SMFM recommendations for routine cervical length screening for preventing Preterm Births.

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A summary of Society for Maternal-Fetal Medicine  recommendation for Cervical Length ( CL) screening to prevent Preterm births. 

Preterm birth (PTB) remains a major cause of neonatal death and short and long term disability across the globe. The current global preterm birth rate is 5% to 18% and statistic shows a steady increase recently. CDC data quotes that in the year 2014, every 1 in 10 infants was born preterm in US. 

 Nearly 2/3 of 15 million preterm births are spontaneous with a very high recurrence. A history of previous PTB is the strongest risk factor for preterm delivery in the current pregnancy. Among various other contributing factors prior uterine surgery, especially those performed on cervix (induced termination of pregnancy (I-TOP) or spontaneous abortion (SAB)) has been implicated in its causation.

There are few laboratory tests to predict preterm birth in a pregnancy.

Transvaginal Cervical Length measurement helps in predicting the risk of preterm birth so the obstetrician and his team gets time to intervene, delay or transfer the patient to high risk units.

At present, a single mid-trimester transvaginal CL measurement is the best clinical predictor of a preterm birth. Those with the shortest cervix has the highest risk of prematurity.

The Society for Maternal-Fetal Medicine issued recommendation about the role of routine cervical length screening in selected high-and low-risk women for preterm birth prevention.

• SMFM recommends “routine transvaginal cervical length screening for women with singleton pregnancy and history of prior spontaneous preterm birth (GRADE 1A).”

Transvaginal assessment at 16 and 24 weeks’ gestation by a trained sonologist or obstetrician should be performed. Routine CL screening is not advisable before 16 weeks and after 24 weeks in asymptomatic women.  

In women with prior history of preterm births serial CL measurements are performed every week beginning at 16 weeks to 24 weeks.

• SMFM recommends “Practitioners who decide to implement universal CL screening should follow strict guidelines (GRADE 2B).”
• SMFM recommends “Routine transvaginal CL screening not be performed for women with cervical cerclage, multiple gestation, PPROM, or placenta previa (GRADE 2B).”

Evidence does not support additional screening for women who have undergone cervical conning or LEEP beyond the standard recommendations.

No additional clinical benefit is derived for repeated CL screening in women who have undergone cerclage operation. Although research demonstrates that progressive CL shortening after the stitch increases the risk for PTB, but no options exist to reinforce the short cervix after cerclage.

Routine cervical screening is not recommended by SMFM in multiple gestation as no additional clinical benefit is derived from it.

Transvaginal CL measurement serves as an adjunct to fetal fibronectin (FFN) in predicting PTB in women with CL of 20-29 mm (the grey zone). Cervical length more than 30 mm and less than 20 mm is independently a strong predictor of minimum chances for PTB or high risk for  PTB respectively. 

Not much clinical benefit is derived from cervical length estimate in Preterm premature rupture of membranes (PPROM). Few observational studies have shown that with a transvaginal CL <2 cm, the positive predictive value of delivery within 7 days was 62%.

Routine transvaginal CL measurement is not performed in case of placenta previa as studies do not show any additional benefit is derived for management.

• SMFM recommends “sonographers and/or practitioners receive specific training in the acquisition and interpretation of cervical imaging during pregnancy”.

Steps for proper cervical length measurement as recommended by SMFM.[1]

(1) Ensure patient has emptied her bladder.

(2) Prepare the cleaned probe using a probe cover.

 (3) Gently insert the probe into the patient’s vagina.

 (4) Guide the probe into the anterior fornix.

 (5) Obtain a sagittal, long-axis image of the entire cervix.

(6) Remove the probe until the image blurs and then reinsert gently until the image clears (this ensures you are not using excessive pressure).

(7) Enlarge the image so that the cervix occupies two thirds of the screen.

 (8) Ensure both the internal and external os are seen clearly.

 (9) Measure the cervical length along the endocervical canal between the internal and external os.

(10) Repeat this process twice to obtain 3 sets of images/ measurements.

(11) Use the shortest best measurement.

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[1] SMFM. Role of routine cervical length screening for preterm birth prevention. Am J Obstet Gynecol 2016.