Calculating the Due Date

Generally speaking, calculating the due date is very easy and can be done in two different ways, namely:

  • either by adding 266 days to the date of conception,
  • or by adding 280 days (exactly 40 weeks) to the first day of the last menstrual period (LMP).

However, unless the exact date of fertilization is known, counting from LMP is the common method of calculating the due date, and it is based on the assumption that fertilization in humans typically occurs 14 days from the onset of the LMP. It is also the method preferred by obstetricians. However, one needs to keep in mind the fact that this 280-day cycle works for the regular monthly cycles, that is to say, cycles that last for exactly 28 days, in which ovulation occurs “by the book” and therefore, the foetus is conceived on day 14 from the LMP.

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