The formula
reliable test = last period + cycle length
Where it comes from
The chain has four steps and each takes time: you ovulate, fertilisation happens, the embryo takes six to twelve days to implant in the womb, and only then does human chorionic gonadotropin start being produced — which is what the test detects. It takes another two or three days to reach a measurable concentration in urine, which is why the day the period is due is the first date that makes sense.
How to work it out by hand
- Place ovulation by subtracting fourteen days from the end of your cycle
- Add six to twelve days: that is the implantation window
- Add two or three more for the hormone to become detectable
- That lands, roughly, on the day the period was due
What is worth knowing
The tests advertised as working five days before your missed period do exist and do work, but only in one direction: an early positive is almost always true, while an early negative says nothing. The difference is sensitivity, measured in milli-international units per millilitre; pharmacy tests run from 10 to 25, and the more sensitive, the earlier they detect. First morning urine is the most concentrated, and drinking a lot of water beforehand can dilute the result enough to change it.