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.