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. There is no proven benefit in assessing the cervix by ultrasound in women with no risk factors for preterm birth.
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 <15 mm at 22-24 weeks gestation is very abnormal and is associated with a significant risk of preterm birth.
In what circumstances might cervical screening be required?
- Prior very preterm birth (<32 weeks gestation)
- 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 depending upon your particular circumstances.
