The size of the follicle on the 12th day of menstruation varies individually depending on each woman, and there is no specific value because the length of each woman’s menstrual cycle varies. Taking 28-30 days of menstruation as an example, on the 12th day of menstruation, the follicle at this time is around 1.4-1.7cm.
If the menstrual cycle usually lasts 28-30 days and there is normal ovulation, the follicle on the 12th day of menstruation will be around 1.4-1.7cm; if the menstrual cycle usually lasts more than 30 days and less than 35 days and there is normal ovulation, the follicle on the 12th day of menstruation will be around 1.1-1.3cm.
There is no way to estimate the size of the follicle on the 12th day of menstruation if menstruation is irregular, especially if the menstrual cycle is long.
If you want to determine how big the follicle is on day 12 of your period, you can have an ultrasound to monitor it so that you can see the size of the follicle more accurately.