Nearly 90 per cent of couples at risk of having babies with devastating genetic conditions have no family history of the disorders and no idea they are potential carriers, a study of 12,000 tests has found.
The finding has astonished the Murdoch Children's Research Institute, which said the likelihood of a baby being born with cystic fibrosis, fragile X syndrome and spinal muscular atrophy was comparable to the chance of having a child born with Down syndrome.
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Pregnant women are routinely offered screening for Down syndrome, but only one in 10 couples planning to have a baby have tests for the other equally serious conditions.
Among those who have been blindsided by a rare genetic condition are Melbourne couple Kirsty and Aaron McConnell.
They had each had a healthy child in a previous relationship and had no reason to suspect they were carriers of spinal muscular atrophy.
But when their daughter Lily was about five months old, they began to notice she was struggling to hold herself up.
Eventually, Lily was diagnosed with the most severe form of the rare muscle-wasting disease.
Children with type 1 spinal muscular atrophy are not expected to live past their second birthday, so while Lily's personality blossomed – and she was able to communicate using her eyes to show what she needed – her body failed.
"It affected her ability to do everything," Mrs McConnell said.
"She couldn't swallow any more. She had to have a gastric tube towards the end. She couldn't breathe comfortably, which was really difficult to watch.
"She never rolled. She never sat up. She never really moved. It was pretty horrific."
Lily died in March, but her parents continue to talk publicly about her so that other families might not go through what they did.
Mrs McConnell said she had never heard about genetic screening for spinal muscular atrophy before she had Lily.
"We didn't know it was an option," she said.
"It just didn't cross our mind that us coming together and having a child of our own could cause the horrendous ramifications that it did."
Researchers are now calling for all would-be parents to be routinely offered genetic carrier testing.
The institute's study examined the results of 12,000 tests carried out on pregnant women and couples planning to have a baby for cystic fibrosis, fragile X syndrome and spinal muscular atrophy.
Fragile X syndrome causes intellectual disability and behavioural and learning challenges, and is also the most common single-gene cause of autism worldwide. Spinal muscular atrophy is a severe muscle-wasting disease, while cystic fibrosis damages the lungs and digestive system.
The study found that one in 20 people screened was a carrier of at least one of the three conditions, and 88 per cent of those carriers had no family history of the condition.
The majority of tests were ordered by obstetricians and fertility specialists, and 69 per cent of the women screened were already pregnant when tested.
Murdoch Institute researcher David Amor said he hoped GPs would offer genetic carrier testing before pregnancy in the same way they might discuss diet or testing for rubella.
"We know that the majority of couples who are found to be carriers prior to pregnancy will choose to use IVF in conjunction with genetic testing of embryos," he said.
"We know overall about 80 per cent of women who are pregnant choose to have tests [for Down syndrome].
"The overall risk of one of these disorders is comparable to that of Down syndrome, and yet this testing is only being done in less than 10 per cent of women or couples who are planning pregnancy."