Women might be more likely to conceive twins thanks to their genetics - and the possibility could be identified through a simple saliva test.
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Brisbane mum Saskia Bollmann had no idea she would have twins until she fell pregnant.
"We found out we were having twins at the first ultrasound and we did not expect that at all," she told AAP.
"It was a huge surprise."
With no one in her extended family having twins, it was not a consideration for Ms Bollmann and her husband Steffen but the couple now has two-year-old bubs Max and Pip running around.
"We have friends who have twins and we would see them and be like wow that is so full on, let's just hope for one, but ended up having two," Ms Bollmann said.
New research by QIMR Berghofer has found at least seven genes responsible for the likelihood of conceiving non-identical twins.
Using DNA samples from thousands of mums of non-identical twins - including Ms Bollmann - in Australia and overseas, researchers identified what genes they have in common.
"I've been trying to find the genes for about 40 years," lead author Professor Nick Martin said.
"(It's) only in the last few years with the advent of gene chip technology that we had the tools to do this."
Five of the genes have known roles in female reproduction - such as affecting the follicle-stimulating hormone pathway - while two have not been previously associated with twins.
It is not just the possibility of twins linked to the genes - the likelihood of lower fertility can also be identified.
Women who struggle with fertility are at the opposite end of the genetic spectrum to those women who have twins, Prof Martin said.
As a result of the finding, researchers believe a simple saliva-based genetic test could reveal a woman's fertility or the likelihood of producing twins.
"It might be possible to predict from a DNA test which women are likely to have trouble conceiving, meaning we can provide valuable knowledge to people which they might use to start planning their families early," Prof Martin said.
The test would not be definitive as to whether a woman would conceive twins or have lower fertility - it would simply reveal the likelihood on either side of the spectrum.
Researchers hope to continue expanding the study to identify more genetic markers responsible for twins or fertility and have called on more women with twins to participate by sending in a saliva sample.
The study was published in Human Reproduction.
Australian Associated Press
