There are few things we can do to prevent preterm birth. One of the primary tools in the toolkit is progesterone. Mothers with a history of spontaneous preterm birth are at increased risk of a repeat preterm birth and are routinely prescribed progesterone to decrease this risk. Another group of women who benefit from progesterone therapy are women with a short cervical length. How would you know if someone has a short cervix? One way is to measure the cervical length with ultrasound- but who should get this ultrasound exam? Everyone? Just high-risk women? Is this strategy cost-effective? And, are women who are eligible even getting their treatment?