DELAYING parenthood raises three especially worrying issues for a would-be mother, says Dr Cheng Li Chang, the medical director of the Thomson Fertility Centre.

These are decreased fertility, a higher risk of pregnancy complications and a greater likelihood of miscarriages.

"First, her fertility rate will decrease as her stock of eggs shrink with age.

In fact, 35 is the cut-off age doctors use to start counselling couples earnestly not to put parenthood off, he says.

"Second, women over 35 have an increased risk of chromosomal abnormalities, the most common among these being Down's Syndrome.

"Third, late-age mothers have a higher risk of miscarriages, as their bodies may expel foetuses in the early stages if they are formed from poor genetic raw material, that is, bad eggs.

"All these risks stem from the steady deterioration in the quality of a woman's eggs as she ages."

In Down's Syndrome, or mongolism, the most notorious chromosomal abnormality, 95 per cent of affected babies have one chromosome too many in their mothers' fertilised eggs.

A chromosome is the rod-shaped component in every human cell which contains genes.

In the remaining 5 per cent, certain cells within the fertilised eggs divide faultily. Down's children are born mentally backward and sometimes with congenital deformities like malformed organs.

One in 600 babies worldwide will be born a mongoloid, and many die in infancy.

More significantly, Black's Medical Dictionary states that 40 per cent of Down's children are born to mothers over 40, while 30 per cent are born to mothers aged 35 to 40.

Also, the risk of having a Down's child is more than one in 60 if a woman is 45, compared to less than one in 1,000 for a woman aged under 30.

Dr Lai Fon-Min, who heads the Antenatal Risk Assessment Unit at Kandang Kerbau Women's and Children's Hospital, says a woman is most fecund when she is in her early 20s, after which her fertility goes downhill.

He stresses: "A point worth bearing in mind is that, while abnormal sperm cannot fertilise an egg, abnormal eggs can be fertilised and continue to grow -- defects and all -- into a full-term baby."