As if the worry over potential miscarriage weren’t stressful enough for newly pregnant women, research released Friday shows that current guidelines for using ultrasound to determine that a pregnancy has ended may not always be accurate.
The four new studies looked at U.K. ultrasound specifications, but when the findings are extrapolated to U.S. guidelines, the researchers suggest 1 in 23 women diagnosed with miscarriage could still have a viable pregnancy.
What that might mean for the future of such incorrectly doomed pregnancies is unclear. Researchers did not analyze whether women in the studies who were told that their pregnancies had ended went on to have surgical procedures to clear out their uterus.
But just the possibility that a pregnant woman would unwittingly end a viable pregnancy is intolerable, says Tom Bourne, a professor of gynecology at Imperial College London and senior author of three of the related studies published in the journal Ultrasound in Obstetrics and Gynecology. “We are concerned that current guidelines can result in a misdiagnosis,” says Bourne. “The current cut-offs for miscarriage are not appropriate.”
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Ultrasound is the gold standard used to determine miscarriage. It’s assumed — at least by expectant mothers — to be exceptionally accurate. Yet one of the studies, which analyzed the existence or size of the gestational sac, found that the presence or absence of a gestational sac, as well as its size, were not foolproof indicators of miscarriage.
Researchers followed 1,060 women who complained of light bleeding or pain — both potential signs that a pregnancy is in trouble — between five and eight weeks of pregnancy and had an initial ultrasound scan. As is standard practice, they returned seven to 10 days later for another scan to measure the sac’s growth. Those who were still pregnant were followed up again between 11 and 14 weeks.
Typically, an embryo larger than 6 mm without a heartbeart or with no detectable increase in the size of the gestational sac between scans is assumed to indicate miscarriage, but Bourne found that it’s possible that a healthy pregnancy may not grow measurably in the course of a week or so. In fact, half a percent of the women in the study — or 1 in 183 — still could have been pregnant even though scan guidelines technically would have classified their pregnancies as ended.
In the U.K., ultrasounds that reveal an empty gestational sac over 20 mm in size result in a diagnosis of miscarriage. In the U.S., the standard set by the American College of Radiology is 16 mm. Based on the researchers’ findings, 8 in 183 U.S. women — or 1 in 23 — would have a miscarriage diagnosed incorrectly.
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Another one of the studies found that measurements of a gestational sac can vary up to 20% depending on who does the measuring; different clinicians get different results. Say, for example, that one sonographer overestimates sac size, then a second sonographer subsequently underestimates it. The resulting confusion could result in a women being falsely told she had miscarried.
To increase accuracy, Bourne recommends ultrasound guidelines be updated and standards for determining miscarriage increased.
Jonathan Schaffir, an associate professor of obstetrics and gynecology at The Ohio State University who was not involved in the research, said the British studies are significant, although it’s likely that most doctors who suspect miscarriage at very early stages of pregnancy would adopt a wait-and-see approach. “It’s important to not miss even one viable pregnancy, but it’s pretty unusual to determine miscarriage based on a scan at these early, early measurements,” he says. “From a practical point of view, the findings of these studies are not going to change clinical practice.”
Bonnie Rochman is a reporter at TIME. Find her on Twitter at @brochman. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.