How to Calculate Due Date of Pregnancy

By Patricia Tomaskovic

Not only is the expectant couple excited to find out about their pregnancy and know exactly when the due date is, but knowing the age of the fetus is important for proper management of the pregnancy. In fact, one of the most important questions that your health care provider will seek to answer is, "How old is the fetus?" Or "What is your due date?" Your due date is known as the estimated date of delivery, or EDD. At one time it was known as the estimated date of confinement (EDC). And others may refer to it as the estimated date of birth (EDB). You may also hear it referred to as gestational age of the fetus. This date is important in maintaining the health of the pregnant woman and also helps to ensure optimum care of the fetus.

Calculate your due date so you can prepare for your baby's arrival. The information is also important for your health care provider, especially in cases where women develop complications of pregnancy. Examples of complications include diabetes or preeclampsia. Knowing your due date can help your health care provider manage the care of your pregnancy. For example, the management of preeclampsia (high blood pressure of pregnancy) will differ depending upon the age of your fetus. If you were to develop preeclampsia at 38 weeks gestation, frequently the treatment will call for delivery. However, if the gestational age of the baby is only 28 weeks, your health care provider will most likely manage your pregnancy more conservatively in an attempt to delay delivery until the gestational age of the fetus has progressed further.

Start with the date of your last period or the date of conception. The more you know about your monthly cycle, the more accurate will be the estimation of your due date. On average, pregnancies last about 267 days from the date of conception. So if you know your date of conception absolutely, then you can simply add 267 days to it to figure out your due date.

Use the following formula to help you figure out your EDD:

Naegele's rule adds seven days to the first day of your last menstrual period (LMP). Then three months are subtracted from that date.

It is easier to use this rule if you substitute numbers for the months and days. One precaution: use the actual numbers of the days in the month of the LMP when you cross over to another month, as the actual number of days in a month will vary. An example of the use of Naegele's rule follows:

5/28 (LMP of May 28) +7 days

6/4 (June 4 -- May has 31 days) -3 months

=3/4 (March 4 of the following year, as nine months have been added)

Add 281 days to the first day of your LMP, to figure out your due date.

Both of these methods are based on an average menstrual cycle of 28 days. If your periods are irregular, if you conceive while breast-feeding and ovulating but without an actual period, or if conception occurs before your regular menstruation has been reestablished after you have either had a termination of a pregnancy or have discontinued birth control pills, then it will be useless to attempt to calculate your EDD using these methods. In these cases, your health care provider will use other methods to determine an approximate EDD.

About the Author

Patricia Tomaskovic has been a nurse for 21 years, specializing in labor and delivery for the last 11 years. She has been studying holistic nutrition for the past five years as well, and is currently writing a paper on NLP and weight loss. She is CEO of and has written several articles for that website.