Due date, or estimated date of delivery, is the approximate date of the baby's birth. It is determined by the date of the woman's last menstrual period and not the date she conceived.
The due date formula is a very basic calculation, and is simply 40 weeks from the woman's last menstrual period. A midwife or doctor will use a chart to determine this date, and many websites can now do this for women who would like to find out before visiting their midwife or doctor.
The chart and subsequent estimate are based on a 28-day cycle. However, as some women do not have a 28-day cycle, and others do not ovulate at the same time during their cycles, variation from the date is always to be expected.
In fact, only five per cent of women give birth on their estimated due date and it is entirely normal to exceed the due date by up two weeks, or give birth two or even three weeks prior to the estimated arrival.
The due date also plays an important part in assessing the development and health of the unborn baby in the scans and screening tests a woman will be offered. Measurements and developmental milestones are all based around the estimated due date.