This little boy was the first baby ever to be 'carried' by two mothers.

Five-month-old Stetson Coulter’s parents, Ashleigh and Bliss, call him their “miracle baby“. For the little boy, from the US state of Texas, is the first child in the world to have been ‘carried’ by two mothers.

Little Stetson came into the world courtesy of a sperm donor and an innovative technology known as an Intravaginal Culture (IVC) device, which allowed both women to play an integral role in the creation of their son.

How it works.

The process is similar to IVF, which involves fertilising eggs with sperm in a lab, incubating them, and transferring the resulting embryos into the uterus.

In this process, though, the sperm and eggs are placed inside a small device, called the invocell, which is inserted into the vagina where it is incubated courtesy of body heat. The process is less costly, though not proven to be any more effective.

The Coutlers, who met six years ago, were offered the option by husband and wife fertility specialists Drs Kathy and Kevin Doody.

“We were just talking one night at home and I said, ‘You know, I think we could use this for a same-sex couple,'” Dr. Kathy Doody told local news network ABC WFAA. “And Kevin said, ‘I think you’re right. I think we could.'”

It was Bliss' eggs that were placed into the device with donor sperm, and she who carried it for those early stages.

"She got the embryo off to an early start," Dr Kathy Doody said. "The eggs fertilised in her body and when they returned five days later, we removed the device and froze the embryos."

Doctors then primed Ashleigh with oestrogen and progesterone, waited for the right time in her cycle, and transferred the embryos into her uterus - "Almost like passing the baton, like it's a relay race," Dr Doddy added.

She fell pregnant the first try.

"She got to carry him for five days and was a big part of the fertilisation, and then I carried him for nine months," Ashleigh told WFAA.

"So that made it really special for the both of us that we were both involved. She got to be a part of it, and I got to be a part of it."

Australian couples will have to wait.

Fertility specialist Dr Joseph Sgroi of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists noted that the US-developed device is not yet available to Australian couples.

Currently, female same-sex couples who both wish to be involved in the process use egg donation, in which one partner's egg is fertilised via donor sperm and the resulting embryo implanted into her partner.

"In this scenario here where one woman is undergoing the ovarian stimulation to collect the eggs and the other woman isn't, then the potential to be able to provide some support to the embryo is quite novel," Dr Sgroi told Mamamia.

"Admittedly it's only for three days, so it depends on how much you as a couple feel that is a [worthwhile] contribution to the whole process.

"But I think any contribution is awesome."

Dr Sgroi notes, however, that should the device become available locally, it still won't be available to all couples. This is largely because it relies on high-quality donor sperm - and donated sperm is in much shorter supply here.

He also notes that the embryo wouldn't be as easy to monitor as it is with conventional IVF.

"We can actually photograph the embryo in real time and see the embryo's development over the course of five days before we implant into the woman. So we're selecting the best quality embryo transfer, knowing that the embryo has achieved its milestones through that development," he said.

Still, he welcomes the possibility of techniques and technology that allow female same-sex couples to contribute more equally to creating a family.

"I think it's technology that we'd no doubt investigates for the future," he said.