Silvia Azaña, embryologist at inviTRA, explains in this video how fertile day calculators can be used to estimate the timing of ovulation:
To use a fertile days calculator, first, it is necessary to know the duration of the woman's own menstrual cycle, keeping in mind that it begins with menstruation and ends with the arrival of the next menstruation.
Ovulation occurs around day 14 within the duration of a regular 28-day menstrual cycle. The fertile window is considered to span 4 to 5 days before ovulation up until the following day, which is what is referred to as the periovulatory days. Therefore, the fertile days would correspond to days 9 or 10 through 15 in this case.
In cases where menstrual cycles are longer or shorter, the best way to estimate the ovulation day would be to subtract 14 days from the date of your next period. In the same way, you would have to consider the 4 to 5 days before and the day after ovulation.
The reason we account for approximately four to five fertile days preceding ovulation is based on the survival duration that sperm maintain within the female reproductive system, which is usually about 4 to 5 days. Therefore, even if intercourse took place 4 to 5 days before ovulation, there would still be a possibility of pregnancy since there could still be live sperm in existence.
