During pregnancy the cervix remains closed and firm
acting as a barrier to infection and supporting the weight of your growing
baby. At around 32 weeks' your cervix naturally begins to soften, shorten,
and become thinner until it is eventually paper thin and almost vanishes
This process of shortening and thinning is called effacement and is
measured by your doctor or nurse during a vaginal examination. Effacement is
reported as a percentage from zero percent (normal length cervix) to 100% or
complete (paper thin cervix).
Because effacement begins at the internal opening of the cervix, effacement
can usually be detected at an earlier stage by ultrasound examination.
When ultrasound is used to measure the length of the cervix, the
length of the cervix is reported in millimeters (mm) or centimeters (cm)
instead of as a percentage.
How the Cervix is Measured Using Ultrasound
The cervical length (CL) is
usually measured when the bladder is empty using a transvaginal
ultrasound (TVU) probe.
The length of your cervix is measured from its internal opening
on the inside of the uterus to its external opening into the vagina.
Normally the cervix is about
3.5 cm or 1 1/2 inches long.
Image The National Institutes of Health
The cervix is considered to be short if the length of the
cervix is less than 25 mm long before 24
weeks of gestation.
The cervix is considered to be very short if the length of
the cervix is less than or equal to 20 mm at up to 24
weeks of gestation
Short Cervix and Preterm Birth
Having a short cervix increases the chances that you will
have a preterm delivery. The chances of having a preterm delivery increase
as the length of the cervix becomes shorter.
The American College of Obstetricians and Gynecologists recommends:
- In women with a singleton pregnancy , no history for preterm birth
, and cervical length is 20 mm or less before or at 24 weeks
Vaginal progesterone 8% gel (90 mg daily) OR
Micronized progesterone capsules (200 mg vaginally daily)
- Progesterone is a substance produced by the ovary and placenta that
acts to prepare for and maintain pregnancy . Progesterone has been shown
to help prevent the cervix from shortening in some women.
- In women with a singleton pregnancy, with a history of preterm birth at less than 34 weeks, already receiving progesterone
since 16 weeks cerclage should be considered if the cervical length
is less than 25 mm before 24 weeks.
- A cerclage is a stitch placed around the cervix to keep it closed.
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Lippincott Williams & Wilkins. All rights reserved
- In women with a twin or triplet pregnancy neither progesterone treatment
or cerclage reduce the rate of preterm birth. Neither is recommended to prevent
preterm birth in these groups of women.
- Some groups are studying the use of a cerclage pessary, a small ring-shaped
device which is inserted around the cervix, to prevent preterm births in twin
An analysis by the Perinatology Research Branch of
the National Institute of Child Health and Human Development found that
vaginal progesterone or cerclage are probably equally effective in the
prevention of preterm birth in women with a singleton pregnancy , with a history
of preterm birth, and who are found to have a short cervix in the second
trimester. The choice of treatment needs to consider adverse events, cost and
If your are having symptoms such as cramping, your cervix is
shortening very quickly, or your cervix is less than 15 mm, your doctor may
admit you to the hospital for additional evaluation or treatment .
and Low Lying Placenta or Placenta Previa
Women who have placenta previa and a cervical length of less than or equal to 30 mm are more likely to have
emergency Cesarean section and experience larger
blood loss before and during surgery.
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