Fertility centers are mandated to report the number of in vitro fertilization (IVF) cycles they perform, but a new study suggests those data may give some practices misleadingly high success rates.

Researchers looked at a database maintained by the Society for Assisted Reproductive Technologies (SART) and found the proportion of cycles that were begun but had no reported final outcome – successful or unsuccessful – increased from 2005 to 2010.

What’s more, 13 U.S. fertility centers out of 341 accounted for half of those excluded cycles, and they reported significantly better overall pregnancy rates than the average clinic.

Researchers said some cycles may disappear from the data when clinics take frozen embryos from multiple rounds of fertility treatments and only implant the one that looks best – either as a treatment strategy, or to make their success rate look better.

That may be done most often with older women or others who have a lower chance of conceiving.

“Among this group of outlier clinics were very well known clinics, many of which are considered leaders in the field of ART in the United States,” said Dr. Vitaly Kushnir, who led the study at New York’s Center for Human Reproduction.

“Clearly, their pregnancy rates are significantly better than those of other clinics in the whole country, and it seems that a large part of that is these clinics have been excluding poor prognosis patients from outcome reporting,” he told Reuters Health.

“It’s probably a combination of clinics that are doing this unconsciously and some that are doing it intentionally.”

Fertility clinics are required to report data on completed IVF cycles to the U.S. Centers for Disease Control and Prevention (CDC), and can choose to report initiated and completed cycles to SART, the researchers noted.

Because they were interested in comparing the number of started and finished cycles, Kushnir and his colleagues used SART data, which covered close to three-quarters of U.S. fertility centers in 2010 and 812,400 cycles during the whole study period.

They found the number of excluded cycles – those that had initiation but not final data – increased from about 3 percent in 2005 to more than 7 percent in 2010.

Among the 13 outlier centers, excluded cycles accounted for more than one-third of all those initiated.