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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 normally about  35 to 48 mm (1.4  to 1.9 inches) long at 24 weeks. 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).
 

Ultrasound of Short CervixHow the Cervix is Measured Using Ultrasound

Because effacement begins at the internal opening of the cervix, effacement can usually be detected at an earlier stage by ultrasound examination. 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.

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.

Illustration of a Normal and Short cervix

 

Normally the cervix  is about 35 to 48 mm (1.4  to 1.9 inches) long at 24 weeks.


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. As the graph below shows, the chances of having a preterm delivery increase as the length of the cervix becomes shorter.

Length of Cervix vs Likelihood of Preterm Deliver.

Cervical Insufficiency

A short cervix is not the same as cervical insufficiency. Cervical insufficiency (sometimes called an incompetent cervix) is the failure of the cervix to maintain a pregnancy when there are no signs or symptoms of labor in the second trimester. A short cervix by itself  is not sufficient for the diagnosis of cervical insufficiency.

Women who have had  biopsy, loop electrosurgical excision procedures (LEEP) , dilation and curettage (D&C), uterine malformations or exposure to diethylstilbestrol may be at increased risk for cervical insufficiency.

Cervical insufficiency can sometimes be treated using cervical cerclage. A cerclage is a stitch placed, under anesthesia, around the cervix to keep it closed. ACOG recommends that cerclage should be limited to pregnancies in the second trimester before fetal viability has been achieved. Activity restriction, bed rest, and pelvic rest have not been shown to be effective treatments for cervical insufficiency [36].


Treatment for the Prevention of Preterm Labor

The American College of Obstetricians and Gynecologists (ACOG) 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.

    Placement of cerclage in women with  a cervical length less than 25 mm between 16 weeks and 24 weeks and no history of preterm birth has not been associated with a significant reduction in preterm birth [36]

  • 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.

McDonald Cerclage

LifeART image copyright 2005  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 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.
 

Treatment of Preterm Labor

If your are having  symptoms such as cramping,  your cervix is shortening very quickly, or your cervix  is less than 15 mm, your doctor or midwife may admit you to the hospital for additional evaluation or treatment . Besides a short cervix,  preterm labor may also be caused by infection, inflammation, vascular disease (poor blood supply to the uterus and placenta) , an overstretched uterus, and abnormalities of the cervix . The cause of many preterm births is never found.

Your provider may collect a sample of fluid  from your vagina to test for a substance called fetal fibronectin (fFN) . Fetal fibronectin (fFN) acts like "glue" holding the fetal sac to the uterine lining during pregnancy. The presence of fetal fibronectin in the vagina between 24 and 34 weeks of pregnancy makes it more likely that you will have a preterm delivery.

If there is enough time you can be given a medication (corticosteroid) to help your baby's lungs mature and reduce other complications of being born prematurely. Medications such as nifedipine , terbutaline, or indocin may be given for up to 48 hours to help control your contractions. You may also be given intravenous magnesium to lessen the chances of your baby having cerebral palsy due to being born prematurely .

Once preterm labor has been stopped bedrest and long term use of medications such as nifedipine , terbutaline, indocin, or magnesium to stop contractions are not recommended  because these treatments are either ineffective and/or may cause harm . The use of progesterone to prolong pregnancy once preterm labor has been stopped is being studied.


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.

Reviewed by Mark Curran, M.D. FACOG


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