healthHealth and Medicine

In Bizarre Case, Mother Pregnant With "Twins" Is Only The Biological Mother Of One Of Them


Robin Andrews

Science & Policy Writer

Superfetation is extremely rare, but not necessairly dangerous. 10 Face/Shutterstock

Well here’s something you don’t see every day. A surrogate mother hailing from California ended up giving birth to two babies, expected them to be twins. However, she was stunned to discover that one of the children was biologically her own.

The strange genetic mishap wasn’t noticed right away, which resulted in both children being given away to the parents. Jessica Allen, the surrogate, then fought to get her own child back.


It’s safe to say that this was a shock to everyone involved – but how did this happen? It's all down to a process known as "superfetation."

There are two types of surrogacy. In “traditional surrogacy”, a woman outside of the couple is artificially inseminated with the father’s sperm. The woman then carries the baby and gives birth to it. She’s the biological mother, but she also gives the baby away to the couple to raise as their own.

In this case, it appears that Allen was a “gestational surrogate”. This involves using IVF techniques to take the eggs from the mother, fertilize them with sperm from the father, and then implanting it into the uterus of another woman, who carries the baby until it’s born.

In this case, the surrogate has no genetic link to the child, because her egg wasn’t used. She may be the “birth mother,” but she’s not the biological mother.


As reported by the BBC World Service program Newsday, Allen appeared to have ovulated while already pregnant, an incredibly rare occurrence. One fertilized egg was from the intended mother, whereas the other was Allen’s own, which was also then fertilized shortly after the surrogacy began.

When this was spotted, doctors assumed it was one embryo that had split into two, and the mistake went unnoticed until long after her pregnancy was over.

OIIVF allows eggs to be fertilized outside of the body. nevodka/Shutterstock

This additional ovulation is what is known as superfetation. In mammals, it involves the formation of a new embryo from a separate pregnancy cycle while another from the original pregnancy cycle is already present and developing.

The risk of this is that the second embryo develops into a baby that is then born prematurely, but fortunately, Allen seems to have avoided this fate.


When both herself and the intended mother had doubts about the identity of one of the babies, a DNA test was ordered and the results revealed that Allen was the genetic mother. Legally, though, she wasn’t one of the parents, and it took 10 months post-birth to finally take back her newborn son.

Superfetation cases like this have been seen before – both during gestational surrogacy cases and in normal pregnancy situations – but they remain uncommon.


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