Pregnancy lasts an average of 280 days or 40 weeks (almost 10 months) from the first day of your last menstrual period (LMP). The first day of your last menstrual period is considered day one of pregnancy, even though you probably didn't conceive until about two weeks later. (Fetal development lags two weeks behind your pregnancy dates.)
Calculating your due date is not an exact science. Very few women (about 4%) actually deliver on their due date, so while it is important to have an idea of when to expect delivery, try not to get too attached to the exact date.
If you have regular menstrual cycles, there are two ways to calculate your due date (otherwise known as the EDC-your estimated date of confinement).
Naegels rule involves a simple calculation: add seven days to the first day of your LMP, then subtract three months.
For example, if your LMP was November 1, 2004:
- add seven days (November 8, 2004);
- subtract three months (August 8, 2004); and
- change the year if necessary (to the year 2005 in this case).
The EDC would be August 8, 2005.
The other way to calculate your due date is to use a pregnancy wheel. This is the method that most health care providers use. If you have access to a wheel, it is very easy to estimate your due date. You or your provider can locate the date of your last menstrual period; when you line up the date with the indicator, the wheel will display your due date (accurate within a couple of days). Please note that the wheel is designed for use by women with regular 28-day menstrual cycles.
One person may tell you your due date is August 8 and another may say August 9. Remember-the due date is only an estimate of when you will deliver. The chances of actually delivering on this date are very slim.
This is more common than you would think. If you can't remember the first day of your last menstrual period there are other ways of figuring out your due date. If you think you know when your period was (within a week), your doctor will probably estimate your due date according to this date as long as other evidence (physical exam or ultrasound studies) doesn't contradict it. If you have no idea when your last period was, your doctor may order an ultrasound to determine your due date.
Some women have cycles that are consistently longer than the average 28 days (for example, 35 days). If this is true for you, you can still use a pregnancy wheel, you just have to adjust it. Generally speaking, the second half of a woman's menstrual cycle (the time from ovulation to the next menstrual period) is consistent and lasts 14 days. Therefore, if your cycle is 35 days then you probably ovulated (released an egg) on day 21.
If you have a long menstrual cycle you can calculate your due date by adjusting the method. Figure out as best you can when you ovulated and then with this adjusted LMP you can determine the EDC that way. Some wheels allow you to enter the date of conception instead of LMP conception-union of the egg and the sperm-which occurs within 72 hours of ovulation.
To "trick" the wheel, you would subtract 2 weeks from the day you ovulated to mark your LMP and then look at the date where the line crosses for your EDC. Some wheels allow you to enter the date of conception instead of LMP.
For example, if your cycle is consistently 35 days and the first day of your last menstrual period was November 1, than you probably ovulated 21 days later on November 22. Subtracting 14 days from the date of ovulation gives you November 8 as the adjusted LMP date that you can use to determine the EDC.
Your doctor may change your due date if the size of your fetus varies significantly from the appropriate size for your calculated age of pregnancy. Generally, your doctor will order an ultrasound to determine gestational age when there is a history of irregular periods, when the date of your LMP is uncertain, when conception occurred during oral contraceptive use, or when the initial pelvic examination suggests a gestational age different from that indicated by the LMP. Ultrasound allows your doctor or provider to measure the length of the fetus from one end to the other in order to obtain the crown-rump length (CRL). During the first trimester, this measurement correlates with actual gestational age, plus or minus seven days.
During the first trimester of pregnancy, if the dates estimated by ultrasound study differ by more than one week from estimates based on your last menstrual period (even if you are relatively sure of when it occurred), it may be reasonable to use the ultrasound dates. In the second trimester, ultrasound is less accurate and your doctor probably won't adjust your dates unless the estimates differ by more than two weeks. The third trimester is the least accurate time to date a pregnancy. Estimates based on ultrasound can be off by as much as three weeks. For this reason, doctors rarely adjust dates during the third trimester. However, it is not uncommon for a doctor to repeat an ultrasound in the third trimester if he is contemplating changing your dates. The repeat study will provide valuable information about growth of the fetus and may reassure you and your doctor that the change in dates is reasonable.
Did You Know?
Ultrasound measurements are more accurate for estimating gestational age early in pregnancy because early development occurs according to a fairly regular schedule. However, as pregnancy progresses, rates of fetal growth begin to vary from pregnancy to pregnancy. Therefore, later measurements cannot be used to accurately predict the gestational age.
When a doctor performs an ultrasound, he or she will write a report on the findings including two estimated due dates. The first date is calculated from the LMP and the second date is based on the ultrasound measurements. These dates are rarely the same. When your doctor evaluates the ultrasound results, he or she will determine whether or not these dates are in agreement. The doctor is not likely to change your due date unless it differs significantly from your ultrasound date.
If you have additional ultrasounds, each time the ultrasound report is written, it will contain a new due date based on the most recent measurements. Generally, an expected due date should not be changed based on data from a second or third ultrasound. Estimating due dates is more accurate earlier in pregnancy. Later ultrasounds are helpful in determining whether the fetus is growing well, but not in determining gestational age.