For patients with no prior preterm delivery, a transvaginal ultrasound cervical length measurement is recommended at 16-24 weeks gestation to assess the risk for preterm delivery. Patients with a normal cervical length are at a lower risk for preterm delivery and require no additional intervention. In contrast, those with a short cervix (ie, cervical length ≤2.5 cm on ultrasound) are at high risk for preterm delivery and are offered vaginal progesterone, which decreases prostaglandin production and maintains uterine relaxation throughout pregnancy, thereby decreasing the risk of preterm labor.
additional measurements are not indicated in patients with normal cervical length.
The higher risk of preterm delivery is likely due to the inability of a shortened cervix to tolerate normal intermittent contractions with increasing gestational age, thereby allowing for preterm cervical dilation and effacement.
Typically, progesterone supplementation is indicated for patients with a prior preterm delivery to prevent recurrence, but this applies only to patients with a prior spontaneous preterm delivery; this patient had an iatrogenic preterm delivery for a specific medical indication (ie, stillbirth).
Bed rest does not decrease the rate of preterm delivery, but it does increase maternal risk of venous thromboembolism and bone loss.
Intramuscular 17-hydroxyprogesterone can be used preventatively in patients with prior preterm delivery as it may reduce the risk of recurrence.
Extremely short cervix is defined as ≤1.5 cm and has even higher risk of preterm labor