A Dutch in vitro fertilization (IVF) center has launched an investigation looking into the possibility that eggs of up to 26 women may have been fertilized by sperm from the wrong man. The University Medical Center in Utrecht (UMC) said the mix-up may have occurred due to a “procedural error.”

The error, the hospital said, occurred between mid-April 2015 and mid-November. Half the couples who underwent the treatment are either pregnant or have already had children. The UMC said all the couples involved have been informed of the error.

“During fertilization, sperm cells from one treatment couple may have ended up with the egg cells of 26 other couples,” the facility reportedly said Tuesday. “Therefore there’s a chance that the egg cells have been fertilized by sperm other than that of the intended father.”

The UMC admitted that the chances of the mix-up happening are small but the possibility “could not be excluded.”

“For some of the 26 couples frozen embryos are still available but the chance remains that they [too] have been fertilized by the sperm from a man other than the intended father,” the UMC said. “The UMC’s board regrets that the couples involved had to receive this news and will do everything within its powers to give clarity on the issue as soon as possible.”

The mistake involved injecting a single sperm directly into the egg using a pipette, a technique called Intracytoplasmic sperm injection. From April 2015 to November 2016, a technician is believed to have used an unsuitable pipette to inject the sperm.

While the pipette was changed each time, the same rubber top containing traces of the sperm still in it was used for all 26 couples. A hospital spokesman told the BBC that the rubber top is usually fitted with a filter but that was not the case here.

Nine of the 26 couples have had children while four women are pregnant and 13 other embryos were frozen. The couples are scheduled to meet doctors from UMC in the following days and can take up a DNA test if they wish to do so.