Calculating Your Due Date

by Jane Foley

Can you explain how due dates are calculated?

First the short version!

calendar pageYour due date is determined from the first day of your last menstrual period (LMP) by adding 40 weeks to determine your "expected date of birth." (For this purpose it tends to consider that everyone has a typical 28 day cycle.) Most women ovulate and conceive around day 14 of their cycle. Since the "clock" started on day 1 of a woman's menstrual cycle, on day 14 she is technically considered 2 weeks pregnant! This is the most important piece of the dating puzzle when it comes to figuring out a conception date.

The most reliable date for most women -- one they are certain of -- is the first day of their last period. This is why the pregnancy is counted from that date.

  • Day 1 -- The first day of your menstrual cycle, the day you start your period
  • Day 14 -- Ovulation. It is presumed that conception happens this day
  • Day 14 or the day of conception, you are considered 2 weeks pregnant, even though you are really only 1 day pregnant. (This is the confusing part!)
  • Day 28 -- the day you are supposed to start your period. If your period doesn't start, you are now consider 4 weeks pregnant

Now for a bit more detail:

Keep in mind that the gestation period (time you carry an infant in the womb) averages 40 weeks from day 1 of your LMP or 38 weeks from the date of conception. First time moms can expect an average 288 days versus 283 (again from LMP) for those that have previously had a baby.

What about those months? Is it 9 months or 10? Actually if you count lunar months (7 days per week with 4 weeks per month) and count from Day 1, then the typical pregnancy does average 10 lunar months. 9 typical months (since some have 5 weeks) just sounds better though doesn't it?

Be sure that you do not get too hung up on plans for your expected due date (EDD) to be "The Day" as only 5% of babies actually arrive as planned. Approximate 80% are born between weeks 38 and 42 -- again counting from Day 1 of your LMP. Yep, you can go early or over!

There are certain factors such as ethnicity, proper nutrition (or lack thereof), smoking and/or substance abuse, mother's age and size that also can impact your expected due date. For example, African-American women averaged 8.5 days fewer than Caucasian with all other socio-economic factors being equal. The most important thing to remember is to seek out prenatal care as soon as you know you suspect you are pregnant (or sooner for those trying to conceive!) Those early steps you take to care for both yourself and your baby can give you both the best start at a long term positive outcome possible.

Want a fun way to calculate your due date? Figure out the date of Day 1 of your last menstrual period. Add 7 days. Subtract 3 months. That's it!

Jane Foley has worked as a Sonographer (Ultrasound Technologist) for over twenty-three years. Being somewhat of a gypsy, Jane has lived and worked in many parts of the world. She pulls a wealth of information from her experience in the field of Radiology and her interactions with such a broad cross-section of cultures. She now makes her home on the island of Maui with her English husband, Michael.

Copyright © Jane Foley. Permission to republish granted to Pregnancy.org, LLC.