They're sometimes called "Snowflake" babies — children who were once frozen as embryos in a thin cane of liquid nitrogen.

They were created when their biological parents used in vitro fertilization in their quest to conceive a child, and these embryos were the leftovers, the ones that didn't get used. The federal government spent nearly $1 million this year trying to get people to adopt them.

But unlike wild burros and older children — which the government also would like Americans to adopt — frozen embryos come packed with ethical baggage. They are people-in-waiting, but governed by property laws. Their numbers are growing, with more than 600,000 embryos currently in cold storage in fertility clinics and commercial storage facilities across the U.S., which has led one Catholic theologian to charge that the clinics are running an "embryo mass-production line." There are even custody battles over them.

It's a dilemma for both the parents of the embryos and the society that endorsed their creation: What should we do with these frozen clusters of cells, sometimes called "souls on ice?"

They can be used for stem-cell research, or they can simply be destroyed by thawing them and allowing them to die. Or they can be "adopted" and implanted in a woman who is not the mother. Each of these solutions, however, involve highly charged political and ethical issues: among them, when life begins, at what point a cluster of human cells contains a soul, and the moral propriety of creating multiple embryos for research or to help infertile couples bear a child.

Even embryo adoption can be a difficult decision for biological parents because another family would raise their genetic offspring and their own children may never know they have siblings.

At the core of the issue, however, shines something of a miracle — that a few human cells, frozen for a decade or more, can be thawed and become a warm, healthy baby in nine months or less.

A less costly adoption

That’s how Maria Lancaster came to be a mother, and to help others bear children the same way.

Lancaster, who lives near Seattle, had suffered several miscarriages when her husband heard an interview with a woman who had given birth to a child who began life as a frozen embryo. Then 47 years old, Lancaster wasn’t even sure the process would work. But she found an adoption agency that offered embryo adoptions, and the cells that would become her daughter were “flown frozen” across the country along with another embryo and implanted in her uterus. She lost one of the twins, but one survived: Elisha, who is 13, healthy and thriving.

Lancaster, now 60, later went on to start an embryo-adoption service through her church, and she now works full-time to match couples trying to have a child with couples who have embryos they no longer need. For her, Embryo Adoption Services of Cedar Park Inc. is part job, part ministry.

Embryo adoption, which costs about $5,000 through Lancaster's service, is less expensive than many other forms of adoption, and for some women, it’s their best chance to raise a child from infancy, Lancaster said. That's because the demand for newborns outstrips the number of available children, and for many couples, adopting infants is prohibitively expensive: A domestic infant adoption can cost anywhere from $7,000 to $40,000, according to the National Adoption Council.

Only about 1 percent of pregnant, unmarried women in the U.S. offer their child for adoption, the council says. Abortion rates have fallen in recent years, but it's still estimated that there are about 900,000 abortions in the U.S. each year.

The supply of embryos, however, is potentially immense, thanks to technology that enables doctors to coax life into being outside the body — and to preserve it indefinitely.

An embryo is born

When couples are unable to conceive naturally, many turn to fertility experts for in vitro fertilization, or IVF. Through this procedure, doctors collect eggs from a woman, then mix them with a man's sperm. When the egg is fertilized and an embryo develops, one or more are implanted in the woman's uterus to develop naturally.

IVF can help couples who were unable to conceive because of poor sperm production, uterine fibroids, damage to the woman's fallopian tube, or other conditions.

The first baby whose egg and sperm united in a petri dish was Louise Brown, the famed “test-tube baby” born in 1978. The advances that led to Brown's birth were watched — and at times, sharply criticized — by religious leaders who found the brave new world of reproductive medicine disturbing. Pope Pius XII said fertilization outside the womb amounted to doctors taking “the Lord’s work into their own hands.” A 1969 Harris poll found that a majority of Americans believed procedures like IVF were against God’s will.

Undeterred, reproductive medicine continued to progress, and in 1984, an Australian doctor delivered the first baby born from an embryo that had been created through IVF and then frozen for two months. Two years later, the first such baby was born in the U.S., and in subsequent decades, the length of freezing time of embryos has been inching up. A record was set in 2010 when a New York woman gave birth to a baby that had been frozen as an embryo for 20 years.

Although freezing and thawing can damage embryos, the success rate of such pregnancies at 45 percent at Nightlight Christian Adoptions, according to Kimberly Tyson, Nightlight's marketing and program director. Nightlight was the world's first embryo adoption service and has trademarked the term "Snowflake" babies.

The Society for Assisted Reproductive Technology reported a similar figure for 2013: 41 percent of 1,201 donated embryos resulted in a baby that year, the society said.

The success rate depends on the quality of the embryos when they were frozen, the age of woman who provided the eggs and the number of embryos that are implanted, The American Society of Reproductive Medicine says.

Tyson points out that these odds are better than those of natural fertilization; couples trying to conceive naturally have only about a 25 percent chance of getting pregnant in any given month.

The 500 Snowflake babies that have been born with Nightlight's assistance are statistically as healthy as children conceived naturally, Tyson said. “There is no research showing that frozen embryos have any more problems than the general population, or a higher rate of birth defects," she said.

There is some evidence, however, that some babies conceived through IVF have poorer outcomes than babies conceived naturally. A 2013 study of babies conceived through assisted reproductive technology found that they were slightly more likely to be underweight and have birth defects and epigenetic disorders. But most children conceived through IVF and other technology are healthy, the authors noted.

Moreover, some research has found that frozen embryos in IVF procedures have better outcomes than embryos that have not been frozen.

More than 5 million babies have now been born through assisted reproductive technologies, primarily IVF, worldwide; in the U.S., about 1.6 percent of all babies born in 2013 were conceived using such procedures. In Utah, the figure was 1.4 percent, or 718 out of 50,957 babies.

Accordingly, popular opinion on the subject has shifted. In a Pew poll conducted in 2013, 79 percent said they either approved of IVF or said it wasn't a moral issue at all.

The ongoing battle between "Modern Family" actress Sofia Vergara and her former boyfriend, however, could ensure that morality remains an issue.

Vergara and Nick Loeb have two embryos left over from an IVF procedure in 2013; in 2015, Loeb sued for custody of them. "In my view, keeping them frozen forever is tantamount to killing them," Loeb wrote in an op-ed for The New York Times.

In December, a pro-life group in Louisiana took up Loeb's cause, filing a lawsuit in which the embryos are named as plaintiffs entitled to life and and a trust fund established for them. The suit accuses the embryos' mother of neglect and asks that their father raise them.

Legal analysts say the case is extraordinary but don't expect it to be summarily dismissed since under Louisiana law, a fertilized egg is a "juridical person" that cannot be intentionally destroyed.

It also says that "in the matter of disputes, the judicial standard for resolving such disputes is to be in the best interest of the in vitro fertilized ovum."

The Society for Assisted Reproductive Technology takes a different view, saying that embryos have "special significance" because of their potential to become persons but "should not be afforded the same status as persons." The group discourages the use of the word "adoption" in the matter of embryos because it is suggestive of legal rights.

Ethics and religion

Because conception is not guaranteed even when doctors unite sperm and egg in a culture dish, multiple embryos are created. The average number of embryos left over after a couple finishes IVF is five. Storing them costs from $350 to $1,000 a year, according to ReproTech Limited, a business that offers embryo, egg and sperm storage. The embryos are maintained in tanks at temperatures below -300 degrees Fahrenheit, according to W. Brent Hazelrigg, ReproTech's president and CEO.

In medicine, the term "embryo" applies to a child up until 8 weeks after conception, when he or she is recognizably human. But embryos that are frozen are, at most, a few days old, and visible only under a microscope. In fact, they're not really embryos, but pre-embryos known as blastocysts, about .1 to .2 millimeters in size, Hazelrigg said.

"This truly is at the edge of the ability of a human to see. However, given these are either vitrified or frozen in a device of some sort, that makes seeing them truly impossible," he said.

To some people, however, the embryos are already human, possibly already endowed with a soul, which has led to conflicting thought about what should be done with them.

Frozen embryos, the Vatican has said, represent a "situation of injustice that cannot be resolved," and the Catholic Church has said embryos should not be destroyed. But the church is troubled by embryo adoption, however, because of the potential for abuse, said the Rev. Tadeusz Pacholczyk, a Catholic priest and Yale-trained neuroscientist who is director of education for The National Catholic Bioethics Center in Philadelphia. Among the Vatican's concerns is that businesses could cultivate embryos for profit.

Pacholczyk says the destruction of frozen embryos, however, is "morally indistinguishable" from leaving a baby to die in a dumpster. He believes that unused embryos should be left in storage indefinitely, and even suggests that parents of such embryos who are unable to bring them to birth leave a trust fund to provide for the embryos' continued maintenance after their parents' death.

The logical solution to the problem that Pacholczyk calls "orphans in liquid nitrogen" is for fertility clinics to produce only enough embryos that they will use.

"Legal measures such as those which exist in Germany and Italy would go a long way towards preventing the serious injustices associated with orphaning human beings in liquid nitrogen," he said.

For hundreds of parents, however, a frozen embryo turned living child is a God-given blessing, and some Christians believe that for parents who cannot bring their embryos to life themselves, allowing their adoption is the only ethical solution.

"You do not have 'frozen embryos.' You have children, frozen in this cruelly clinical world of suspended animation," the Rev. Russell Moore, a prominent Southern Baptist leader, has written.

Current policy of The Church of Jesus Christ of Latter-day Saints discourages surrogate motherhood and IVF using sperm or eggs from anyone other than the husband and wife, but the church does not address embryo adoption specifically in its guidance on adoption.

Property, not people

Couples can obtain embryos through specialty adoption services and some fertility clinics; one nonprofit organization, Miracles Waiting, works like a dating service, allowing families to select each other through internet postings.

There are more than 440 fertility clinics in the U.S., according to the Centers for Disease Control and Prevention, and more than 99 percent offer freezing services. Some help arrange adoption for parents who don't want to keep embryos indefinitely in storage.

At Utah Fertility Center, there is a waiting list for embryos that is from 6 months to a year, and the adoptive parent must be a patient of the clinic. As at most fertility clinics, the adoptions are anonymous. Adoptive parents can review the medical records of the embryo, but no further information is disclosed.

This is one reason people turn to faith-based adoption services like Nightlight Christian Adoptions that encourage the sharing of information between families. The parents of the embryos select the couples who will receive their embryos, Tyson said, and all the embryos left over from a procedure go to the same couple, instead of being split up among multiple families.

Most families who work with a Christian adoption agency prefer to place their embryos in a two-parent home, but occasionally, single women are permitted to adopt.

The age of the parents, however, is a factor. Nightlight requires that parents be 45 or younger; Lancaster's Embryo Adoption Service of Cedar Park has an age limit of 48.

And the recipients must undergo a home study and interviews by social workers as if they were adopting a child who has already been born.

A key difference, however, is that embryos are considered property, not people, and contracts are written accordingly. The classification has been been upheld in courtrooms all over the country, most recently in Missouri, where a divorced woman wants to give birth to two frozen embryos she has named Noah and Genesis.

The embryos were created when the woman and her former husband were married and undergoing IVF. The father, however, maintains that he should not be forced to have children with a woman to whom he is no longer married, and two judicial bodies have agreed. In November, Missouri's Court of Appeals upheld a lower court's ruling that said the embryos must remain frozen unless both parties agree to thaw and implant them. They are "marital property of a special character," the court order said.

While that position may be disturbing to some people, Tyson said she understands why so many American judges take this view.

"Think about our society — we love women to have a choice over their reproduction. If embryos become people, that has some very significant impacts in the reproductive world," she said.