 The trial will have an observational portion and an interventional portion. In the observational portion, 1,000 women with a history of a spontaneous preterm birth at 17 to 32 weeks gestation will undergo biweekly vaginal ultrasounds beginning at 16 to 18 weeks gestation and ending by 22 weeks. Prior research indicates that one third of these women will either have or develop a cervix less than 25 mm long. Women will be seen at a frequency determined by the cervical length as determined by ultrasound examination starting at 16 weeks gestation. If cervical length remains greater than 30 mm, the next visits will occur in 2 week intervals until 22.6 weeks gestation. If the cervical length is 25 mm to 29 mm, the visits will be scheduled weekly. If the cervical length shortens to less than 25 mm, the woman is eligible for randomization, and will undergo one more study visit. 