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