Day one of your cycle is the first day of your period. This is when your uterus starts shedding the lining it has built up over the last 28 days. After your period is over, the lining of your uterus starts to build up again to become a thick and spongy ‘nest’ in preparation for a possible pregnancy.

Once you start seeing blood toward the end of your menstrual cycle, mark it on your calendar. If bleeding continues and gets heavier the next day, then that spotting day was Day 1 of your new menstrual cycle.

The first day of your menstrual cycle starts on the first day of your period. On average, you’ll usually bleed for 3 to 5 days, but the length of your cycle could fluctuate. The first day of your next period typically starts within 21 to 35 days of your last cycle.