31 August 2026: Twins can have different parents. How is that possible? A fertility expert explains | The Conversation
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Key takeaways
- Two genetically unrelated babies developed during the same pregnancy following an embryo transfer and natural conception around the same time.
- One baby was genetically related to the intended parents, while the other was the biological child of the surrogate and her husband.
- The babies resulted from two separate conceptions, making the case very different from the way identical or fraternal twins ordinarily develop.
- Natural conception can potentially occur around an embryo transfer when the transfer takes place in a cycle in which ovulation occurs.
- The case created an unusual legal question because Queensland surrogacy legislation includes provisions intended to prevent children from the same multiple birth being separated.
- The court ultimately found the children were not “birth siblings” under the relevant provision of Queensland's surrogacy legislation, allowing each child to be raised by their respective biological parents.
How can twins have different biological parents?
A rare surrogacy case in Queensland has resulted in two babies being born from the same pregnancy despite being genetically unrelated to each other.
One baby was conceived through an embryo created for the intended parents and transferred to the surrogate as part of fertility treatment. The other was conceived naturally by the surrogate and her husband around the same time.
The extraordinary case is believed to be the first reported in Australia and only the third reported worldwide.
In an article for The Conversation, Genea Melbourne City Medical Director and Fertility Specialist A/Prof Alex Polyakov explains how two separate conceptions could result in a single twin pregnancy and why the unusual circumstances raised complex legal questions.
What happened?
The case involved a 27-year-old woman who had agreed to act as a surrogate for a couple after the intended mother, who was born without a uterus, was unable to carry a pregnancy.
An embryo belonging to the intended parents was transferred to the surrogate as part of fertility treatment.
Later that month, an ultrasound revealed she was carrying two babies.
DNA testing during the pregnancy subsequently established that the babies had different biological parents. The baby girl was genetically related to the intended parents, while the baby boy had been naturally conceived by the surrogate and her husband.
Two separately conceived embryos had therefore implanted and developed during the same pregnancy.
How do twins usually develop?
To understand why this case is so unusual, it helps to understand the two main ways twins ordinarily develop.
Identical twins, also known as monozygotic twins, occur when one fertilised egg develops into an embryo and then splits into two. Because both babies originate from the same egg and sperm, they are genetically identical.
Fraternal twins, or dizygotic twins, develop when two separate eggs are fertilised by two separate sperm during the same cycle. Genetically, they are no more alike than other siblings and can be the same or different sexes.
The Queensland case doesn't fit neatly into either of these scenarios.
Instead, one embryo had been created through fertility treatment for the intended parents, while another embryo resulted from natural conception involving the surrogate and her husband.
Both implanted and continued developing at the same time.
What is superfecundation?
A rare reproductive phenomenon known as superfecundation occurs when two eggs released during the same menstrual cycle are fertilised following separate acts of intercourse.
An even rarer form, known as heteropaternal superfecundation, can occur when the eggs are fertilised by sperm from two different biological fathers.
As A/Prof Polyakov explains in The Conversation, the Queensland case represents an unusual variation on this phenomenon.
Rather than two natural conceptions, the pregnancy combined an embryo transferred through fertility treatment with an embryo conceived naturally during the same reproductive window.
The result was two babies developing together despite having entirely different biological parents.
How could this happen following an embryo transfer?
Embryo transfers can be performed using different treatment protocols depending on a person's individual circumstances.
In some cases, the transfer is coordinated with natural ovulation.
When ovulation occurs, natural conception may still be biologically possible if an egg released during that cycle is fertilised.
In this exceptionally rare case, the transferred embryo successfully implanted while an embryo conceived naturally around the same time also implanted and continued developing.
This is why patients undergoing fertility treatment should follow the individual instructions provided by their fertility specialist and clinical team, including any advice about intercourse during a treatment cycle.
Why did the case raise legal questions?
The unusual biology of the pregnancy also created a situation Queensland's surrogacy legislation had not specifically anticipated.
The legislation includes protections intended to prevent children from the same multiple birth being separated through a surrogacy parentage order.
In this case, however, the two babies had been conceived separately, were not genetically related and had different biological parents.
The Children's Court of Queensland therefore had to consider whether they should legally be regarded as “birth siblings” for the purposes of the legislation.
The court found they were not “birth siblings” within the meaning of the relevant provision of the Surrogacy Act 2010.
This allowed each child to continue being raised by their respective biological parents.
An exceptionally rare reproductive event
While this case demonstrates what is biologically possible, it represents an extraordinarily unusual combination of circumstances.
The pregnancy required two independent events to occur within a similar reproductive window: the successful implantation of an embryo transferred through fertility treatment and the successful fertilisation and implantation of an egg following natural conception.
It also illustrates how developments in assisted reproductive treatment can occasionally produce circumstances that existing legal frameworks may not have specifically anticipated.
For people undergoing fertility treatment, individual treatment protocols and circumstances can vary. Following the advice provided by your fertility specialist and clinical team throughout treatment is important.
A/Prof Alex Polyakov, Genea Melbourne City Medical Director and Fertility Specialist, explores this rare case and explains the reproductive science and legal considerations in his article for The Conversation.
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