Things You'll Need:
- Calendar
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Step 1
Determine your average menstrual cycle; this is the number of days from the first day of your period until the first day of your next period. For most women this is 28 days. Be sure to notify your obstetrician if you find that you cycle is irregular or tends to be longer or shorter on average.
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Step 2
Use a calendar to find the first date of your last menstrual period.
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Step 3
Add 7 days to this date.
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Step 4
Subtract 3 months.
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Step 5
Add or subtract the number of days your period is under or over the 28 day average. If your cycle is 27 days long then subtract one day from your due date, but if your period averages 30 days, add 2 days to your due date.
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Step 6
Adjust the year accordingly. Calculate the due date in the following year if the first day of your last period was in March through December. So if your last period started in February, then your pregnancy due date will be the same year, but if your last period started in April, then your due date will be in the following calendar year.
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Step 7
Remember that this is the gestational age for your baby and it is calculating your due date based on the day of your last period and not the date of conception.
