Large numbers of women are putting off having babies, resulting in more pregnancies affected by Down's syndrome and more terminations of those pregnancies.
A recent study in England and Wales has found the number of women aged in their 40s having children has nearly doubled in a decade.
In New Zealand, Peter Stone, professor of maternal foetal medicine at Auckland University, told NZPA the median age for women in this country having babies was also increasing.
Prof Stone said that New Zealand did not have easy access to statistics comparable with the British figures, but "we are likely to have the same issues".
Diane Ormsby, a Victoria University lecturer who specialises in reproductive biology, said the median age of women giving birth in New Zealand had increased to 30, from 25 in 1969, and the frequency of babies born with Down's syndrome increased with maternal age.
She said in student newspaper Salient that with the tendency to give birth later in life, New Zealand was likely to see more terminations of foetuses with Down's syndrome.
That did not take into account changes in antenatal testing.
The International Clearing House for Birth Defects reported that in 2006 63 New Zealand babies were born alive and affected by Down's syndrome. The numbers of affected stillbirths and terminations were not reported. From 1987 to 1991 -- the last period for which both births and abortions related to Down's syndrome were reported -- the incidence of both rose rapidly with age from 9.28 per 10,000 in women aged 30 to 34 years, to 34.3 for women aged 35-39 years, and 452 for women aged 40-44.
Since then the number of older women having babies in New Zealand has increased substantially.
In Britain, University of London researchers found the number of confirmed prenatal or postnatal diagnoses of Down's syndrome increased by 71 percent between 1989-90 and 2007-08 -- from 1075 to 1843 -- because of delayed motherhood.
But the researchers, Joan Morris, and Eva Alberman, reported in the British Medical Journal that the number of babies actually born with Down's syndrome remained relatively steady -- there were 752 in 1989 and 743 in 2008 -- because of increased terminations following prenatal testing showing a high risk of the baby being affected.
Prof Morris said that without the improved screening, the increase in maternal ages would have boosted the number of British babies born with Down's by 48 percent.
The proportion of mothers diagnosed with a Down's syndrome baby who decided to terminate their pregnancy remained constant at 92 percent, but the increased numbers of babies being diagnosed with the syndrome meant the actual number of abortions rose.
The researchers noted that some of the cases discovered in earlier screening would have naturally miscarried without being aborted.
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