Being an "older" mother is a risk for having twins. So is using reproductive technologies such as ovarian stimulation or in vitro fertilization (IVF).

Data suggest that about 50 out of every 1000 women ages 40 to 45 will have twins. For 20 to 24 year olds, the numbers are 20 out of every 1000. But these numbers include women undergoing IVF or ovarian stimulation (drugs that induce the release of eggs from the ovary) and those who conceive without reproductive technologies. The risks of multiple pregnancies are very different for the two groups. Using IVF or ovarian stimulation is a much bigger risk factor for twins than age.

Woman undergoing IVF should ask about the number of embryos the doctor expects to transfer during each cycle. The higher the number of embryos, the more likely it is that you will have twins, triplets or more.