Working mostly over the Internet, Ms. Kehoe handpicked the egg donor, a pre-med student at the University of Michigan. From the Web site of California Cryobank, she chose the anonymous sperm donor, an athletic man with a 4.0 high school grade-point average.

On another Web site, surromomsonline.com, Ms. Kehoe found a gestational carrier who would deliver her baby.

Finally, she hired the fertility clinic, IVF Michigan, which put together her creation last December.

“We paid for the egg, the sperm, the in vitro fertilization,” Ms. Kehoe said as she showed off baby pictures at her home near Grand Rapids, Mich. “They wouldn’t be here if it weren’t for us.”

On July 28, the Kehoes announced the arrival of twins, Ethan and Bridget, at University Hospital in Ann Arbor. Overjoyed, they took the babies home on Aug. 3 and prepared for a welcoming by their large extended family.

A month later, a police officer supervised as the Kehoes relinquished the swaddled infants in the driveway.

Bridget and Ethan are now in the custody of the surrogate who gave birth to them, Laschell Baker of Ypsilanti, Mich. Ms. Baker had obtained a court order to retrieve them after learning that Ms. Kehoe was being treated for mental illness.

“I couldn’t see living the rest of my life worrying and wondering what had happened, or what if she hadn’t taken her medicine, or what if she relapsed,” said Ms. Baker, who has four children of her own.

Now, she and her husband, Paul, plan to raise the twins.

The creation of Ethan and Bridget tested the boundaries of the field known as third-party reproduction, in which more than two people collaborate to have a baby. Five parties were involved: the egg donor, the sperm donor, Ms. Baker and the Kehoes. And two separate middlemen brokered the egg and sperm.

About 750 babies are born each year in this country through gestational surrogacy, and twice that many surrogacies are attempted. Most are less complicated than the arrangement that resulted in the birth of Ethan and Bridget.

But as the dispute over the Michigan twins reveals, surrogacy arrangements that go badly can have profound implications, particularly for the children. Surrogacy is largely without regulation, with no authority deciding who may obtain babies through surrogacy or who may serve as a surrogate, according to interviews and court records.

Instead, surrogacy is controlled mainly by fertility doctors, who determine which arrangements are carried out and also earn money by performing the procedures. And while some agencies that coordinate surrogacies and some clinics that carry them out strictly adhere to guidelines, others do not, the interviews and records show.

The lax atmosphere means that it is now essentially possible to order up a baby, creating an emerging commercial market for surrogate babies that raises vexing ethical questions.

In some cases, parents must go through adoption proceedings to gain legal custody of the children. But even in those situations, the normal adoption review process is upended. In surrogacy, prospective parents with no genetic link often create their own baby first, then ask for legal approval, potentially leaving judges with little alternative. Some states allow prebirth orders that place the parents’ names on the birth certificates without any screening.

When disputes arise after the babies are born, the outcome can vary from state to state. In California, considered a friendly state for surrogacy, courts have upheld the validity of surrogacy contracts, meaning that the people who hire surrogates are very likely to keep the babies if a dispute arises.

But a statute in Michigan, where Ethan and Bridget were born, holds that surrogacy is contrary to public policy and that surrogacy agreements are unenforceable, giving the woman who gives birth a strong case if she decides to keep the babies.

A handful of other states have similar laws, according to an analysis by the Center for American Progress, a liberal research group.

About 10 states have laws that allow for surrogacy but impose restrictions; several of those states require at least one parent to have a genetic relationship to the baby. But the majority of states are silent on surrogacy, according to the analysis. Legal uncertainty in some states means that babies are sometimes left in limbo, their parentage left up to courts.