Generally a woman releases her eggs 12-16 days before her next menstruation. An egg naturally survives up to 4 days in the uterus before being flushed, while a sperm will survive for 3 days within the same uterus in a bid to fertilize the egg. Conception is therefore not immediate and may take time before achieved. It is for this reason that some people have come up with another form of natural birth control where the moon cycle of a woman is calculated in order to know those periods when she is fertile and those when she is not. This method is referred to as calendar rhythm method and is done by subtracting 8 days from the month where she experiences her shortest cycle within the year- this will determine her first fertile day. Subtraction of 11 days from her longest cycle will determine her last fertile day. This method has been shown to be about 80% effective. The method is subject to fallibility like irregular menstrual cycles, menstrual cycle shifts, and error in counting and so on.
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