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Unable to make babies on their own, more than two dozen couples went to a top-flight fertility clinic in the Netherlands for help.

But the clinic has announced it may have been leaving old sperm on its tools for more than a year — and now those couples face the prospect of raising a stranger's biological child.

“The chance is small, but cannot be ruled out” that women were “fertilized by sperm cells from a man other than the intended father,” officials at the University Medical Center in Utrecht said in a statement this week.

It's not the first mass case of artificial insemination potentially gone awry, though most high-profile bungles have mixed up anonymous donors.

In this case, the medical center said that it was trying to contact 26 men and women — half of whom already have or are expecting a baby — for 26 very difficult conversations.

“The supervisory board regrets having to burden the couples involved with the news,” the statement said.

The New York Times reported that the women were contacted after the statement's release and were offered DNA tests.

The patients didn't use the most common method of in vitro fertilization, in which egg and sperm are combined in a petri dish.

Rather, they used a newer procedure called intracytoplasmic sperm injection, which can help men with scant or sluggish sperm by shooting it directly into an egg cell.

A technician had just finished an injection in November, a hospital spokesman told the Times, when she discovered sperm from a previous patient on the end of the syringe.

The syringe tips are disposable, the Times reported, so the hospital suspects the rogue cell hitched a ride on a rubber bulb that wasn't supposed to be attached to the device.

The clinic had been improperly using those bulbs since April of the previous year. Some of those couples now have a child — the oldest might be a year old, if the first patient conceived right away.

Other women treated during that period are pregnant, according to the hospital. And 13 couples have frozen embryos at the clinic, which they must now decide whether they want to replace.

Meanwhile, the hospital is trying to figure out if any stray sperm actually managed to make a baby.

It's also unknown how many contaminant sperm cells from how many men may have been used on patients.

“Everything will be done to provide clarity for everyone as soon as possible,” the statement said.

David Keefe, who chairs New York University's department of obstetrics and gynecology, said his own clinic is hesitant to perform the injections, unless absolutely necessary. The procedure was considered risky by many doctors until a few years ago — mostly because it was new.

Now that it's been tested, Keefe said, it tends to be overused by patients in a hurry to have a child.

“This is one of many examples where it can go awry,” he told The Washington Post. “It's very sobering that one of the very best fertility centers in the most regulated country in the world still had risk.”

But he said that he suspected Utrecht was simply being cautious in halting the procedure and announcing its error.

Technicians usually pick up a single sperm cell at a time in a syringe, Keefe said. “The thought that one of them would have gotten up into the micro-manipulator is interesting, but I think theoretical. There aren't that many sperm to get up there.”

“I suspect they're going to find out every one of those kids is from the right father,” he said.

Keefe was at odds with his friend and colleague at New York University: Arthur Caplan, who founded NYU's bioethics division and heads its medical ethics department.

“If there was contamination from a healthy donor, versus a male making barely any sperm . . . then the odds of the baby coming from contaminant sperm is like 100,000 to 1,” Caplan said.

And where Keefe described the Netherlands' fertility clinics as the very strictest in a highly regulated industry, Caplan has argued that the fertility industry isn't watched closely enough.

Accidents are bound to happen, he said, and they often do.

But he called the Utrecht case “very unusual.”

He recalled many one-off and obvious mishaps — like a couple who sued a Chicago-area clinic, claiming they ordered sperm from a white donor but ended up with mixed-race child.

Caplan remembered cases of mass malfeasance, too, like an Indiana doctor accused of impregnating at least eight patients with his own sperm.

But those cases involved anonymous sperm banks.

The Utrecht mishap could be “deeply troubling to the male donor, because they were looking for their own biological child,” he said. “Most people would say, what choice do you have? You have to accept a child that was born in error.”

On the other hand, Caplan wondered what Utrecht will do to satisfy the unwitting source or sources of the errant sperm.

“What about the man or men? Does he find out?” he said. “Does he have 26 children now?”