You may have heard the happy news: Tyra Banks and her boyfriend, Erik Asla, had a boy! On Instagram last week, the new mom revealed the birth of her son, York, whose arrival came as a complete surprise since Banks, 41, had never announced that she was expecting—only she was, via gestational surrogate. So what is that again, exactly?

In short, using a gestational surrogate, or gestational carrier (the technical term), is an option if a woman is unable to carry her own pregnancy, or doing so would be too risky for her health. (Banks spoke out about her struggle with infertility last fall, telling People, “I’ve had some not-so-happy, traumatic moments.”) But who serves as the carrier? And which woman is the baby’s biological mom? Because the arrangement can get a little complicated, we asked two specialists to share the basic facts.

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The child is born with his parents’ DNA

In most cases, this is how the process unfolds: “Through an IVF cycle, you take the egg from the mom and fertilize it with the dad’s sperm, then transfer the embryo into the uterus of a gestational carrier,” explains reproductive endocrinologist Naveed Khan, MD, of Shady Grove Fertility in Leesburg, Virginia. The gestational carrier then carries the baby to term, and mom and dad resume their rolls as parents after the baby is born. So a gestational carrier has no biological relation to the child.

A gestational carrier is not the same as a surrogate

A traditional surrogate becomes pregnant with her own egg and the father’s sperm—via IVF or intrauterine insemination (in which sperm are placed directly in the uterus)—and she is the child’s biological mother. But this arrangement isn’t very common anymore, says Dr. Khan. “Most parents want their child to have their genes, so they use a gestational carrier.”

Yes, you know other stars who’ve had a gestational carrier

Giuliana and Bill Rancic’s son, Edward Duke, now 3 and a half, was born with the help of a gestational carrier. So was Lucy Liu’s baby Rockwell Lloyd, who arrived last summer. The list goes on: Ellen Pompeo, Sarah Jessica Parker, Elizabeth Banks, Nicole Kidman—they’ve all used surrogates to carry their children.

Gestational surrogacy isn’t done for convenience

“When word gets out there, people start thinking ‘oh I should do it,’ because they don’t want stretch marks, or they’re too busy. But those aren’t medical reasons,” explains Dr. Khan, who points out that all pregnancies involve the potential for complications, and gestational carriers are assuming risk that shouldn't be taken lightly.

There are a variety of reasons a couple might need a carrier

For example, the woman may not have a uterus after a hysterectomy to treat a disease like cancer. Or she might have suffered recurrent miscarriages, or tried several cycles of IVF that were unsuccessful. Or she could have a health problem that makes pregnancy dangerous, such as pulmonary hypertension, says Michael Cackovic, MD, an obstetrician specializing in maternal fetal medicine at The Ohio State University Wexner Medical Center. Homosexual male couples who are using donor eggs also turn to a gestational carrier to have a child.

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You can ask a friend—or find a stranger

Choosing the person who will carry your baby is a big deal, no doubt. She may be someone you know, like a sister or trusted friend or even, in some cases, your mom. (That means grandma gives birth to her grandbaby. Yep, it’s happened before.) Or you can go through an agency that will match you with a carrier. Agencies consider the medical and pregnancy history of a prospective carrier, as well as her emotional wellbeing. They also do lab workups, and may have other requirements, like a limit on past cesarean sections, says Dr. Cackovic. Carriers are heavily screened, he explains, in a process that’s similar to selecting an egg donor.

Couples are advised to lawyer up

It’s all about a healthy baby in the end. But you’ve got to protect yourself, says Dr. Khan. Find a lawyer who specializes in reproductive law and create a legal contract with your gestational carrier. Surrogacy remains in a bit of a legal vacuum. (Sherri Shepherd made headlines last year in a sad, complicated case involving a child conceived from a donor egg and her ex-husband’s sperm, and carried in a surrogate’s womb.) What’s more, laws vary state by state, says Dr. Khan. Some don’t recognize gestational carriers at all, which makes it more difficult to legally transfer the baby to her parents. His advice: Make sure your legal advisor is experienced and well versed in the law to help ensure your baby’s birth is the incredibly joyous occasion it’s meant to be.