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Natural History of Cervical Funneling in Women at High Risk for Spontaneous Preterm Birth
Abstract & Commentary
By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.
Dr. Hobbins reports no financial relationship to this field of study.
Synopsis: Authors' attempt to estimate the natural history of funneling in the second trimester by transvaginal ultrasonograms, and whether funneling increases the risk of spontaneous birth.
Source: Berghella V, et al. Natural history of cervical funneling in women at high risk for spontaneous preterm birth. Obstet Gynecol. 2007; 109:863-869.
There is no doubt now that the cervical length (CL) measurement, obtained with transvaginal ultrasound between 20 and 24 weeks, can give the clinician a very reasonable idea about which patients have the greatest and smallest chances of delivering preterm. Data generated from the NICHD Maternal-Fetal Medicine Network surfaced recently in a paper published in the American Journal of Obstetrics and Gynecology which have shed light on the meaning of another ultrasound finding: cervical funneling.
Five hundred and ninety scans were undertaken in 183 women who had a previous spontaneous birth between 16 and 32 weeks of gestation. The authors were careful to exclude patients whose histories were suggestive of causes for preterm birth (PTB) other than spontaneous labor, such as substance abuse, uterine anomalies, etc. Transvaginal ultrasound examinations were conducted at 16 to 19 weeks and every 2 weeks thereafter, up until 24 weeks. During these examinations careful attention was directed to the presence or absence of cervical funneling, in addition to the shape and size of the funnel. The managing clinicians were not privy to the investigators' findings.
The results indicated that funneling was strongly associated with birth before 32 weeks. Sixty women had funneling at least once during the examination, and they delivered earlier than those without funneling. For example, those with a "V" shaped funnel on the first exam delivered on average at 33.2 weeks, and if it was only present on the last exam, the average time of delivery was 33.5 weeks. The finding of a "U" shaped funnel on the first exam was an ominous sign since the average age at delivery was only 19 weeks, and if the "U" shape was preceded by a "V" the average gestational age at delivery was 21 weeks. Even when the "U" was preceded by a completely normal-appearing cervix, the patients delivered on average at 27 weeks. Most importantly, if no funneling was observed during the study period, the patients delivered on average at 37 weeks.
The above results would lead the reader to think that the finding of funneling was a powerful independent predictor of PTB. However, the authors actually found, surprisingly, that funneling did not add anything statistically to the predictive ability of CL alone.
Zilianti first reported an observation that the cervix goes through a typical sequence in cervical shape prior to preterm delivery, which he described by the phrase "Trust Your Vaginal Ultrasound." Normally, there is the T-shape to the junction where the cervical canal meets the uterine cavity. However, those who are predisposed to, or are in, preterm labor will first have a "Y" shape at this junction. Later, this gives way to a "V" shaped wedge into which membranes protrude. Last in this continuum, there is a U-shaped funnel which precedes complete effacement, and, ultimately, delivery.
The authors of the above paper certainly have validated the ominous meaning of a U-shape at any time in the second trimester. However, the "V" conveys less meaning, especially since, on occasion, the V shape reverted back to a T, and this was associated with a term delivery. In all of the patients in the V category destined for PTB, the CL was less than 2.5 cm (measured from the tip of the wedge to the exocervix).
Cervical length is more easily standardized, less subjective, and not affected by "false funneling," as has been noted in patients with concentric lower uterine contractions. So this and other studies in the literature have strongly suggested that in patients at historical risk for PTB, at least one CL measurement between 20 and 24 weeks is particularly helpful in identifying which patients have the strongest predisposition for this complication of pregnancy. Also, a CL of greater than 2.5 cm significantly diminishes a patient's risk of PTB, and should allow us to lighten up on her surveillance. A U-shaped cervix has a very bad prognosis, and a V shape may or may not, depending upon its effect on the CL.
Unfortunately, the question that continues to haunt us is, "What do we do to prevent PTB in those who, by CL, are at greatest risk?" Perhaps weekly IM progesterone injections are the answer: Tocolytics have a horrible therapeutic track record, especially in those who are not contracting. Last, most randomized trails have shown no benefit to cerclage. Therefore, we are not there yet.