Almost all human population exhibit seasonal variation in births, owing mostly to seasonal variation in the frequency of conception. Although humans have the capacity to reproduce continuously, Monsoon has always been a season of copulation in ancient literature.
Brazilian researchers did a study on Seasonal variation in assisted conception. They studied 1,932 patients undergoing IVF/ICSI and assessed fertilization 18 hours after ICSI, which is a procedure in which a single sperm is injected into the egg. They found that the fertilization rate was maximum in Spring 73.5 percent however there was no effect on pregnancy rate.
Monsoon brings with it right food, right temperature, right environment to optimize fertility and its definitely one of the best times to plan your pregnancy.