Incidence rate refers to the probability of developing a particular disease or disorder. The incidence rate for a congenital or prenatal condition is calculated by dividing the number of new incidences of that condition by the total number of pregnancies and/or babies born. Incidence rates provide data on the number of new cases of the condition, and they provide a look at how a condition has changed or progressed over time. Incidence rates allow for comparisons between populations as well. 
You can find the references annotated in each definition above at this link.