The human gestation period, or pregnancy, lasts for an average of 280 days, according to MayoClinic.com. However, 280 days is an average, so any due date is an estimate. It is normal for the birth of a baby to occur up to 10 days before or after the due date. The gestational period and due date are calculated from the day you started your last period prior to conception, not from the actual date of conception. The time since the start of your last period prior to conception is called the gestational age.
Step 1
Check your calendar to determine the date your last period started before you became pregnant. If you aren't sure, but have an idea of the date that conception occurred, subtract two weeks from the date of conception.
Step 2
Find a calendar that covers at least the next nine months. If necessary, get a calendar for next year as well as the current year. Mark the date of the start of your last period prior to the beginning of the pregnancy. Make a note of the day of the week.
Step 3
Count forward from the date you marked by weeks until you reach 40. Count the day your last period started as zero. For example, if the date of the start of your last period was a Monday, count forward week by week until you reach 40 Mondays. This is your estimated due date.
Tips and Warnings
- The time since the date of conception is called the fetal age. Normally, conception occurs about 14 days after the start of your last period. If you are not sure of the conception date, you can estimate fetal age by subtracting two weeks from the gestational age. This does not affect due date estimates.
Things You'll Need
- Calendar
