Your Pregnancy Matters

Should I have a transvaginal ultrasound to measure cervical length and help prevent preterm delivery?

Your Pregnancy Matters

The yellow and green dotted lines in this sonogram image are measuring the length of a normal, 3.5 cm cervix.

Delivering your baby early is a scary thought. 12 percent of all births in the United States are considered preterm (delivering before 37 weeks of pregnancy), and for the families involved it can mean significant additional anxiety and expense.

Previously having a preterm birth is the biggest risk factor for delivering early again. But if this is your first pregnancy, how can you gauge your risk for preterm delivery?

Many studies have examined predictors of preterm birth, some looking specifically at asymptomatic, first-time mothers. Cervical length can be used to identify women at risk of preterm birth. The shorter the cervix is in the midtrimester, particularly if it’s less than 2.5 cm, the higher the risk of delivering early. Knowing the length of your cervix gives your obstetric provider an opportunity to offer interventions that can reduce or mitigate that risk.

But is the potential risk high enough to warrant a screening test that can be invasive, expensive, and not really accurate at predicting when or whether preterm birth will occur?

A screening test to measure cervical length

In this post, I’m talking specifically about a screening test that measures the length of your cervix using ultrasound, typically when you are between 18 and 20 weeks. The best measurement is done with a transvaginal approach, where the ultrasound transducer is placed in the vagina right against the cervix for the most accurate measurement.

Many studies recommend that if your cervix is measuring less than 2.5 cm, it might be time to implement more surveillance for other signs of preterm birth. If your cervix is 1.5 cm, the studies indicate it might be time to intervene with medical or surgical options to prevent preterm birth.

However, those options are not considered universally helpful. Whether those interventions are beneficial or not helps us understand if measuring cervical length with transvaginal ultrasound is a good screening test at all.

Let’s talk about what screening tests should do in order for them to be considered helpful and worthwhile:

1. The test must correctly identify a problem before it happens

This test will identify the short cervix, of course. But having a short cervix does not necessarily mean that you will deliver early – only that the risk of delivering early is higher.

A recent study by Esplin and colleagues looked at more than 9,400 women, all during their first pregnancy, and screened them with transvaginal ultrasound for cervical length in the midtrimester. The rate of spontaneous preterm birth in that study was 5 percent. The test was able to identify only 23 percent of the women who went on to deliver early.

It is far more likely that you will make it to term than deliver early, even with a short cervix. So what we’ve found is that routine universal cervical length screening in the midtrimester did not accurately predict subsequent preterm birth.

2. An intervention to correct the problem must exist

There are no proven treatments to prevent preterm birth, or at least none that have shown conclusive evidence of benefit.

Countless studies have examined interventions to prevent preterm birth when a short cervix is found in an asymptomatic patient. These include treatments with injectable progesterone, or vaginal progesterone suppositories, as well as stitching the short cervix closed, in a procedure called a cerclage.

A study out of Pennsylvania showed that when comparing a population of screened patients versus another that did not have any screening, both had very similar rates of preterm birth, despite the fact that those screened were offered interventions to try to prevent preterm birth. This suggests that the interventions they followed were not useful at all.

Another recent study by Norman and colleagues did not show any difference in the rate of preterm birth in patients treated with progesterone versus those who were not treated. And finally, a study done at our center at UT Southwestern by Dr. David Nelson showed that when given to patients at risk of preterm birth, injectable progesterone concentrations were not different between women delivered prematurely and those delivered later in pregnancy.

3. The test must be cost effective

Cost effectiveness is critical for a screening test to be considered a good test.

The United States health care system is one of the most expensive in the world. In 2015, health care spending in the United States accounted for 17.8 percent of the country’s gross domestic product. Compared to other wealthy countries, we spend 50 percent more than the next highest-spending country. Even though as a country we spend that much on health care, we still have a higher rate of infant mortality than any of these other countries. Clearly, higher spending does not necessarily equate to more effective care.

Mandating cervical length screening for the low-risk population would add approximately $350 million to our health care costs as a nation. And, as mentioned above, it is not particularly effective at predicting preterm births. Could it be that we think technology can fix our problems, when we really haven’t evaluated these new technologies to make sure that they are safe, useful, and cost-effective?


Universal cervical length screening for all women, including those who have a low risk of preterm birth, is currently not widely used.

I suspect this will remain so for the time being, for multiple reasons: cost of this additional ultrasound testing and the additional testing and surveillance it may generate; limited efficacy of vaginal progesterone and other treatment options; and the reluctance of women to undergo transvaginal ultrasound given its invasive nature.

But I also believe that cervical length screening, as it currently stands, has the potential to benefit only those patients at highest risk and not the general low-risk population of pregnant women. Until we have refined our screening methods, or identified truly efficacious interventions, this screening test will only add to expense and anxiety and benefit the pregnancy very little, if at all.

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