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The "safe window"

My partner and I have been together for 6 years. In the first year, we used condoms and afterwards for the other 5 years, he used the withdrawal method. I have never been on birth control. I know some people might frown on the withdrawal method but we have a lot of trust for each other and he has excellent control. We recently heard that our friend often cums in his wife and she has never become pregnant and is not on any birth control of any form. Our friend and his wife have been together for 8 years. I asked our friend how this was possible and he said that they chart the phases of her period and there is a "safe window" where he can come in her.

Now, as a physiology / anatomy student, I know enough about the body but not nearly enough to understand it completely. I understand that a woman is most fertile before her period. But what about after her period ? In between her period ? Can anyone point me in the right direction or provide me more information on this.

Not that my partner and I want to try having him cum in me during a "safe window" but since hearing of our friends doing this, we just want more information on this just to merely educate ourselves.