The cervix is the opening or passage through which the baby must pass before being born vaginally. Ultrasound can detect early changes of the cervix, such as shortening of the cervical length, to predict preterm birth. On identifying a short cervical length, interventions can be applied to prevent preterm birth. These interventions include giving the expectant mother progesterone to relax the uterus, or applying a stitch, known as a cerclage, to tighten the opening of the cervix.