There are few bigger surprises than learning that you are pregnant with twins. We can all picture the shocked mother and father looking at an ultrasound of two growing babies with wide eyes. The incidence of twins is on the rise, mostly due to the increased utilization of ART. With this increase, researchers have taken another look at the mechanisms that lead to twins, both "typical" and "atypical." This week, we cover the traditional and some new models for how twins form.