MOMS-TO-BE: What You Should Know About Cervical Length

February 27th, 2013

by Elizabeth T. Jordan, DNSc, RNC
Johns Hopkins School of Nursing


As an expectant mom, your prenatal care provider may advise you to have an ultrasound scan to measure the length of your cervix. Your cervix is where your uterus, also known as your womb, opens into the vagina. Women with a short cervix in the mid-trimester may be at increased risk for preterm delivery.  Several studies have shown that women with shorter cervical length, as measured by an ultrasound, are at higher risk for having a preterm birth.

Your cervical length can be measured by a transabdominal ultrasound, and/ or a transvaginal ultrasound. CerviLenz is a disposable device, that looks like a long cotton swab, that also measures cervical length and can be used by the provider during an examination in the office or clinic. The transvaginal scan is currently thought to be the best way to measure the cervix because it can be seen much more clearly this way.

The cervix should be between 3 cm and 5 cm long, with one end at the top of the vagina (the external os) and the other end inside the uterus (the internal os). If the cervix measures less than 2.5 cm, your health care provider may request you have additional scans and recommend further treatment based on your obstetrical history. The care provider may also look at the shape of the cervix and watch to see if the cervix changes from V-shaped to U-shaped. This is called “funneling” of the cervix.  If you cervix is short (<2.5 cm) and shows funneling, it puts you at a higher risk for delivering your baby early. Your provider may recommend additional treatment.

Your cervical length measurement may also be calculated when an ultrasound is being performed to confirm your due date and look at the growth of your baby. Routine ultrasounds are usually done between 19-24 weeks’ gestation.

Your cervix should remain closed throughout pregnancy until just before birth.  If the cervix begins to open before you are full-term, this may be a danger sign and may place you at risk for infection or premature dilation. The cervix has a mucous plug, also known as show, and, it protects your womb. Cervical length measurement is recommended if you are expecting twins or have had surgery to your cervix.

Cervical length is currently not routinely being performed on every pregnant woman. However, results from several studies build a strong case that pregnant women with a short cervical length are at higher risk for delivering preterm.  Knowing if you are at risk for preterm birth can help guide your prenatal care and minimize your risks.