The findings challenge the widely held belief that assisted reproduction increases the risk of birth defects in all women, according to the researchers at the University of Adelaide.
"There's something quite remarkable occurring with women over the age of 40 who use assisted reproduction," study lead author Michael Davies said in a university news release. He is a professor and epidemiologist at the university's Robinson Research Institute.
The researchers looked at information from births in South Australia between 1986 and 2002. The study included more than 301,000 naturally conceived births, 2,200 in vitro fertilization (IVF) births and nearly 1,400 births from intracytoplasmic sperm injection (ICSI).
IVF takes place when a woman's egg and a man's sperm are combined outside the body in a laboratory dish so that fertilization can take place. In ICSI, the sperm is directly injected into the egg. Once the egg is fertilized, it is placed into the woman's womb.
Among women of all ages, the average rates of birth defects were 6 percent for naturally conceived births, 7 percent for IVF births, and 10 percent for ICSI births.
But when the researchers looked at the births by maternal age, a different picture emerged. The rates of birth defects ranged from a high of 11 percent for women younger than 30 using ICSI to a low of 3.6 percent for women 40 and older using IVF to conceive.
In naturally conceived births, birth defect rates ranged from about 5.6 percent among young women to 8 percent among those 40 and older, the researchers said.
Previous studies have shown that women who undergo assisted reproduction have an increased rate of birth defects overall compared to women who conceive naturally, Davies said. It's also been known that the rate of birth defects increases exponentially from age 35 and on for naturally conceived pregnancies.
"Therefore, it was widely assumed, but untested, that maternal age would be a key factor in birth defects from assisted reproduction," he said.
"However, our findings challenge that assertion. They show that infertile women aged 40 and over who used assisted reproduction had less than half the rate of birth defects of fertile women of the same age, while younger women appear to be at an elevated risk," Davies said.
Further research is needed to determine the reasons for the findings, the study authors said.
The study was published Oct. 17 in BJOG: An International Journal of Obstetrics & Gynaecology.
The U.S. National Institute of Child Health and Human Development has more on assisted reproductive technology.
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