Short Cervix in Pregnancy                                           

What is A Short Cervix?

The cervix is the narrow,  lower end of the uterus that opens into the vagina.  The cervix  has a firm texture, similar to the tip of your nose, and is n
ormally about  35 mm or 1 1/2 inches long. 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 chances  for  having a  preterm birth are higher in women who have a short cervix

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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  • 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 pregnancy.

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 patient/clinician preferences.

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 .

Cervical Length
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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