1) Cervical length is measured between the 11th-24th week of pregnancy to assess the risk of preterm delivery. It involves using transvaginal ultrasound to measure the length of the cervix along its longitudinal axis, avoiding excessive pressure from the probe. 2) Multiple measurements should be taken and the shortest used. A full bladder can artificially lengthen the measurement so an empty bladder is preferred. The internal and external cervical os must be clearly identified. 3) If over 25mm, the cervix may curve so the standard straight line measurement may underestimate length. For those under 16mm, the cervix will always be straight. The presence of amniotic fluid sludge in the cerv

1. How to measure
cervical length
Karl Oliver Kagan and Jiri Sonek

2. How to: measure the cervical length
Indication: routine pregnancy care and for patients with
an increased risk of preterm delivery
When? At 20–24 weeks’ gestation and anytime in
patients with an increased risk of preterm
delivery (11–13 weeks)
Ultrasound approach: transvaginal or transperineal
Ideal maternal position: dorsal lithotomy

3. Videoclip S1:
Normal.avi
The cervix should be measured along its longitudinal axis. The cervix should occupy
approximately 50–75% of the image. Excessive pressure on the cervix by the probe should be
avoided, as the cervix artificially appears to be longer and the presence of a funnel will be
obscured. The examination should last for 3–5min. Several (at least three) measurements
should be obtained during the course of the examination and the shortest measurement
should be used for counseling.

4. A full bladder can artificially increase the cervical length. To et al. showed that mean difference
between the cervical length measured with an empty and full bladder is about 4mm. In
addition, a full bladder can obscure the presence of cervical funneling by compressing the two
halves of the funnel together.
27mm 14mm
Full bladder Empty bladder

5. The cervical canal and the surrounding cervical mucosa need to be identified.
In order to avoid inclusion of the isthmus into the cervical length measurement, care must be
taken to identify the internal, as well as the cervical, os. The external cervical os is identified
as the point where the anterior and posterior lips of the cervix come together. The
identification of the internal cervical os can be more challenging. Identification of the cervical
mucosa as a homogenous and hypoechoic structure, compared to the surrounding stroma,
can be helpful in these cases, as the internal cervical os is located at the point where the
cervical mucosa ends.

6. This is particularly important when measuring the cervical length in the first trimester.

7. Videoclip S2:
Ex_pressure.avi
Excessive pressure on the cervix by the probe should be avoided, as the cervix artificially
appears to be longer and the presence of a funnel will be obscured.

8. Videoclip S3:
Dyn_cervix.mov
The cervix is not a static structure and the length can vary, for example due to uterine
contractions or a different position of the patient. Therefore, sufficient time should be allowed
for the examination to detect these changes. It may be helpful to manually press on the uterus
or to ask the patient to push downwards to assess the cervical stability. Some even suggest
that the patient stands up during the examination, with the ultrasound probe inside the vagina.

9. If the cervix exceeds 25mm, it will be curved in more than half of the cases. The standard
method of measurement, using a straight line between the internal and the external ora, will
underestimate the cervical length in these cases. However, this is of little clinical significance
as these patients are at low risk regardless of the exact measurement. In the high-risk group
of patients with a cervical length < 16mm, the cervix will always be a straight line.
Long curved cervix Short straight cervix

10. Funneling, defined as protrusion of the amniotic membranes into the cervical canal, is
considered by some as an additional risk factor of preterm delivery. Various criteria for the
diagnosis of true funneling have been published. However, using a logistic regression analysis
that includes both funneling and cervical length, funneling has been shown not to be an
independent risk factor.

11. The cervical canal may have a thin layer of hypoechoic contents. This is especially true in the
third trimester. In all likelihood, this represents accumulation of mucus. This finding needs to
be differentiated from a thin cervical funnel. This is best done by delineating the course of the
fetal membranes: if they are not prolapsing into the cervical canal and are located at the level
of the internal cervical os, the presence of a true funnel is unlikely.

12. Videoclip S4:
Sludge.avi
Amniotic fluid sludge can be found as echogenic aggregates close to the internal os or within a
funnel. This appears to be associated with microbial invasion of the amniotic cavity. Sludge is
an independent risk factor for spontaneous preterm delivery, preterm premature rupture of
membranes, microbial invasion of the amniotic cavity and histological chorioamnionitis in
asymptomatic patients at high risk for spontaneous preterm delivery.

13. Care should be taken to identify vasa previa (left), placenta previa (right), or a low-lying
placenta.

14. In general, the cervix should be assessed transvaginally. In cases where transvaginal
ultrasound should be avoided, such as premature rupture of membranes, the cervical length
can be measured by a transperineal approach. The transabdominal assessment of the
cervical length can be used as an initial evaluation but a proper risk assessment should be
based on a transvaginal or transperineal measurement.