BabyPlan is an Excel spreadsheet designed to calculate the best days for conceiving a child. BabyPlan is based on the use of natural physiological cycles and is also suitable for calculating the days that are least likely to conceive. In the process of using the program statistically takes into account the data that are entered into it, which increases the accuracy of the physiological cycles of the female body. It is possible to estimate the accuracy of the forecast. There's a detailed reference.