Can You Become Pregnant When You're Already Knocked Up? Absolutely, But It's Rare

A pregnant woman holds her stomach in Sydney, on June 7, 2006. A women in Bakersfield, California, has given birth to a stillborn girl with a fractured skull. Police say the woman asked the father to bash her stomach until the baby stopped moving. Ian Waldie/Getty Images

When a 31-year-old surrogate mother learned that she was carrying twins instead of the single child she initially signed up for, she thought nothing of it. That was, until the "twins" were born and she quickly realized they were anything but. In truth, the mother had unknowingly conceived naturally despite already carrying another couple's child. The case is rare, but biologically speaking, completely plausible.

The biological term for becoming pregnant while already pregnant is superfetation, and this can occur when a pregnant woman continues to ovulate even after conception. If a woman has sex during this period she can conceive a second child, which would grow next to the first in its own amniotic sac. If a woman has more than one sexual partner, or, as in this case, is a surrogate, the two children can have different parents despite being carried and born at the same time.

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In 2016, Jessica Allen decided to become a surrogate mother, and that April she became pregnant via IVF with another couple's baby, Science Alert reported. Six weeks later, her ultrasound revealed that the pregnancy had actually yielded twins, which she delivered nine months later via C-section. Although Allen was not able to see the babies initially after birth, as they were immediately taken into a neonatal care unit, a few weeks later when she Skyped with the babies as a follow-up, Allen realized something was off.

According to Allen, one baby had striking Asian features like his ethnically Chinese partners, referred to by the pseudonym "the Lius" in the original New York Post article. The other child looked more similar to Allen, who is Caucasian, and her husband, Wardell Jasper, who is black. A DNA test revealed what had occurred. In a freak chance, Allen had become pregnant with her own biological son not long after receiving IVF for her surrogacy pregnancy.

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Getting rights to their own biological child proved to be far more difficult than the couples had predicted. At first, the Lius agreed to relinquish parental rights over the child who was not biologically their own, but demanded $22,000 in repayment, since they had paid extra for the surrogacy thinking it would have yielded twins, The Washington Post reported.

After hiring an attorney, Allen and her husband were able to have their biological son without having to pay back the Liu family any money. The couple was reunited with their son nearly two months after he was born. Although the story had a happy ending, it still stands as a reminder nothing is ever a 100 percent guarantee against pregnancy—not even already being pregnant.