Why is FSH tested on day 3, and not 1, or 28?
To test your FSH on day 1 would mean that you would have to get to a lab on the first day of your period, it’s just not very practical. To test it in anticipation of a period when your cycle might not be regular is also not practical. It happens that FSH is highest in the beginning of the cycle and it’s been shown in several studies that it’s elevated between days 2-5 about similarly , so those are the best days to check it.
