Usually, it’s just parents who anxiously await test results from their fertility doctors. But for one particular IVF clinic in the Netherlands, the doctors, too, are sweating this one.

After technicians at Utrecht's University Medical Center reproductive technology clinic discovered a problem with a rubber apparatus used in a fertility treatment, the clinic and 26 couples are nervously waiting to see if eggs were fertilized with the wrong sperm. Half of the couples had their fertilized eggs frozen, while the other half are either pregnant or have already had their babies. The oldest baby affected by the rubber apparatus fiasco is around one year old.

“In each of these cases there is a small risk—small but it cannot be ruled out—that during the procedure, sperm cells of the mother’s own partner have been mixed with remaining sperm from a previous procedure,” center spokesperson Paul Geurts told the New York Times.

The procedure to which Geurts is referring is intracytoplasmic sperm injection, or ICSI, which is when doctors use a pipette tip to inject a single sperm cell into an egg for fertilization. ICSI is often used in cases involving male infertility.

Technicians discovered in November that they had been using the wrong type of rubber apparatus at the end of the pipette, which apparently left sperm lingering at the tip instead of shooting clear. They admitted the wrong part may have been in use for as far back as April 2015.

“The hospital regrets, of course, what has happened to the couples involved, and we will provide counseling and a meeting with the medical staff with all of these couples to work their way forward,” Mr. Geurts said. “It was just an unfortunate event.”

The clinic has informed all of the couples affected and offered them DNA tests to determine whose sperm fertilized their eggs.

Geurts declined to say whether the technicians involved were disciplined.