Children conceived with the aid of fertility treatments are more likely to be born with serious physical defects, according an Australian study published on Saturday.

Conception using treatments like ovulation induction, in-vitro fertilisation or the injection of sperm directly into an egg, resulted in serious defects in 8.3 percent of cases studied, the research team said.

The corresponding ratio in spontaneous conceptions was 5.8 percent -- a "very" significant difference, lead researcher Michael Davies told AFP of the University of Adelaide study published in the New England Journal of Medicine.

"Something that is not often talked about in the clinic, I suspect, is the risk of having an abnormal baby. And so this emphasises this is something that must be talked about between patients and clinicians.

"They must discuss the risk for this when choosing the treatment."

Davies, from the university's Robinson Institute for fertility, said the research had focused on serious defects, "things that either require treatment or if there is no treatment they are going to be considered handicapping", like a heart condition or cerebral palsy.

The study covered 308,974 births registered in South Australia between January 1986 and December 2002, of which 6,163 had resulted from assisted conception.

"I don't think there is any reason it wouldn't be applicable to the majority of clinics around the world," said Davies, calling for further research.

More than 3.7 million babies are born every year as a result of fertility treatment.

The survey, which researchers said was the most comprehensive of its kind yet, found that not all treatments were equally risky.

The scientists noted birth defects in 7.2 percent of children born from in-vitro fertilisation (IVF) and 9.9 percent from intracytoplasmic sperm injection (ICSI).

For IVF, the percentage dropped significantly when taking into consideration factors like parental age, smoking and other factors, but for ICSI it remained high.

ICSI, in which a sperm cell is injected directly into an egg, is a form of IVF -- which involves the fertilisation of an egg outside the body, in a laboratory.

Davies said there were several theories on why ICSI was more risky -- possibly involving the use of damaged sperm or damage caused by manipulation of the sperm and egg in the lab.

With IVF, the sperm entered the egg of its own accord.

"There are factors associated with ICSI that require further research," he said.

The researchers also found a tripling of risk in women using clomiphene citrate, a drug for ovulation induction.

"While confined to a small group in our study, this is of particular concern as clomiphene citrate is now very widely available at low cost," said Davies.