I would actually challenge the need for Heartworm "Prevention" at all.
My current Frenchies do not get flea, tick or Heartworm as these are all poisons.
There is some very clear studies that Heartworm prevention is unnecessary and Heartworm testing would provide
just as much effectiveness.
Heartworm cycle and mosquitoes must be understood, as @Honeys Owner pointed out;
Most areas don't require Heartworm (Mosquito) protection until June.
Frenchies specifically can really benefit from limited poison, chemicals and vaccinations.
The following document from Doctor Dobias DVM, this has the map for Mosquito lifecycle.
https://s3.amazonaws.com/dogsnaturally/HeartwormGuide.pdf
The Heartworm drug companies are the ones suggesting monthly.
The dosage is also doubled than what is needed because they assume you will forget.
Continual Heartworm "prevention" has created Heartworm medication resistant worms.
Make informed decisions and not just follow the Heartworm drug companies.
I test my girls twice a year for Heartworm.
Just my 2 cents.