The composition of the biological clock defining the duration of human pregnancy remains a central question in reproductive biology. Our goal is to understand the molecular signals comprising this biological clock to prevent preterm birth. We have generated and analyzed mice with defects in prostaglandin biosynthesis, prostaglandin catabolism, circadian clock molecules, and oxytocin production in efforts to begin to define the key physiological pathways. These genetic studies in mice reveal essential functions for cyclooxygenase-1-generated prostaglandin F2 alpha for labor onset and the degrading enzyme hydroxyprostaglandin dehydrogenase in the maintenance of pregnancy. While these studies have elucidated the pathway for parturition timing in rodents, the findings have resulted in limited understanding for mechanisms of human parturition. In this presentation, data that genetic factors contribute to human preterm birth will be summarized. We have analyzed DNA from families with recurrent preterm birth and control families without preterm infants through genome-wide association and whole exome sequencing. These efforts have identified potentially interesting new contributors to birth timing from the maternal genome, such as the follicle stimulating hormone receptor gene, novel phospholipase isoforms, and pathways harboring rare, predicted deleterious mutations. New data regarding contributors to birth timing from the fetal genome will also be discussed.