There's a reason why doctors and midwives regularly emphasize the "estimated" aspect of the due dates they give their pregnant patients, and that's because it is hardly an exact science.
In fact, only 5 percent of women actually deliver on their estimated due dates.
But a new meta-analysis, released this week in BJOG: An International Journal of Obstetrics & Gynaecology, suggests doctors and midwives may finally be able to help full-term women hone in on their actual delivery dates by using ultrasounds to measure how long their cervixes are. (The cervix is the lower part of the womb, which becomes thinner and opens during labor.)
Researchers with Thomas Jefferson University in Pennsylvania analyzed five studies that included a total 735 women whose babies were in the head-down position.
When a woman's cervix was relatively long at her due date -- so, measuring more than 30 millimeters -- she had less than a 50 percent chance of delivering within the next seven days.
But if her cervix was relatively short -- measuring 10 millimeters or less -- she had more than an 85 percent chance of delivering in the next week. And a woman whose cervix was just 5 millimeters on her due date had a 94 percent chance of going into labor within the week.
"We doctors, nurses and midwives always see these patients [who are at or post-term], and they're big, they're uncomfortable, and they want to know when they're going to deliver. And I've always felt we look like stooges, because we have no idea," laughed study author Dr. Vincenzo Berghella, director of Maternal Fetal Medicine at Thomas Jefferson University Hospital. "Finally, this data seems to put an end to it."
The length of the cervix, which is the lower part of the womb, may be a good predictor of impending labor because it softens and changes its shape as woman's body prepares for what lies ahead. Transvaginal ultrasound, the method used to measure it in these studies, is a relatively simple, minimally-invasive test.
Last year, the American College of Obstetricians and Gynecologists (ACOG) joined with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine to release new guidelines for estimating women's due dates, which, to many women's disappointment, seemed like more of the same. A high-quality ultrasound in a woman's first trimester is the most accurate method of establishing or confirming the gestational age of a fetus, the groups concluded. When information on a woman's last known period is available, her care providers can also use that, but they should defer to the results of the first trimester ultrasound if the estimated due dates differ by more than seven days.
And ultimately, those methods still leave women with a several-week window when labor may start.
"The ability of ultrasound to detect abnormalities, and to accurately measure a baby has greatly improved with time," said Dr. Sindhu Srinivas, director of obstetrics at the Hospital of the University of Pennsylvania. "But it's true that our ability to predict an individual's delivery date beyond the estimated due date has not significantly improved, largely because the triggers for labor and what makes a woman go into labor are not fully understood."
Srinivas believes the new meta-analysis shows some promise, but said it is too soon for the method to be routinely used by all women -- particularly before studies directly compare the efficacy of transvaginal ultrasound with a traditional hands-on exam.
"The potential for using this in specific circumstances is interesting, but I don't know that it's ready for full-blown adoption," Srinivas cautioned.
Berghella was more enthusiastic about the prospects of his team's findings, saying the method will not simply help cut through late-pregnancy frustrations, but help inform women's care. Measuring the cervical length of a full-term woman who had a cesarean section with her first and was considering a vaginal birth after cesarean section or "VBAC" with her second could provide clues about how likely she was to go into spontaneous labor soon and how her care team should proceed, Berghella said. A woman with a higher risk of stillbirth may want to have labor induced if her cervix is still quite long at her due date, suggesting the possibility of spontaneously going into labor is some ways off.
"We talk about what women want, and this is something women want. It would be very helpful to them," said Berghella. "It's a test most OBs should be able to provide, so that's what I'm hoping will happen."
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