How to Calculate a Pregnancy by a Calendar

Calculate your due date by counting days on a calendar.

Calculating a pregnancy due date is an inexact science based on the beginning date of your last menstrual cycle. If you know the exact date, and the date of conception you can come up with a more accurate due date. You may also calculate the approximate date of conception by counting back from the due date given by your obstetrician. These dates are always approximate, and do not guarantee that you will deliver on your exact due date.

Instructions

Due Date: Based on Last Menstrual Cycle

1. Mark the first day of your last menstrual period on the calendar. If you know your date of conception, count back and verify that the first day of your period falls within 11 to 21 days of your conception date.

2. Count out 40 weeks from the first day of your last period.

3. Note your estimated due date. It will fall on the same day of the week, 40 weeks later.

Date of Conception: Based on Due Date

1. Mark your due date on your calendar.

2. Count back 37 weeks and mark the calendar. This is the latest possible date of conception.

3. Count back 10 more days and mark the calendar. This is the earliest possible date of conception.

Date of Conception: Based on Last Menstrual Cycle

1. Mark the first day of your last menstrual period on the calendar.

2. Count forward 11 days and mark your calendar. This is the earliest possible date of conception.

3. Count forward another 10 days and mark your calendar. This is the earliest possible date of conception.

4. Compare the due date given by your obstetrician with your possible dates of conception.

References

• American Pregnancy Association: Pregnancy Due Date Calculator

Resources

• Read this Article in Spanish