A doctor or midwife will figure your initial estimated due date, or EDD, by figuring from your last normal menstrual period (LNMP). They will take the first day of your last period and add 266 days (or 38 weeks) to figure your estimated due date. This gives an estimate of your due date but it isn't 100% accurate as we'll explore later.
Figuring Your Own Estimated Due Date
You can figure out a rough due date for yourself by using this simple calculation. Take the first day of your last menstrual period. Let's say that is August 1st. Now minus 3 months, making it May 1st. Now add 7 days for an estimated due date of May 8th. While your health care provider will be able to give you a more accurate due date later, this can give you a good idea of when to expect your new arrival.
Factors Effecting the Due Date Accuracy
The length of your menstrual cycle can effect the accuracy of your due date. A woman with a longer menstrual cycle is more likely to go over her EDD, while a woman with a shorter menstrual cycle is more likely to deliver early compared to her estimated due date. Irregular cycles, or a tendency to miss cycles as some athletes experience, can make it even more difficult to determine an estimated due date so your health care provider may recommend an early ultrasound to check for dates.
Will You Deliver Your Baby on Your Due Date?
Probably not! Only 5% of mothers give birth on their "due dates" but the vast majority of pregnant women deliver within two weeks in either direction. Some women don't even tell their friends and family their true estimated due date for this reason. If you are given a due date of May 10th you might tell your friends "I'm due mid-May", just in case you go to 41 weeks so you don't get too many questions and comments.
Studies done with first time mothers show that the average first pregnancy goes 41 weeks and 1 day so primiparas (first time mothers) should automatically add a week to whatever estimated due date they are given. All mothers should also keep in mind that they are not considered "overdue" until week 42 of pregnancy so you're encouraged to try to relax and keep in mind that the babies come when they are ready.
Using Ultrasound to Estimate a Due Date
If you are unsure of when your last menstrual period was, or if you are prone to irregular menstrual cycles, your health care provider might ask you to consider having an early ultrasound. No ultrasound is 100% accurate in determining a due date but an ultrasound within the first trimester will give a better estimate of gestational age than later ultrasounds will.
Ultimately the key to using a due date is to mentally block off a week in either direction of whatever date your doctor or midwife gives you. After getting so many questions and feeling frustrated the last two weeks of my pregnancy with my first born, I now tell people "I'm due the end of September" or "Sometime in May".
The last thing a woman wants to hear when she is 40 weeks pregnant is, "You haven't had that baby yet?" Remember that recent studies suggest the labor process is triggered by hormones produced in the baby's brain so when the baby is ready, the baby will come. They don't know about due dates and calenders.
More Resources: The Pregnancy Book by Dr. Sears
