Pregnancy due date - How can you calculate
by Ivana Stamenkovic
Calculating due date isn't an exact science, but knowing your expected date of delivery (EDD) is important. Just don't get too emotionally attached to it, since most likely you won't deliver on that day. Only about 5% of women give birth on their due date.
Calculating Due Date
The first day of your last menstrual period (LMP) is considered the first day of your pregnancy, so it's important that you know it. If you don't know or you have irregular periods - your doctor will estimate it for you.
If you have regular 28-day menstrual cycles, here are two ways to calculate your due date - Naegele's rule and pregnancy wheel.
Calculating Due Date
This rule is named after the German obstetrician, Franz Karl Naegele (1778–1851), who devised it. It's quite simple and there are two ways of calculating. In both ways, you are supposed to add seven days to the first day of your LMP, but in one way you subtract three months and in another you add nine months. If necessary, adjust the year.
Just to make sure, here's an example:
The first day of your LMP: November 15, 2015
Add seven days: November 22, 2015
Add nine months: August 22, 2016
or
Subtract three months: August 22, 2015
Adjust the year: August 22, 2016
The result is approximately 40 weeks (280 days) from the start of the last menstrual period. The calculation method doesn't always result in 280 days because not all calendar months are the same length and it doesn't account for leap years. 280 days from the start of the LMP is found by checking the day of the week of the LMP and adjusting the calculated date to land on the same day of the week. November 15, 2015, was Sunday, and August 22, 2016, was Monday, so the adjusted due date was August 21, 2016.
Pregnancy Wheel
It's also known as a gestational calculator. Most doctors use it since it's very easy to estimate your due date on a pregnancy wheel. You just need to locate the date of your LMP, line up that date with the indicator and the wheel will display your due date.
Calculating Due Date if You Don't Know the Date of Your Last Menstrual Period
Your doctor will probably order an ultrasound to determine your due date or estimate your due date accordingly if you remember in which week was your LMP.
Calculating Due Date if You Have Irregular Periods or Long Cycles
If you have long cycles you can still use pregnancy wheel, you just need to calculate your adjusted LMP date and mark it on. No matter how long your cycle is, from ovulation to the next period always will be 14 days. Calculate your ovulation day, add that number to the first day of your LMP and subtract 14 days and you'll get your adjusted LMP.
If your cycle was 37 days long, your ovulation was on day 23 of your period. If the first day of your LMP was November 15, and your cycles are 37 days long, your adjusted LMP date would be November 24.
LMP: November 15
Cycle Length: 37 days
Ovulation Day: 37 - 14 = 23
November 15 + 23 = December 8
December 8 - 14 = November 24
Adjusted LMP: November 24
There's another way, too. Just subtract 28 from your cycle length and then add that number to the first day of your LMP.
37 - 28 = 9
November 15 + 9 = November 24
Changing a Due Date by a Doctor
Your doctor may change your due date if your fetus is significantly smaller or larger than the average fetus at that gestational age. The highest chance for changing a due date is in the first trimester, especially if a difference between your due date estimated by a doctor according to you LMP and a due date estimated by an ultrasound is more than one week. How the pregnancy growths an ultrasound is less accurate, so doctors rarely adjust dates during second and third trimesters. The difference needs to be more than two weeks in the second trimester and more than three weeks in the third trimester. Sometimes a doctor might request a repeat ultrasound or some other tests, which will provide him more information about the progress of your fetus so he could determine if he should or shouldn't change your due date.
