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.