One of the questions which all of my patients ask is that when they are going to deliver their babies. The answer is not entirely straightforward. We as Obstetricians would be more than happy if we know when exactly your baby will be born. However, studies have shown that only 1 in 20 babies were born on his or her due date. Although textbooks says that normal healthy pregnancy lasts about 280 days or 40 weeks from the first day of the last menstrual period, most babies were born within either way of that date (anywhere from 38 to 42 weeks), and therefore it keeps everyone guessing right up to the delivery day and makes life interesting for the Obstetricians.
Therefore, the medical term for due date is called an ‘Estimated Date of delivery’ or EDD. It is calculated based on the first day of the last menstrual period (LMP), the estimated due date is 40 weeks from that date. It can also be calculated by adding 7 days and subtracting 3 months to the date of the last menstrual period. For example: if a woman’s last menstrual period is on 01/12/2009, the estimated date of delivery will be on 8/09/2010. This can be calculated using the Obstetric Calender
Picture 1: An Obstetric Calender
However, this method is base on the assumption that a woman has a regular menstrual cycle of 28 days. For women who has irregular menstrual period or women who cannot remember the first day of their last menstrual period, it can be difficult to determine the gestational age. Therefore, it is important that they have a scan early to determine the gestational age and to estimate their due dates. The baby can be measured as early as 5-6 weeks after the mother’s last menstrual period and for the most accurate estimation, measurement of the baby should be made before 12 weeks of pregnancy. This is important for the Obstetrician who looks after you as accurate dating helps doctors to ensure that your pregnancy is progressing well. 
Picture 2: An ultrasound showing a fetus at 12 weeks
Gestational age or the duration of the pregnancy, is calculated from the first day of the mother’s last menstrual period and not from the date of conception. The term is also interchangeable with ‘Period of Amenorrhoea’. Since the exact date of conception is not usually known, the first day of the last menstrual period is used to measure how old the baby is. However, in women who undergo infertility treatment such as artificial insemination or in vitro fertilization, the exact date of conception is known.
Reference: Pitterpatter.com.my