DALLAS » First they made a commitment to serve their country, and then they made a commitment to each other.

But when Christy and Mike Trabun, who married in the fall of 2005, had difficulty conceiving a child, they pursued a commitment of a different kind: They "adopted" embryos donated by another couple that had been produced through in-vitro fertilization.

"We had never heard of anything like that," said Christy, who learned about the possibility on a radio show. "We thought, ‘That’s a really cool way to go about building a family.’"

It’s been 37 years since the first baby conceived through in-vitro fertilization was born. Pregnancies achieved through assisted reproductive technologies now account for 1.5 percent of U.S. births.

But couples who conceive through IVF often end up with more embryos than they need. The unused embryos are put in frozen storage, and parents pay regular fees — an average of $600 annually — to keep them viable.

Some couples will use them to try to have more children later, but those who decide their families are big enough face another choice: What to do with the remaining embryos?

Across the country, an estimated 600,000-plus unused embryos rest in liquid nitrogen at places like Dallas-Fort Worth Fertility Associates — some released by parents for "adoption" or still in the possession of those who can’t bear the thought of discarding or giving them away.

"It’s tough," said Dallas attorney David Cole, whose practice, called Little Flower, focuses on adoptions and assisted productive technologies. "For some couples, it’s just difficult to donate them to someone else. But on the other hand, it’s very difficult for some to discard them."

Sometimes, unused embryos can spark legal battles, as in a well-publicized case this year between actress Sofia Vergara and ex-fiancee Nick Loeb, who is suing the Modern Family star to keep her from destroying frozen embryos the couple created during their relationship.

For wannabe parents Mike Trabun, 50, and wife Christy, 34, who had met in the U.S. Marine Corps, the decision was easy. They’d already considered traditional adoption but found the idea of taking on already-created embryos appealing.

While the term "adoption" is largely used to make the concept relatable to prospective parents, the legal term in Texas and elsewhere is "embryo donation," which skirts the red tape tied to traditional adoption.

"It’s truly a hybrid between IVF and traditional adoption," said Dr. Karen Lee, of Dallas-Fort Worth Fertility Associates.

The Trabuns had known people whose efforts to be parents — including IVF, traditional adoption and fostering — had ended in heartbreak. But Mike, who already had two children from a previous marriage, wanted to give Christy a chance to experience pregnancy.

"Her dream was to be a wife and mom," said Mike, a student at Dallas Theological Seminary. "That was the biggest thing for me."

With the help of a national program called Snowflakes Embryo Adoption, which has facilitated such placements since 1997, the Trabuns took on seven embryos donated by a couple from the Pacific Northwest.

Three of those seven embryos were thawed and transferred to Christy’s uterus. That resulted in a pregnancy and a son, Cade, now 4.

They’d let go of the idea of having a genetically connected child who looked like them, but looking back on it now, they can smile. That’s because a little more than a year later, Mike and Christy conceived a child: Gage was born in June 2012.

But the couple wasn’t done yet.

For parents looking to conceive, embryo donation can be an attractive option for several reasons. For one, it’s cheaper: According to embryoadoption.org, total expenses range from $6,000 to $15,000, compared with $20,000 and higher for IVF.

Embryo transfer also offers a simpler medication regimen, and for some, like the Trabuns, who believe human life begins at conception, it’s a choice that allows them to do some good.

At DFW Fertility Associates, embryo donation is an option addressed with couples pondering traditional adoption when other methods don’t work or become undesirable.

"There’s something noble about it. These are embryos that would otherwise be left in the lab," Lee said

Rates of success vary widely, with estimates ranging from 28 to 50 percent depending on the clinic. Factors might include the egg donor’s age or lesser quality embryos, but Lee said what’s most at play is the technology used to freeze and thaw the fertilized eggs and how developed they are when transferred to the prospective mother.

In 2002, faced with the issue of surplus embryos, Congress established funding to promote embryo adoption as a family building option, and the number of babies born through such placements has risen 18 percent a year since 2012.

However, the surplus has also grown. And while some nations impose limits on how long embryos can be frozen before they must be donated or destroyed, the U.S. has no such rules.

While the Trabuns, with son Gage, had at last naturally conceived a child, they still wanted to grow their family. They returned to their remaining four embryos. Two failed to survive the thaw, but the other two were transferred to Christy.

This September, twins Coen and Shiloh, a boy and girl, will celebrate their first birthday.

Cade, Coen and Shiloh are genetic siblings — and though they are part of a different family, they have a biological brother and sister in Oregon, where the couple who created the original embryo batch had twins of their own. The Trabuns regularly email photos of their kids to them.

The Trabuns want to be honest with their children about their narrative from the start.

"There should never be a time in his life when he looks back and says, ‘Huh, that’s the day my parents changed the story on me,’" she said.

That the Trabuns opted to use their remaining embryos instead of trying to conceive naturally again was puzzling to some, but for Christy, it was a chance to make a difference.

"It’s just a reminder that families are built in so many different ways. … We felt it was a step in the right direction."