A growing number of women under 35 are terminating pregnancies because their foetus has Down syndrome, researchers have found—suggesting prenatal screening is occurring widely, even in younger age groups.
Veronica Collins, an epidemiologist in public health genetics at Melbourne’s Murdoch Children’s Research Institute, said the age of women giving birth to live children with the syndrome, the leading cause of intellectual disability, had shifted.
According to statistics from the Victorian Birth Defects Register, when diagnostic testing was first introduced for older women in 1986, 71 babies with Down syndrome were born in the state to mothers younger than 35—three-quarters of the total live births.
By 2002, only 30 babies with Down syndrome were born to younger mothers, representing just over half of the total. The number of such children born to older women stayed constant at about 25 live births annually.
Altogether, the number of babies with Down syndrome born alive fell by almost half—to 50 in 2002—and had been between 50 and 60 for the past seven years, Dr Collins said.
But the total of all notifications for the condition—including abortions and stillbirths—increased from about 115 in 1986 to 190 in 2002. Those figures indicate only a quarter of Down syndrome pregnancies now result in a live baby.
“Younger women are taking up the option of prenatal screening. The number of live births among younger women is coming down,” said Dr Collins, who presented her work this month to the annual scientific meeting of the Human Genetics Society of Australasia in Fremantle.
The study—the first long-term examination in Australia of the demographics of Down syndrome births—comes amid intensifying debate about the role of screening and tests on foetuses, embryos and even before conception.
In NSW, long term figures are unreliable because a legal requirement to report birth defects was not introduced until 1998. But the recent trend is similar to Victoria’s. Since 2000, the proportion of terminations associated with birth defects has fallen among older women and risen among the under-35s.
Any pregnancy in an older woman is more likely to be affected by Down syndrome, but because more young women become pregnant, the total is higher in this age group.
According to a report published last year on prenatal testing in Victoria, the type of diagnostic tests women choose is also changing. The proportion of women over 35 who had a diagnostic test had declined from 40 per cent at its peak in 1995 to 28 per cent in 2002.
This was presumably because older women were choosing non-invasive blood tests and ultrasound screening, which can determine if a foetus is at elevated risk of a chromosomal disorder such Down syndrome but do not definitively diagnose it. Those whose results reassured them it was very unlikely their baby was affected might then decide not to have a diagnostic test, because these can occasionally trigger a miscarriage.
Conversely, the proportion of young women who underwent an invasive diagnostic test had risen to 2.5 per cent. Earlier blood or ultrasound screening might have indicated those women were at increased risk of having an affected child, Dr Collins said.