A simple formula is to take the first day of the menstrual period, subtract three months and add one week.
For instance, if your period starts on the 5th April, then your estimated due date for delivery is the 12th January.
Even when you can remember when your last menstrual period began, ultrasound can still be more accurate than relying on your last period date alone.
The American College of Obstetricians and Gynecologists state the following: “Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks gestation) is the most accurate method to establish or confirm gestational age” (Committee Opinion, No. Even though using your last period date (LMP) is a traditional dating method, the accuracy of this practice depends on a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day of the cycle.
They are usually very accurate within 3 days as far as dating goes in the middle of the first trimester (less accurate later in pregnancy).