Modern women spend 3,000 days of their life menstruating, on average. For something that takes up that much time, menstruation is still enticingly mysterious.

Here is the biological bit: your period may be late for all sorts of reasons. Periods, like many bodily functions, are governed by the hormone-emitting pituitary gland in the brain. Oestrogen, progesterone, FSH (follicle-stimulating hormone): our endocrine system sends hormones around the body to regulate all sorts of processes, procedures and activities, including the monthly act of preparing for a possible foetus by having your ovaries release an egg. Your uterus prepares by making a thick, comfortable, nutrient-rich extra lining called an endometrium that a foetus – which is after all a kind of parasite – can grow in and feed from. The endometrium is filled with blood vessels to feed a growing human. Think of it as a nutrient mattress-topper. No foetus, no topper needed, and the endometrium will be discarded as your period: up to 90ml of bloody discharge released over, usually, up to seven days.

via Why is my period late? You asked Google – here’s the answer | Rose George | Comment is free | The Guardian.