Pregnant women often want to know their expected date of delivery or EDD. This is your estimated due date, or the date when spontaneous onset of labor and delivery may occur. This information is often given to you by your doctor on your first prenatal visit. However, this estimated date of delivery is not always exact and it usually represents the midpoint in a 4-week period when your baby may arrive two weeks before or after the estimated date. If you want to know your EDD, go to your doctor who will evaluate your menstrual history and help you calculate this date. You may also try to learn how to do it yourself using an LMP calculator.
Part 1: How to Calculate Expected Date of Delivery
One simple and common LMP calculator for your delivery date is as follows:
Record the first day of your last menstrual period (LMP)
Add seven to this number
Now count back three months from the month of your last period
Depending on the result, you may have to add a whole year to the final date.
Here's an example:
If your LMP date was October 10, 2014, add 7 days to make the date October 17, 2014. Count back 3 months, and you get July 17, then add a year, and your estimated date of delivery becomes July 17, 2015.
Calculating methods using a pregnancy wheel estimates the EDD by adding 280 days (or 9 months plus 7 days) to your LMP.
Ultrasound EDD Calculation
An ultrasound can help determine your expected date of delivery in some situations:
When you are not sure of your LMP
When you have irregular periods
When your uterus size does not match the size that is expected for your LMP
When you register late for your prenatal care
Ultrasound (UTZ) helps determine the age in weeks of your baby at the time of testing. A transvaginal UTZ can estimate your baby's age as early as 6 weeks. From then on, the technician (sonographer) measures the length of your baby (the embryo) from crown to rump and estimates the gestational age. The most accurate period when the ultrasound can be used as a dating estimation device is between 7 and 10 weeks or between 12 and 18 weeks of pregnancy.
EDD Calculation Using Uterus Size
The size of your uterus can help the doctor assess your expected date of delivery, but with an error of plus or minus two weeks. Furthermore, if it is your second or third pregnancy, your uterus may be bigger than it was during your first pregnancy. Therefore, the estimated date of delivery is just a rough guess. Nevertheless, if your uterus feels like the size of a normal lemon, you may be about six weeks pregnant. By the 12th week, your uterus may be the size of a cantaloupe.
Calculate EDD From First Fetal Movements
Another way to estimate your expected date of delivery is to count 18 to 20 weeks from the time you first feel fetal movements, which can be as light as a fluttering movement. This usually occurs at about 20 to 22 weeks for your first baby or at 16 to 18 weeks with your next babies.
This method, however, is very crude and inaccurate because it depends on your perception of your baby’s movement, which may be quite vague or unsure.
Other Approaches of Expected Date of Delivery Calculation
Heart tones: Your baby's heartbeat can be heard using a Doppler beginning at 9 to12 weeks and by using a stethoscope at 18 to 20 weeks.
Fundal height: After your 20th week of pregnancy, your doctor will use a measuring tape to evaluate the size of your belly in inches, from your pubic bone to the top of your uterus. The length equals the number of weeks of gestation from week 24 to 34. As an example, a 28-week gestation can have a fundus at 28 cm. Yet, the fundal height can be affected by several factors.
Click HERE to calculate your EDD.
Part 2: How Accurate Is the EDD Calculation?
Will Your Baby Born at the EDD?
The expected date of delivery and actual date of delivery can be different in most cases. Knowing the exact dates of conception and the gestational age of a baby is not a guarantee that you can predict the exact date of a baby’s birth. Pregnancy normally lasts from 37 to 42 weeks. Very few babies (4%) are born on the dates they were expected to be due. Up to 10% of babies are born earlier than 37 weeks, while others are born after 42 weeks.
So, while predicting your EDD is not an exact science, just try to get everything ready for your baby’s birth two weeks before the predicted due date.
The Accuracy of LMP Calculation
Estimating your expected date of delivery based on your LMP depends on how accurately you can recall that day, assuming that you have regular 28-day menstrual cycles, because conception usually occurs on the 14th day of the cycle. However, using your LMP to estimate your due date may result in overestimation of the duration of your pregnancy, because it can have a margin of error of more than two weeks.
In some cases, the exact date of conception is known, such as in women who have in vitro fertilization. Their EDD may be calculated by adding 266 to their date of conception.
The Accuracy of the Ultrasound Calculation
The ability of the ultrasound to accurately estimate your baby's age depends on the gestational age. Generally speaking, ultrasound measurement of a fetus is most accurate in the 1st trimester or up to about 13 weeks of gestation.
Other Factors to Consider
You can also calculate your expected date of delivery through these methods:
Birth weight: One study found that heavier babies (those weighing > 10 lbs) tend to be born about one or two weeks past 280 days.
Multiple births: Twins are more likely to be delivered earlier (about 4 weeks) than single babies, but triplets and quadruplets, even much earlier.