a consistently increased risk of preterm birth among women in both the lowest and highest GWG categories. It is difficult to summarize the quantitative impact because the studies used varying definitions of high and low rates of weight gain and different analytic methods to characterize the relationship with preterm birth. In those studies that provided relative risks comparing higher and lower GWG to the middle range, the relative risks were on the order of 1.5-2.5 for both the higher and lower GWG groups, with greater consistency for the influence of lower GWG on preterm birth.
Effect modification by prepregnancy BMI (Siega-Riz et al., 1996; Spinillo et al., 1998; Schieve et al., 1999; Dietz et al., 2006; Nohr et al., 2007) was examined in 5 of these 12 studies. The authors of these studies consistently reported a stronger effect of a lower rate of GWG on preterm delivery among underweight women. As prepregnancy BMI increased, the magnitude of increased risk associated with a lower rate of GWG diminished. There was some evidence that the increased risk of preterm birth associated with a higher rate of GWG was greater with increasing BMI, so that the optimal GWG shifted downward with higher prepregnancy BMI. Four of the five studies that applied the IOM (1990) guidelines to define adequacy of GWG reported increased risk of preterm birth associated with inadequate GWG among underweight and normal weight women.
Several studies considered the clinical presentation of preterm birth (Siega-Riz et al., 1996; Spinillo et al., 1998; Nohr et al., 2007), and several studies considered severity of prematurity (Dietz et al., 2006; Stotland et al., 2006) in their analyses. Though limited in quantity, the results of these studies do not provide a clear suggestion that the association between GWG and preterm birth differs by clinical presentation or severity. More recently, Rudra et al. (2008) considered preterm birth subtypes in relation to prepregnancy BMI and GWG. They reported that greater GWG during gestational weeks 18-22 was weakly associated with lower risk of spontaneous preterm birth and higher risk of medically indicated preterm birth, with some variation in these patterns in relation to prepregnancy BMI.
Although the pathogenesis of spontaneous preterm delivery has not been clearly elucidated, researchers have postulated at least five possible primary pathogenic mechanisms (IOM, 2007):
Activation of the maternal or fetal hypothalamic-pituitary-adrenal (HPA) axis.
Amniochorionic-decidual or systemic inflammation.
Uteroplacental thrombosis and intrauterine vascular lesions.
Pathologic distention of the myometrium.