Woman becomes pregnant - while already pregnant

Signing up to become a surrogate mother has seen a surge in popularity within the US, despite it being legal in only some states.

The number of infants born to gestational surrogates almost doubled between 2004 and 2008, according to data from the Centers for Disease Control and Prevention and the Society for Assisted Reproductive Technology.

But what happens if a woman finds herself pregnant with her own child - whilst she is carrying someone else's?

A woman in the US recently revealed her story of becoming pregnant while already pregnant, resulting in her giving birth to two kids with different sets of parents.

Jessica Allen told the New York Post that she had agreed to become a surrogate mother for a Chinese couple back in April 2016 for the price of US$30,000 (S$40,755).

The 31-year-old California native found out she was carrying twins at an ultrasound session during the sixth week of her pregnancy.

The Lius, the Chinese couple that hired her, were thrilled at the sound of that.

"My US$30,000 payment, including expenses - which I received in installments by check each month - was increased by US$5,000 for the second child," Allen told the Post.

Little did anyone involved realise that that the babies were in separate sacs - which meant that the babies weren't identical twins.

What had actually happened was that Allen had gotten pregnant naturally, despite using protection, after an In-Vitro Fertilisation (IVF) procedure that she had gone through.

It is a rare medical case termed as superfetation, which can still occur despite it being a rare condition. There have reportedly been only an estimated 10 known cases around the world of superfetation that were documented as medical reference.

Even so, there are experts who still question the reliability of some stories.

In December 2016, Allen gave birth via C-section at Riverside Community Hospital in California.

She didn't get so much of a glimpse of the two babies she had carried for 38 weeks after they were delivered. Everything was done behind an opaque screen, she added.

Things became more complicated when Mrs Liu sent Allen a photo of the two babies from the hospital. By then, Allen was already home and recovering from her surgery.

The San Diego-based agency Omega Family Global that Allen had worked with for her surrogacy then informed her that baby Mike was a biological match to the Lius while baby Max had her genes.

The Lius made it clear they wanted nothing to do with Max, according to Allen's revelation in the Post.

They even wanted to put him up for adoption.

After some negotiation as well as getting a lawyer to help them, Allen and her husband, Wardell, were reunited with their son on Feb 5 - whom they have renamed Malachi.

The couple are already proud parents to Jairus, 2, and Jeramiah, 7.

Ultimately, Allen told the New York Post: "I don't regret becoming a surrogate mom because that would mean regretting my son.

"I just hope other women considering surrogacy can learn from my story."