You go to the bathroom, pull down your underpants and, nothing. You expected your period to be here by now, but it’s not. Before you start to worry about why it could be late, take a deep breath and listen to this podcast.

Your body is not a clock, and variation in your menstrual cycle is normal, especially if you are pregnant, teenager, breastfeeding or nearing menopause. Stress, travel, diet, illness , and medication (including birth control) can also affect your cycle length, symptoms and the length and heaviness of your period. Up to 7–9 days of variation cycle-to-cycle is considered normal for adults.