Prospective Evaluation of Pathways for Preterm Birth

Inclusion Criteria: * Maternal age from 18 to 45 years of age. * Singleton gestation. Exclusion Criteria: * Treatment with vaginal progesterone or intramuscular progestogen within the previous 4 weeks. * History of congestive heart failure, chronic renal failure, or uncontrolled diabetes mellitus, or diabetes mellitus with evidence of end organ dysfunction secondary to vascular disease. * Current pregnancy with a major fetal anomaly or known chromosomal abnormality. * The subject has a marked uterine anatomic malformation that may alter pregnancy duration such as a septated uterus, unicornuate uterus, or uterine didelphys. * The subject is considered not capable or unwilling to undergo study procedures and requirements. * The subject is symptomatic with vaginal bleeding at enrollment visit.