and where the future parents may have to be married and heterosexual to be eligible for treatment.
When Jennifer and Aaron Wilson found they could not get pregnant, they knew exactly what they wanted to do.
The couple from North Carolina had the choice of starting in vitro fertilisation (IVF), in which mature eggs are fertilised with sperm in a laboratory. Or they could have tried to adopt a child already in need of a home.
Instead they applied to a specialist Christian fertility clinic in Knoxville, Tennessee - the National Embryo Donation Center (NEDC) - which promised to help them "adopt" an embryo.
Doctors often create extra embryos when a couple undergoes IVF, in case multiple rounds of treatment are needed. But this can leave many left over. More than 600,000 are currently being held in frozen storage in the US, most of them waiting to be used by the couple that created them the next time they want to try to have a child.
But not all of these embryos are needed, and it is estimated that one in 10 are available for embryo donation.
