One set of twins - two fathers. There's a rare chance that might happen but it can and does.
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There's a lack of knowledge in the intricacies of DNA and a rare phenomenon known as heteropaternal superfecundation adds to the twist.
An estimated one in 80 Australian births are twins.
We often categorise twins as either identical or fraternal, based on their genetic makeup. Identical twins result from the splitting of a single egg, while fraternal twins occur when two separate eggs are fertilised.
Heteropaternal superfecundation is the third category of twins. It happens when a woman becomes pregnant more than once in the same menstrual cycle from different sexual encounters.
Forensic scientist and founder of Identilab Kate Pippia explained how twins could have two different fathers.
According to Ms Pippia a woman's menstrual cycle typically lasts around 28 days, with ovulation occurring approximately in the middle. Heteropaternal superfecundation describes the situation where a woman becomes pregnant more than once in the same menstrual cycle from different sexual encounters.
"In this unique scenario, two separate eggs are released, and each one is fertilised by a different sperm cell from different men. The time frame for this to happen is within a few hours to a maximum of 4-5 days after the initial encounter," she said.
Ms Pippia said they've seen this rare occurrence twice at their Brisbane-based parentage lab Identilab.
"We had a family in Queensland who did the at-home test. The dad tested himself against his two children and the tests said he was only the dad of one of the twins."
"The mum was suspicious so they came into our office and we collected their samples in the labs.
"The mum gave a sample as well, we had the mum the two kids, and dad, and the test result came back - he was the dad of one but not the dad of the other."
"There's speculation that the number of heteropaternal superfecundation twins may increase in the future due to factors such as the use of fertility drugs, which can increase the number of eggs released during a cycle, as well as the rising frequency of multiple concurrent partners and sexual encounters," she said.
IN OTHER NEWS:
Another unique DNA occurrence is human chimerism. In human beings, a chimera is a person who has two totally different sets of DNA inside their body.
Ms Pippia said people can double up on DNA when a woman is pregnant with fraternal twins and one embryo dies very early on. The other embryo can "absorb" its twin's cells, leading to the "absorbed" twin being the biological father many years later.
Superfecundation is relatively uncommon among humans. A study conducted in 1992 found that superfecundation twins were involved in over 2% of paternity tests in the United States.
But Ms Pippia said many cases go unreported due to the absence of specific confirmation or the need for a paternity test.
