Lisa, a mom desperate to give her son a little brother, is looking to trade embryos with another mom. Doing so, she’s wading into legal territory colored in shades of gray, experts say.

Only a handful of states — including Florida, Louisiana, Oklahoma and Georgia — have passed legislation regarding embryo donation. (New York is not among them.)

Georgia was the first to allow embryo-adopting parents to file for a final order of adoption once a child is born. In states without laws, couples who adopt embryos rely solely on private legal contracts.

According to Kimberly Tyson, program manager of the Embryo Adoption Awareness Center in Colorado, the general “law of the land is that the woman who gives birth to the baby is legally the baby’s mother on the birth certificate.” (This is complicated by surrogates, although typically a pre-birth order is drafted to assign parentage.)

Compared to the United Kingdom, which has a 10-year limit on embryo storage and prohibits people from discriminating against embryos based on race or religion, the United States is one of the most lax countries in the world when it comes to the creation, storage and donation of embryos.

According to the Society for Assisted Reproductive Technology donated embryos were used in 1,092 transfers in 2014, up from 596 in 2009. Rebecca Mendel, a New York attorney, said she has seen people interested in embryo donation even turn to Craigslist.

Despite the rise, Dr. Brian Levine, practice director for the fertilitiy clinic CCRM New York, said embryo donation isn’t very common in US due to the complicated legalities and contracts and because “egg and sperm donation is so easy to come by.”

Plus, he warns, it’s unlikely that the embryos put up for donation are the most viable. “You may not be getting the highest quality, which means the pregnancy rate might be sacrificed.”

Both Levine and Mendel both say they’ve never heard of embryo swaps, but, Mendel admitted, “It wouldn’t surprise me.”