no, the beginning of the cycle is counted from when they bleed. when they bleed they are not fertile, the ground on which the egg would attach is being shed

they are fertile for about a week to 11 days immediately prior to the bleed... so if they bleed for 5 days (typical) and are fertile for a week (typical) then out of 28 days that's 12 days they can't have sex for, and it should be noted that during that fertile period their hormones fuck with their brains and make them want sex more

so, that's 16 days or a bit over 2 weeks of the month they can fuck with impunity, if they know what the state of their system is