Cervical insufﬁciency is characterized by asymptomatic dilation of the internal os during pregnancy. This generally leads to dilation of the entire cervical canal during the second trimester with subsequent risk of rupture of the membranes and/or expulsion of the fetus. This affects 1/54 to 1/1842 pregnancies (resulting from uncertain diagnostic criteria). Though uncommon, it is thought to be involved with as many as 20% to 25% of all second-trimester pregnancy losses.
Cervical insufﬁciency may come from
iatrogenic sources, most often damage from cervical dilation at the time of
dilation and curettage or other manipulation, or damage caused by surgery
(conization). Other possible causes include congenital tissue defect, uterine
anomalies (uterus didelphys), prior obstetric lacerations, and in utero
exposure to diethylstilbestrol.
Generally cervical insufﬁciency is
suggested by a history of second-trimester pregnancy loss accompanied by
spontaneous rupture of the membranes without labor, or rapid, painless preterm
labor. The ﬁnding of prolapse and ballooning of the fetal membranes into the
vagina without labor would strongly suggest cervical insufﬁciency. Cervical
insufﬁciency must be differentiated from the presence of uterine anomalies,
chorioamnionitis, and other sources of midpregnancy loss.
When the patient is at high risk for
cervical insufﬁciency (generally by history) or cervical change is suspected,
ultrasonography should be used to assess cervical length. Ultrasonography must
also be performed before cervical cerclage to assess for abnormal fetal
development. Although cervical length can be measured by ultrasonography,
routine use of this has not proven to be an effective screening tool except in
the face of a high-risk history. (Normal cervical length is approximately 4.1
cm [±1.02 cm] between 14 and 28 weeks and gradually decreases in length to 40
weeks, when it averages between 2.5 and 3.2 cm.) Signs of cervical funneling
and cervical shortening are associated with an increased risk of preterm delivery,
but management in the absence of other risk factors is unclear.
Currently the best screening
technique remains frequent vaginal examinations beginning around the time of
previous cervical change or the second trimester, whichever is earlier.
Attempts to deﬁne or identify cervical insufﬁciency by hysterosonography,
pull-through techniques with inﬂated catheter balloons, measurement of cervical
resistance to cervical dilators, magnetic resonance imaging, and others have
not gained clinical acceptance.
Treatment of cervical insufﬁciency
is by cervical cerclage (placement of a concentric nonabsorbable suture close
to the level of the internal cervical os) generally performed between 10 and 14
weeks of gestation. When the suture is placed vaginally, it is generally
removed at 38 weeks of gestation. If labor occurs before this point and cannot
be stopped, the suture should be removed immediately because of the risk of
uterine rupture with an obstructed outlet. Cervical cerclage is occasionally performed transabdominally. When
placed in this manner, these sutures are intended to remain permanently and
they preclude vaginal delivery. The use of lever pessaries (such as the
Smith-Hodge) has been reported to be associated with outcomes similar to those
obtained by cerclage, but this modality is infrequently used. Bleeding, uterine
contractions, obvious infection, or rupture of the membranes are
contraindications to cerclage. Because of scarring after cerclage, some
patients require cesarean delivery. With correct diagnosis
and cervical cerclage, fetal survival increases from 20% to 80%.
Restriction of activity is often
suggested, but evidence that this alters the outcome of pregnancy is lacking.
After 24 weeks of pregnancy, bed rest may be the only therapy available because
the risk of cerclage to trigger labor may outweigh the potential beneﬁt.
Prophylactic antibiotics and β-mimetics
(tocolytics) have not been shown to be effective in prophylactic cerclage.