Test-tube babies have higher rates of birth defects, and doctors have long wondered: Is it because of certain fertility treatments or infertility itself? A large new study from Australia suggests both may play a role.

Compared to those conceived naturally, babies that resulted from simple IVF, or in vitro fertilization -mixing eggs and sperm in a lab dish - had no greater risk of birth defects once factors such as the mom's age and smoking were taken into account.

However, birth defects were more common if treatment included injecting a single sperm into an egg, which is done in many cases these days, especially if male infertility is involved. About 10 percent of babies born this way had birth defects versus 6 percent of those conceived naturally, the study found.

It could be that the extra jostling of egg and sperm does damage. Or that other problems lurk in the genes of sperm so defective they must be forced to fertilize an egg.

"I don't want to scare people" because the vast majority of babies are born healthy, said the study's leader, Michael Davies, of the University of Adelaide in Australia.

Couples could use simple IVF without sperm injection, freeze the embryos and implant only one or two at a time, he said. All of those can cut the chance of a birth defect.

The study was published online on Saturday by the New England Journal of Medicine and presented at a fertility conference in Barcelona, Spain. Health agencies in Australia paid for the research.

More than 3.7 million babies are born each year through assisted reproduction. Methods include everything from drugs to coax the ovaries to make eggs to artificial insemination and IVF. Fertility treatments account for about 4 percent of births in Australia and as many as 8 percent of them in Denmark, where costs are widely covered, Davies said.

In the United States, more than 60,000 babies were born in 2009 from 146,000 IVF attempts. About three-quarters of them used ICSI, or intracytoplasmic sperm injection.

ICSI was developed because of male infertility. But half the time, it was not done for that reason but to improve the odds that at least some embryos will be created from an IVF attempt. Many clinics do it in all cases.

IVF costs around $10,000 to $12,000 per attempt and another $2,000 for sperm injection.

Associated Press