during delivery. Cervical competency depends on the
anatomical integrity as well as the biochemical composition
(ground substance) of the cervix. One of the earliest indicators
of cervical incompetency or onset of labor is shortening of
the cervix. Interest in the assessment of cervical length using
ultrasound as a predictor of preterm labor arose after Iams et
al 22 established the normal distribution of cervical lengths
after 22 weeks of gestation. It is widely accepted that a
cervical length of less than 25 mm between 24-28 weeks
may increase the relative risk of preterm delivery. A
prospective trial involving 2915 women evaluated by serial
cervical ultrasonography reported a relative risk of preterm
delivery of 9.57, 13.88 and 24.94 for cervical lengths of <
26 mm, < 22 mm and < 13 mm, respectively at 28 weeks of
gestation. The results of various studies using cervical length
assessment as a predictor of preterm delivery were not always
reliable or reproducible. There is also a wide variation in
predictive values. A systemic review of 35 studies involving
cervical length assessment revealed a very wide variation in
sensitivity (68-100%) and specificity (44-79%) 23. Hence,
currently there is no strong evidence to support routine cervical
assessment using ultrasound between 24-28 weeks for the
purpose of predicting preterm delivery. However, it may have
a place in high-risk pregnancies or in combination with fFN
assessment.
Combination of fFN and cervical ultrasonography
Cervical length assessment in conjunction with fFN estimation
in cervicovaginal secretions in women with high risk of
preterm delivery may be useful. A study to determine the
risk of recurrence of spontaneous preterm delivery in women
with prior preterm birth reported a risk of 65% if the cervical
length is less than 25 mm and the fFN is positiuve 24. However,
if the fFN is negative, the risk of preterm delivery was only
25%. As shown in Table 1, the risk of recurrent preterm
delivery in women with cervical length > 35 mm and negative
fFN was only 7%. Hence, a combination of cervical length
assessment using ultrasound scan and estimation of fFN may
help predict the recurrence of preterm delivery in high-risk
women.