Consanguinity promotes homozygosity of recessive susceptibility gene variants and can be used to investigate a recessive component in diseases whose inheritance is uncertain. The objective of this study was to assess the association between consanguinity and preterm birth (PTB), stratified by gestational age and clinical presentation (spontaneous vs. medically indicated). Data were collected on 39,745 singleton livebirths without major birth defects, admitted to 19 hospitals in Lebanon, from September 2003 to December 2007. Deliveries before completed 33 weeks' gestation and deliveries at 33-36 weeks' gestation were compared, with respect to cousin marriage, with those after completed 36 weeks' gestation by using multinomial multiple logistic regression. Overall, infants of consanguineous parents had a statistically significant 1.6-fold net increased risk of being born at less than 33 weeks' gestation compared with infants of unrelated parents. This association was statistically significant only with spontaneous PTB. There was no increased risk of being born at 33-36 weeks' gestation associated with consanguinity for both clinical presentations of PTB. Our findings support a genetic contribution to early onset PTB and suggest that early PTB should be targeted in future genetic studies rather than the classic lumping of all births less than 37 weeks' gestation.