"We may have given the wrong information, that there was no pregnancy while in fact the woman was pregnant," says chief doctor Per Weitz at the clinic.
The error was spotted by the Karolinska Hospital in Stockholm, where staff alerted the manufacturers, ANL.
"We still don't know how this happened," says the company's CEO Björn Gillberg. "We're currently looking into it."
The pregnancy tests in question have been sold to hospitals and clinics but are not for sale in drug stores and pharmacies.