How to calculate the day of conception?
The day of conception can only be calculated retrospectively. Since it is only clear after pregnancy that conception and fertilization must have taken place, this type of calculation is necessary. If, on the other hand, the term “day of conception” in the context of family planning refers to the determination of the optimal time window for conception, this is a prospective, i.e. forward-looking calculation.
However, since this fertile time window is not always associated with fertilization, i.e. conception, the term “conception day calculation” should be omitted for better differentiation in this type of calculation and instead the fertile days or ovulation should be determined. There are numerous online calculators for both the pro- and retrospective calculation, in which either the first day of the last bleeding and the length of the cycle or the planned date of birth are conveniently entered. However, it is important to emphasize that both the online calculators mentioned above and the conception date determination presented below can never determine the exact day of conception or planned conception, but always a period of several days is determined.
If pregnancy has occurred, the calculation is based on the planned date of birth. However, this type of calculation is also possible if the child has already been born, in which case the starting date of the calculation is the birthday. Subsequently, 267 days are deducted, which corresponds to the average duration of a pregnancy.
However, a few days more or less must be deducted in the case of a transfer or premature birth. Another form of calculation is used for prospective calculations. To illustrate and simplify the following calculation of ovulation and thus a potential subsequent conception, a 28-day cycle is assumed.
Ovulation takes place about halfway through the cycle, i.e. around day 14.Since sperm have a maximum lifespan of about five days, the time window is extended accordingly to five days before ovulation, which explains the beginning of the fertile days from the ninth day of the cycle. The calculation also takes into account the maximum lifespan and fertilisation readiness of the egg cell of 24 hours, which results in the extension of the fertile time window to the 15th day of the cycle. In summary, this type of calculation leads to an increased probability of conception and readiness to conceive between the ninth and fifteenth day of the cycle. For longer or shorter cycles, the fertile days are different.
