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Length of Pregnancy
The average length of a pregnancy is 280 days, which is calculated from the first day of the last menstruation. This calculation assumes that every woman ovulates on the 14th day of her cycle, which is not always the case. For more accuracy, we can start with the probable date of conception, and then add 266 days to obtain the expected birth date.

Irregular Cycles and Cycles after the Discontinuation of the Pill
Most of the variation in cycle length occurs in the pre-ovulatory phase. This means that the more irregular a woman's cycles are, the more likely it is that calculating from the first day of the previous menses will give a delivery date that is too early.

A woman's hormonal system may take a while to return to normal after the discontinuation of hormonal contraceptives, particularly if there is a history of irregular cycles.

Without the information provided by the Sympto-Thermal Method (or other methods of determining the fertile phase), the time of conception may be difficult to establish.

Any woman who charts her cycles using Sympto-Thermal graphs can determine when ovulation likely occurred, regardless of how irregular her cycles are. This means that she will be able to use the more accurate calculation, which is to add 266 days to the probable date of conception.

Expected Date of Birth
Sympto-thermal user calculator:...
If you are a sympto-thermal user,

Enter the probable date of conception as determined by your Sympto-Thermal chart
Day: Month: Year:

Or to use a traditional calculator:..

Enter the date of your last of menstruation before pregnancy
Day: Month: Year:
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