Cervical length assessment

Cervical screening for preterm delivery

Preterm birth occurs in 6-8% of pregnancies and is a very serious complication of pregnancy. The risk of spontaneous preterm birth increases as the length of the cervix (neck of the womb) decreases. Ultrasound may be used to assess the length of the cervix in women at high risk of preterm birth, usually on at least two occasions at 14-24 weeks gestation. A transvaginal ultrasound measurement of the cervical length at 18-22 weeks gestation has been advocated by some researchers as a screening test for preterm birth risk for all pregnant women.

What is a normal cervical length in pregnancy?

• At 20 weeks gestation the average cervical length is 40mm
• At 34 weeks gestation the average cervical length is 34 mm

How short is too short for cervical length?

A cervical length < 15mm at 22-24 weeks gestation is abnormal and associated with a significant risk of preterm birth.

In what circumstances might cervical screening be required?

• Prior very preterm birth
• Previous cervical surgery (eg. cervical cone biopsy)
• Women with a cervical suture
• Women with suspected cervical incompetence
• Multiple pregnancies

How is the cervical length assessed in pregnancy?

A transvaginal ultrasound examination is required to accurately measure the length of the cervix. The bladder should be empty. There is no known risk to a transvaginal ultrasound assessment in pregnancy.

What features are assessed in cervical screening?

• Length of the cervix (normally >25mm)
• Funnelling of the membranes into the cervical canal
• Dilatation of the cervix
• Location of a cervical suture (if present)

Normal Cervix on transvaginal scan

Abnormal cervix demonstrating dilation of the internal cervical os and funnelling

Normal appearance of a cervix with a suture in place

Abnormal appearance of a cervix with a suture in place demonstrating shortening and funnelling

What may happen if the cervix is shortened or dilated in pregnancy?

A very short (< 25 mm) or dilated cervix is abnormal and is suggestive of an increased risk of preterm birth. Your obstetrician may consider modification of activity, bed rest, hospitalisation, medications or a cervical suture if cervical screening is abnormal. The specific intervention will depend upon your particular circumstance.

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