Transvaginal cervical-length ultrasound is indicated for women with risk factors (prior spontaneous preterm birth, cervical surgery) and for symptomatic patients (vaginal bleeding, uterine contractions). Routine screening is recommended for high‑risk pregnancies around 16–24 weeks. It’s done before cerclage or after cervical procedures. A cervical length <25 mm before 24 weeks is short and prompts intervention and serial scans. Not routine in low‑risk women.

A cervical length scan, usually done transvaginally, is generally not painful. Most people feel mild pressure or brief discomfort when the ultrasound probe is inserted; some experience light cramping or an urge to urinate. The procedure is quick and doesn’t require anesthesia. If you have significant pain, bleeding, or severe anxiety, tell your clinician so they can adjust the exam or offer support.

Cervical length is measured mainly using transvaginal ultrasound: a probe is inserted into the vagina to visualize the cervix and measure distance from the internal to external os in millimetres. A full bladder or transabdominal scan can give less accurate readings, so transvaginal imaging is preferred. Measurements are typically done around 16–24 weeks; under about 25 mm suggests increased preterm-birth risk.

On ultrasound the cervix is usually measured by length. In non‑pregnant women the cervical length is typically about 2.5–3.5 cm (25–35 mm). In pregnancy, a mid‑trimester cervical length of 25 mm or more is generally considered normal; shorter measurements define a short cervix and raise preterm birth risk. Values vary with parity and scanning technique.

Normal cervical length when not in labor is about 3–4 cm. In pregnancy, a transvaginal cervical length ≥25 mm (2.5 cm) after the mid‑trimester is generally considered adequate and linked to lower preterm risk. At term, the cervix normally effaces (shortens) and then dilates to about 10 cm to permit vaginal delivery; shorter early lengths may signal preterm risk.

Bed rest has not been shown to prevent preterm birth from a short cervix and is not routinely recommended. Evidence-based options—vaginal progesterone, cervical cerclage (in selected cases), or a pessary—are used depending on gestation and findings. Strict activity restriction can raise risks (blood clots, muscle loss, emotional distress). Discuss individualized management and risks with your obstetrician or maternal-fetal specialist.