My patient was 22 years old and midway through her first pregnancy when she arrived for an ultrasound examination. It was a routine evaluation in every way, and in the darkened room, the ultrasound technologist moved the ultrasound probe over the patient's gel-covered abdomen, cataloguing a long list of normal fetal structures.

Then the tech did something incredibly important: she aimed her ultrasound probe toward the patient's cervix.

"I think the cervix is short," she told me.

It can be difficult to measure the cervix using the same external ultrasound probe that is used to examine the fetal spine, heart, and other anatomic structures. The cervix can easily become lost among the other pelvic organs that look similar on ultrasound.

"Let's do a vaginal ultrasound," I suggested. This approach allows for reasonably easy measurement of the cervical length.

On the grainy, black-and-white screen, the cervix in cross-section appeared like two loaves of bread placed bottom to bottom. The tech was correct. My patient's cervix was only 1.8 cm long, well below the 3 cm threshold we use to define normal.

Incorporating the routine measurement of cervical length into the 'anatomy survey' ultrasound that most women have midway through pregnancy will be critical if the United States is going to see a further decline in the rate of preterm delivery.

After watching preterm birth rate in the United States inexorably climb for 20 years, the rate dropped slightly in 2009 to 12.2 percent. The two major contributors to the decline were increased use of injectable progesterone for women who delivered early with their previous pregnancy, and a seriousness among obstetricians about avoiding elective deliveries before 39 weeks of gestation.

We have known for years that a short cervix is a risk factor for a preterm delivery. It makes sense: the cervix has the challenging task of being a physical barrier and protecting the pregnancy for nine months, and then miraculously melting away and dilating to allow the fully developed baby to emerge.

Although we knew a short cervix was bad news, my colleagues and I didn't have many tools to use when we diagnosed a short cervix. Putting patients on bed rest didn't help, and sewing the cervix closed -- a surgical procedure called a cerclage -- was controversial because decades of research had failed to prove it worked.

But in the past two years, several well-designed clinical trials demonstrated that women who start vaginal progesterone after they are diagnosed with a short cervix are less likely to deliver early than those who don't take the medication.

This is great news, but now for the hard part: getting a screening program up and running so that women with a short cervix come to our attention in time to be treated.

Cervical insufficiency, which is what we call it when the cervix inexplicably shortens early in pregnancy, is a silent problem without symptoms (rarely women report an increase in vaginal discharge).

Measuring the cervix using the standard external ultrasound probe is challenging, and there is often a lot of reluctance to do a vaginal ultrasound, particularly as a screening test: it can be uncomfortable and time-consuming. As a consequence, guidelines have not recommended cervical length measurement on all women -- only those who are at high risk for a short cervix due to a prior preterm delivery or some other reason such as surgery on their cervix.

But the time for routine cervical length measurement during the 'anatomy survey' ultrasound has come.

My patient started taking progesterone that very day, and we made a plan to see her again in two weeks. When she returned, her cervix was even shorter, and we recommended a stitch be placed to sew her cervix closed. In spite of the weak medical evidence in support of that procedure, we had nothing else we could offer, and for a few weeks it looked as though the cerclage was working.

Had the ultrasound technologist not been compulsive, I am confident that my patient's cervix would have silently dilated and she would have delivered her baby many weeks before survival was even possible.

As it is, she is still pregnant. We monitor her cervix, and she has been in and out of the hospital, but as the days and weeks tick by, it is becoming clear that this pregnancy will end happily.

For years we have known the consequences of a short cervix. Now, we have a medication proven to work. It's time for obstetricians to put the pieces together and make measurement of the cervical length part of our routine.