A better way to predict baby’s due date?

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Of all the pregnant women you’ve known in your life, how many delivered their precious bundle of joy on their due date?

Probably not many. Only 5% of pregnant women deliver on their estimated due date, with most celebrating baby’s arrival one to three weeks early or late. But a new meta-analysis from Thomas Jefferson University in Philadelphia says a routine test might help moms late in pregnancy narrow the window when baby is expected.

“Measuring cervical length via ultrasound at around 37-39 weeks can give us a better sense of whether a mother will deliver soon or not,” Dr. Vincenzo Berghella, a senior author of the analysis, said in a press release.

The study analyzed the use of transvaginal ultrasound measurements of cervical length for 735 women with single-child term pregnancies and found when the cervix was 10 millimeters or less, there was an 85% chance the baby would be born within the next seven days. When the cervix was less than 30 millimeters, there was a 50% chance.

“Women always ask for a better sense of their delivery date in order to help them prepare for work leave, or to make contingency plans for sibling-care during labor,” said Berghella, who is director of maternal fetal medicine at Thomas Jefferson University Hospital. “These are plans which help reduce a woman’s anxiety about the onset of labor,” allow “providers and birth locals” to better plan staff and coverage, and help women decide if they should go with repeat caesarean delivery or attempt vaginal birth.

Doctors contacted by CNN weren’t so sure about the study’s conclusions.

“This seems more like a party trick, rather than a useful tool,” said Dr. Katie Babaliaros, an OB-GYN with Peachtree Women’s Specialists in Atlanta. “At the very best, there’s still 15% of these women who are going to be saying ‘Where’s my baby?'”

“Checking a cervical length at term only gives a window of time, in weeks, in which a woman may deliver,” said Dr. Dian Tossy Fogle, a perinatologist at Northside Hospital Center for Perinatal Medicine in Atlanta. “Most patients are already prepared that delivery will be occurring in the near future and have taken precautions. I do not see a real use for this in daily clinical care.”

How due dates are determined

To determine a due date, doctors use a simple calculation using the first day of a woman’s last menstrual period. They then add 280 days to get to what would be considered a “term” baby: 40 weeks of gestation.

“This can be confirmed, ideally, by a first trimester ultrasound,” Fogle said. “If the menstrual period is unknown, then an ultrasound may be performed to establish a due date. First trimester ultrasounds are very accurate in determining the due date.”

The problem with knowing if baby will actually be born on that due date is that science isn’t sure about all the reasons why a woman goes into labor.

“There are many factors that are unknown and beyond the control of women or the best medical experts that will predict when a baby will deliver,” said Dr. Pratima Gupta from Physicians for Reproductive Health.

“We just don’t know how all the pieces all fit together,” Babaliaros said. “In the weeks leading up to delivery, there are a ton of hormones and chemicals that are released by the mom and the baby to trigger labor and those occur in very complicated ways.”

Potential downsides to ultrasounds

To further burst the bubble of women hoping for a more accurate delivery date, the experts CNN talked to mentioned several downsides to transvaginal ultrasounds.

“My concern with this study is that it will increase health care costs with no clinical benefit,” Fogle said. “If the goal is to reduce ‘maternal anxiety’ regarding timing of delivery, a simple physical exam of the cervix performed at a routine OB visit can also give a patient an idea of how soon they will deliver.”

The transvaginal ultrasound also won’t necessarily produce a more accurate measurement, experts said.

“Cervical length measurement can vary depending on the provider performing it,” Gupta said of the ultrasounds. “Therefore, the potential downside to this new technique is that a woman is given an (estimated due date) that is inaccurate due to the subjectivity of who did the measurement.”

Fetal medicine specialist Dr. Ashley Roman, director of the division of maternal fetal medicine at NYU Langone Medical Center, said it might be helpful for some patients with rare planning needs.

“The patient who has a partner who is traveling out of town or who is serving abroad in the military,” Roman said. “If the cervical length is less than 10 millimeters, it might be time to hang around.”

“But even if the cervical length is greater than 30 millimeters, the chance of delivering in the next week is like flipping a coin. ‘Who knows?’ This data isn’t that helpful because this is what we already tell our patients.”