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Trans-vaginal ultrasound

Starting at: 9 weeks to get a baseline cervical length
Frequency: Every 2 weeks; every week from 15-24 weeks
Purpose: To monitor for cervical weakness or irritability (often called “incompetent cervix” or IC); particularly changes in the upper half of the cervix that cause “funneling”.

Cervical weakness is one of the few identifiable causes of PROM. Health care practitioners routinely monitor for weakness in the lower half of the cervix to prevent preterm birth. However, most practitioners do not routinely screen for changes in the upper half of the cervix which can also dilate and shorten, giving it a funnel-like appearance. ThisĀ cervical “funneling” occurs when the internal portion of the cervix closest to the baby begins to open. A funneling cervix can allow the bag of waters to slip down into the cervix and rub against it, which could cause PROM. Members of the PROM list have reported cervical funneling as early as 12-13 weeks into the pregnancy.

Cervical funneling cannot be detected during a speculum or vaginal exam, which only evaluates the lower half of the cervix. Trans-vaginal ultrasound is the best way to detect cervical funneling because it evaluates the entire cervix, including the upper half.

In addition to identifying cervical funneling, trans-vaginal ultrasound and monitoring will allow your practitioner to detect other cervical change such as:

  • Dilation: When the cervix begins to open. The opening can be anywhere from the size of a fingertip (approx. 1 cm) or larger.
  • Effacement: When the cervix shortens in length, usually indicating approaching labor.
  • Ripening: When the physical makeup of the cervix changes from firm to soft or “ripe,” usually indicating approaching labor.

If your practitioner detects any cervical change, including funneling, he or she may recommend strict bedrest and/or placing a cerclage to help keep the cervix closed. (See the cerclage section of this article for more information.)

Monitoring for cervical changes is important even if your practitioner believes that you do not have an incompetent cervix (IC) because you had a previous full-term pregnancy or a normal-length cervix after you PROM’d. The cervix is a dynamic organ and it may look very different after PROM than before the rupture occurred. Members of the PROM list have found that cervical change is often detected between 15 and 24 weeks of pregnancy, perhaps from the growing weight of the baby and amniotic fluid. During this period, more frequent (weekly, instead of bi-weekly) monitoring is ideal.

For a more in-depth discussion about the advantages of trans-vaginal ultrasound and the implications of detecting cervical changes, click here.

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Article by Kay Squires & Holly Norman, January 2005, updated July 2011
Special thanks to the members of the PROM list