he recurrent, familial, and racial nature of preterm birth suggests that genetics may playra causal role. Accumulating literature on genetic variants buttresses this concept (Gibson, 2007; Hampton, 2006; Macones, 2004; Velez, 2009). Several such studies have also implicated immunoregulatory genes in potentiating chorioamnionitis in cases of preterm delivery due to infection (Varner, 2005).
Gum inflammation is a chronic anaerobic inflammation that afects as many as 50 percent of pregnant women in the United
States (Goepfert, 2004). Vergnes and Sixou (2007) performed a metaanalysis of 17 studies and concluded that periodontal disease was signiicantly associated with preterm birth. To better study this relationship, Michalowicz (2006) randomly assigned 813 pregnant women between 13 and 17 weeks' gestation who had periodontal disease to treatment during pregnancy or postpartum. Treatment during pregnancy improved periodontal disease and was safe. However, treatment failed to significantly alter preterm birth rates. his position was reafirmed by a joint workshop of the European Federation of Periodontology and the American Academy of Periodontology (Sanz, 2013).
he intervals between pregnancies are linked with adverse perinatal outcomes. In a metaanalysis, intervals < 18 months and > 59 months were associated with greater risks for both preterm birth and small-for-gestational age newborns (Conde-Agudelo, 2006). he causal efect of short interpregnancy intervals, however, has been questioned (Ball, 2014).
he most important risk factor for preterm labor is a prior preterm delivery. Data from nearly 16,000 women delivered at Parkland Hospital are instructive. Namely, the recurrent preterm delivery risk for women with a preterm first delivery was threefold greater than that of women whose first neonate was born at term. More than a third of women whose irst two newborns were preterm subsequently delivered a third preterm newborn. Most-70 percent-of the recurrent births in this study occurred within 2 weeks of the gestational age of the prior preterm delivery. he causes of prior preterm delivery also recurred. Although women with prior preterm births are clearly at risk for recurrence, they represented only 10 percent of the total preterm births in this study. Expressed another way, 90 percent of the preterm births at Parkland Hospital could not be predicted based on a history of preterm birth. Laughon and associates (2014) confirmed the importance of prior spontaneous preterm birth. Notably, these investigators also found that prior indicated preterm birth was strongly associated with subsequent spontaneous preterm birth. Variations in the deinition of spontaneous and indicated used may explain this association.
Ultimately, risk of recurrent preterm birth is influenced by three factors: the frequency of prior preterm deliveries, severity as measured by gestational age, and the order in which the prior preterm delivery occurred (McManemy, 2007). That is, an individual woman's risk for recurrent preterm birth is inluenced by her past number and sequence of preterm and term births. For example, a risk of recurrent preterm birth for a gravida 3 para 2 woman with a prior preterm birth followed by a term birth difers from a woman with a prior term birth followed by preterm birth. Thus, the inluence of reproductive history has a profound prognostic significance for risk of recurrence. Moreover, this may also inluence the supposed benefit attributed to various interventions described later.