As more Americans than ever before turn to in vitro fertilization to create families, a surprising ethical question — what to do with thousands of embryos stuck in a sort of frozen limbo — is dividing US Catholics who call themselves pro-life, with some calling for the embryos to be adopted, while others say: Not so fast.

In vitro fertilization, in use since the early 1980s, is pretty straightforward, at least medically. Doctors combine human eggs and sperm outside of the womb to create embryos and then implant one or more in a woman’s uterus. To increase the chances of success, doctors produce several embryos at a time, not all of which are implanted.

About 65,000 children were born in the United States last year using the procedure.

The cost can run into tens of thousands of dollars, not to mention the potential emotional toll on parents waiting to see if the implantation leads to a successful pregnancy.

For some Catholics, IVF is a last-resort option that has enabled them to have a family. But the issue is not as clear-cut for other Catholics who consider themselves pro-life. They are troubled by the creation of fertilized eggs, a potential human life, that are not carried to term. It’s one of the reasons that the procedure is condemned by the Church.

When the owners of the embryos decide their families are complete, embryos often remain, and the owners must decide what to do with them.

That’s where it gets tricky.

“We have an unmitigated tragedy that we have probably close to a million frozen human embryos in the US,” said the Rev. Tadeusz Pacholczyk, the director of education at The National Catholic Bioethics Center in Philadelphia.

About 85 percent of the estimated 600,000 to 1 million embryos that are frozen could still be used by the individual or couple who created them. But the rest face a less certain future.

Most Catholics would agree it’s a less-than-ideal situation. But the debate over what to do with those remaining 15 percent is where fault lines appear.

The owners can choose to thaw and discard them, donate them for scientific research, pay about $600 per year to keep them on ice, or give them back to clinics that offer them to other couples who are unable to conceive naturally.

But since the mid-1990s, another option has emerged: treating the frozen embryos as babies available for adoption.

Adopting frozen embryos differs from acquiring them from a clinic. With adoption, the embryos’ owners have a say in who will receive the embryos. The couple putting the embryos up for adoption and the couple adopting them exchange family histories, and the decision to move forward is mutual.

In cases where the embryos are simply given back to a fertility clinic, it is treated like a transfer of property, with the transaction ending there. The creators of the embryos usually have no say in what happens next.

One of the largest agencies that arranges embryo adoption, Nightlight Christian Adoptions, has received about $5 million in federal funding from the US Department of Health and Human Services to promote embryo adoption, acting as a sort of matchmaker for the donors.

“They want to know something about the person who will receive their embryos,” said Kimberly Tyson, marketing and program director of the non-denominational Christian organization, which has offices in eight states. “How long have they been married? What type of faith do they practice? What is their financial situation? How much contact are they willing to have?”

That’s exactly how Andrea Alexander and her husband formed their family.

A self-described “pro-life, conservative Catholic,” Alexander, 34, said her 5-year-old twins and 6-month-old baby are all the result of embryo adoption.

She said she and her husband, who live in Fort Hood, Texas, where her husband is in the Army, spent months researching to see how their faith might inform their decision.

“We were prepared that if the Church had a definitive stance, that we would stop what we were doing and change to traditional adoption,” she said. But after reading encyclicals, letters, and consulting theologians and priests, “We decided that we could move forward.”

Since then, Alexander said she has counseled other women considering embryo adoption, including a Catholic friend who is about to give birth to her third child as a result of the procedure.

The debate over the fate of frozen embryos certainly isn’t unique to Catholicism.

Earlier this month, for example, a judge in San Francisco ruled that a set of frozen embryos must be destroyed after the couple that created them split up. The judge said the contract the couple had signed when they created the embryos took precedence over the woman’s plea to let her take custody of the embryos because she was no longer able to have children after cancer treatment.

Then there was Nick Loeb, the ex-fiancé of Modern Family star Sofia Vergara, who filed a lawsuit in 2013 seeking custody of the embryos the pair had created before they broke up and Vergara married actor Joe Manganiello. A judge has not yet ruled in that case.

The complexity in Catholic circles arises from the reason the Church is opposed to IVF in the first place: The way the embryos were created, in a glass dish outside of a sexual encounter — separating procreation from the conjugal act, thereby depersonalizing the act and also leading to embryos that don’t survive.

The Vatican hasn’t ruled definitively on embryo adoption. The closest it comes to offering guidance are two paragraphs in “Dignitas Personae,” a 2008 document that calls the existence of frozen embryos “a grave injustice.”

It condemns using embryos for research as well as giving them to fertility clinics. But on the issue of embryo adoption, theologians and ethicists say the Church has left the question open.

In “Dignitas Personae,” the Church says that “the thousands of abandoned embryos represent a situation of injustice which in fact cannot be resolved.”

The solution, the document says, was proposed by Pope St. John Paul II: Halt the production of human embryos outside the womb because “there seems to be no morally licit solution regarding the human destiny of the thousands and thousands of ‘frozen’ embryos which are and remain the subjects of essential rights and should therefore be protected by law as human persons.”

A 2009 letter from the US Conference of Catholic Bishops about the topic offered little clarity, repeating JPII’s call for an end to IVF generally.

Alexander, the mother of three from Texas, said the lack of information about embryo adoption means it’s difficult for Catholics considering the process to make informed decisions.

“I wish there were more education on the pastoral level,” she said. “Of all the priests I’ve talked to, only one has heard of it before.”

But others say there is plenty of information out there already.

“My opinion is that this is something that [Catholic couples] should not do. It’s something I’m convinced is always morally disordered and not acceptable,” said Pacholczyk, the priest in Philadelphia who also holds a PhD in neuroscience from Yale University.

He concedes, however, that the Church hasn’t ruled definitively on the issue. He suggested that couples considering embryo adoption should rely on prayer, research, and ethicists to come to their own conclusion.

And as for all those frozen embryos?

Pacholczyk said the owners “have certain parental duties toward their frozen embryos, toward their frozen children.”

“The only thing you can really do to protect them and to respect their integrity is to pay the bill each month or each year to the company that is pouring fresh liquid nitrogen into the tanks to preserve them,” he said. He suggested that the owners of frozen embryos consider creating a trust to pay the bills.

Other Catholic theologians, however, are on board with embryo adoption.

“In principle, it’s moral for women to gestate an embryo that is not their own biological embryo and I think it can be a very generous and charitable act,” said Janet E. Smith, an ethics professor at the Sacred Heart Major Seminary in Detroit. “These are human beings, and these are human beings who need a certain environment in order to come to full fruition.”

Charles C. Camosy, author of “Beyond the Abortion Wars” and a professor at Fordham University, said he doesn’t understand how some Catholics can on one hand claim frozen embryos are human beings, but on the other oppose embryo adoption.

“I wonder if some pro-lifers actually believe our own rhetoric about the personhood of the embryo,” he said.

“If there were hundreds of thousands of frozen 4-year-olds in need of rescue and adoption, wouldn’t the tone of the theological and ethical discussion sound quite different?” he asked.

Another theologian, Kent J. Lasnoski of Wyoming Catholic College, believes embryo adoption is morally permissible only if a couple is doing it to save a life, not to solve their infertility problems. For that reason, he said, infertile couples should refrain from adopting embryos. He pointed to historical events in the Church as justification.

“Is embryo adoption really a fertility question or is it a work of mercy?” he asked. “Is it somehow really giving my body for the sake of the captive?”

He said religious orders of the Middle Ages that raised funds to pay ransom for captive Christians held in Islamic lands, actions approved by the Church, provide a model for how embryo adoption could be viewed as a positive Catholic response to frozen embryos.

“They would go to lands where Christians were held captive and buy them back, in the same way that you would buy an embryo from the fertility clinic,” he said.

But Smith, the seminary professor, called the notion that embryo adoption should be closed off to infertile couples “a very foolish condition.”

“If [embryo adoption] meets the needs of both the embryo and of the infertile couple that wants to have a child, that’s a good thing, not a bad thing,” she said.

In addition to the issue of removing the sexual act from fertilization, Catholic opponents of embryo research also are wary that it bolsters the loosely regulated, multi-billion dollar IVF industry, which the Church opposes.

An increase of embryo adoptions, Pacholczyk said, “would play into the hands of those who promote IVF and backfire.”

“What we have in the US is an assembly-line manufacturing of human beings, and nobody is batting an eyelash at it,” he said. “This will be just another way to play into the market dynamics. That’s what is ultimately driving all of this.”

But Tyson, the marketing director at Nightlight Christian Adoption, said she is focused exclusively on the fate of frozen embryos — and wishes more Catholics would join her cause.

“We’re not trying to bring an end to IVF. We’re trying to solve a problem that is brought about by IVF,” she said. “I’m about rescuing babies from frozen storage.”

Only a few agencies offer the service, so embryo adoption has led to little more than 1,000 births, according to estimates.

But, she said, “If I got my Catholic brethren to participate, I could maybe make a bigger dent.”