When is cervical length screened for preterm birth risk?

What does the evidence say about short cervix screening timing during pregnancy?

1. A review describes transvaginal ultrasound as the reference method for cervical-length assessment and identifies 16 to 24 weeks as a useful screening period in singleton pregnancies. 1
2. A review of asymptomatic singleton pregnancies describes a cervical length of 25 mm or less as a commonly used short-cervix threshold and supports transvaginal assessment in the mid-trimester. 2
3. For an asymptomatic singleton pregnancy without prior spontaneous preterm birth, SMFM recommends vaginal progesterone when transvaginal cervical length is 20 mm or less before 24 weeks; measurements of 21 to 25 mm prompt shared decision-making. Twin pregnancies and prior preterm birth need separate pathways. 3
4. In women first found to have a short cervix after 24 weeks, a recent cohort did not find vaginal progesterone associated with lower spontaneous preterm-birth rates, so later decisions need individualized counseling. 4
5. A study of borderline mid-trimester measurements found that some cervixes shortened on follow-up before 24 weeks, supporting why the follow-up plan depends on the original measurement and clinical history. 5

Is short cervix screening timing safe in each trimester?

• First trimester. First-trimester history can identify people who may need a cervical-length plan later, especially after a prior spontaneous preterm birth or cervical procedure.
• Second trimester. Mid-trimester is the usual window for transvaginal cervical-length assessment. The exact schedule differs by pregnancy history and local practice.
• Third trimester. A newly short cervix later in pregnancy needs individualized assessment because evidence for interventions after the mid-trimester window is less certain.