard physical labor are probably linked to a higher risk of preterm birth (Luke, 1995). However, aerobic exercise in normal-weight women with uncomplicated singleton pregnancies appears to be safe and not associated with preterm birth (American College of Obstetricians and Gynecologists, 2017d; Di Mascio, 2016). One metaanalysis of physical activity found that leisure-time physical activity was associated with a reduced risk of preterm birth (Aune, 2017).