If you forgot the tablet and didn't start taking it again then carry on with just the oestrogen and on day 15 start the progesterone and take it for 12-14 days. After that if you are post-meno then you can start the utrogestan on the same date of each calendar month to give a monthyl bleed. If you are still very much peri then stick to a 4 week ( 28 day cycle). Day 1 is the start of the bleed ( irrespective of the date).
Progesterone changes the structure of the womb lining that has been stimulated by oestrogen ( ready for fertilised egg). If the progresterone level falls the lining breaks down because no egg implnatation, and is shed as a period/withdrawal bleed. HRT mimics the natural cycle in this way.
There can be abnormal bleeding from too much oestrogen (lining too thick), too much progesterone (lining too thin) and other reasons but with the HRT bleed it is the addition and then withdrawal of progesterone that causes the intended bleed. Other types of bleed are termed abnormal.
Hope this helps
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Hurdity x