In "Timeless Secrets of Health and Rejuvenation" (if my memory is correct), Andreas recommends using the methods described in "Mental Birth Control" by Mildred Jackson. He & his wife use this method successfully.

It's true, during the transitional period is is difficult to monitor the cycles.

The most natural birth control is completely non-physical and requires trust in oneself and the partner. The bottom line is that a soul cannot incarnate in a human body unless it receives permission by both the parents or at least the mother. This is true also for unwanted pregrancies, such as occur as a result of rape (past lives play the main role in such incidences). Some pregancies occur due to guilt, shame from raping others in previous lives. To bring this to balance, the rape has to occur. The child that is born is giving the mother the opportunity to love the child and end the vicious cycle, although it is fathered by someone who played the villain. However, most pregnancies are consciously desired. Some are desired unconsciously. A woman may love to have a child, but tells everyone and herself she doesn't want children in her life (because she is afraid of the responsibilities that come with it, or because her mother treated her badly when she was a child). In such a case, she is likely to become pregant one way or another, because her subconscious longing invites a soul to incarnates through her. No soul can incarnate without invitation. If you and your partner are very clear about not wanting a child at this time, you must clearly state that before sexual intercourse. This serves as a red light for any soul checking out parents that are suitable for them and that have karmic agreements with them. Some souls wait patiently until the right time arrives. No soul incarnates by accident. There are no accidental children. For a child to be born and to live it must have soul consciousness. Just having a fetus in the womb doesn't mean there is anyone living in the fetus. The soul usually "enters" the body at birth. Children that are still born are those where the soul changed its mind and did not enter the body.

By and all, it is about learning to trust in one's own power. You are always in charge/control whether you use your energies in a positive or negative way. This includes conception.

Andreas