Your baby's due date is usually calculated as 9 months + 7 days after the first day of the woman's last NORMAL menstrual period (or 280 days after this day). This is known as 'Naegele's Rule' and is based on a 28 day menstrual cycle. The due date can be different if the woman's menstrual cycle is not 28 days. For example if it was about 21 days the month you conceived, you would deduct 7 days from this date. If your cycle was about 35 days the month you conceived, you would add 7 days to this date.
To calculate from the conception date it is about 9 months minus 7 days after conception or 266 days (+ 3 to 5 days). Therefore taking 14 days off the estimated 'Naegele's Rule' date.
The baby's due date can be unclear if the woman experiences what is called an 'implantation bleed'. This is when the fertilised egg implants in the blood-rich lining of the woman's uterus, about 5 to 7 days after it is fertilised. The implantation bleed may be confused as the 'last period', calculating the woman's pregnancy, as about 3 weeks earlier than the baby would be due. In addition, the woman can experience blood spotting (or light bleeds) around the time her period would have been due, even during her pregnancy. This can give her the impression that she is not pregnant, even though she is!
For the woman, if you know the first day of your last normal period, and have a 28 day cycle, you can calculate your due date using our (and add or minus any days for changes in the length of your menstrual cycle):
'birth' calculator
OR
If you know your conception date, you put this date into the
'birth' calculator and add 14 days to the date that it calculates.
Image 4-53 shows you how to calculate your baby's due date from the first day of your last period, or the conception date.
The woman's due date can be calculated (or adjusted) by the caregiver if: