How to Predict Due Date
Size of Uterus
At around 12 weeks of pregnancy, the top of the uterus (fundus) can be felt above the pelvic rim and after about 18 weeks, the distance between the pubic bone and the fundus (in centimeters) is likely to be about the same as the number of weeks since your last period. Although, it isn't an accurate way of predicting the due date, the fundal height is used to guess the due date.
Last Menstrual Period
The most common method of calculating your due date is to add 40 weeks to the first day of your last menstrual period. Another method is by taking the first day of your last normal menstrual period (LNMP), adding 7 days, and then counting 3 months backwards. For example, if your LNMP started on February 10, add 7 days, and then subtract 3 months to get the due date, which in this case will be November 17.
To predict the due date correctly, it is very important to know the pregnancy period. Ultrasound testing provides an accurate resolve to the problem of finding out how long you have been pregnant, especially if it is done before 20 weeks of pregnancy. During an ultrasound test, a small instrument is moved back and forth over the pregnant woman's abdomen. The instrument sends out sound waves. A computer then converts the sound waves that bounce off the fetus and shows up on the TV screen in the form of a picture. You can predict the due date more accurately with the help of ultrasound results.