Figure 1.

The fertile window (FW) shifts ahead a few days each cycle during the transition. The shifting of the FW shortens the pre-ovulatory fertile phase and lengthens the postovulatory luteal phase by a few days with each cycle. The FW itself remains the same length. For the MM protocol, the FW begins on day ten of the first cycle and is moved forward one day each cycle. That is in cycle two the FW begins on day nine, in cycle three the FW begins on day eight, in cycle four the FW begins on day seven and in cycle five the FW begins on day six.