If you are trying for a boy or girl, the planner will also suggest some actions that many influence the sex of the child, based on the scientific research of Dr. Landrum Shettles and David M. Rorvik, who published findings in a book called "How to Choose the Sex of Your Baby: The Method Best Supported by Scientific Evidence."

• Cycle lengths of more than 35 days are too long for appropriate calculations.
• Cycle lengths of less than 21 days are too short for appropriate calculations.
• This calendar cannot be used to reliably prevent pregnancy.