Assisted reproduction ... associated with higher risk of birth defects.

Assisted reproduction ... associated with higher risk of birth defects.

Freezing embryos can significantly decrease the risk of birth defects in assisted reproduction, a new Australian study has shown.

Researchers at the University of Adelaide's Robinson Institute looked at the risks of major birth defects associated with assisted reproductive technology (ART).

The study's co-author, Eric Haan, said the team was "able to tease out where some of these risks lie".

"It turned out the main source of risk lies in whether you freeze or don't freeze the embryos," he said.

Professor Haan said the group with the highest risk of birth defects was seen in intracytoplasmic sperm injection (ICSI), a fertilisation procedure in which a single sperm is injected directly into the egg.

"When comparing fresh IVF and ICSI, IVF was 36 per cent less likely to result in birth defects than ICSI," he said.

"However, when the embryos were frozen, the risks of birth defects in both procedures decreased significantly."

Researchers compared 6100 ART births in South Australia over a 16-year period from 1986 to 2002, with a registry of more than 300,000 births and 18,000 birth defects.

According to the study's lead author, Michael Davies, the risk of birth defects in pregnancies involving ART was higher than for spontaneous conception.

The risks were 7.2 per cent in IVF, compared with 9.9 per cent for ICSI, Associate Professor Davies said.

Professor Haan said it was also important to look at background factors when assessing risk.

"Even when you don't use ART, you [can be] at increased risk of birth defects anyway, just because you are infertile," he said.

"Assisted pregnancies only increase the risk by a relatively small amount. You're not suddenly going to go from low risk to high."