Incidence is the % of new events among population at risk while the prevalence is the % of an event (new + old) among population studied, either at a specific point of time (point prevalence) or throughout a specific period of time (usually one year) (period prevalence). It is practically difficult to calculate these rate worldwide through population surveys. The best approach is the extracting data from national registry systems (if available) of pregnancy outcomes. one can find the number of molar pregnancies (numerator) from clinic records (if available & comprehensive). the problem is with denominator. As the number of all pregnancies is definitely unknown, one can used the number of live birth (or even total births) as a proxy measure of total pregnancies. In this case we are calculating the proportion (not a rate) of molar pregnancy.

Researchers can refer for textbooks in bio statistics for more  details

Technically the numerator is number of molar pregnancies diagnosed according to FIGO guidelines over denominator total number of pregnancies. This denominator includes miscarried, terminated and stillbirth and live birth pregnancies. However, it is not routinely collected as many countries do not collect statistics on terminations, and miscarriage is variously reported. Therefore, most publish with live birth and stillbirth as denominator.