If you know the first day of your last menstrual period, you may have already estimated your baby's due date. At your first prenatal exam, your health care provider will ask questions to try to predict your due date as precisely as possible.
Knowing your baby's due date will help your provider monitor your baby's growth more accurately. Because certain laboratory tests change throughout your pregnancy, knowing an accurate due date will also allow your provider to better track these tests, and, if it occurs, to manage preterm labor.
How Is My Due Date Determined?
Normally, your due date is 280 days (40 weeks or about 10 months -- also known as 10 lunar months) from the first day of your last period. However, if your periods are not regular or are not 28 days apart, your due date may be different from the "280-day rule." Your health care provider may order an ultrasound to more accurately determine your due date.
If you are certain of your conception date (the date when you got pregnant), please tell your health care provider. This information can be helpful in determining your estimated date of delivery (EDD).
A full-term pregnancy ranges from 37 weeks to 40 weeks and 6 days, so your actual date of delivery can be different from your estimated date of delivery, which is sometimes called estimated date of confinement, or EDC. A very small number of babies are actually born on their due dates. Typically, only 5% percent of women deliver on their due date.