Pregnancy
- The first day of the woman's last normal menstrual period, and the resulting fetal age is called the gestational age.
- The date of conception (about two weeks before her next expected menstrual period), with the age called fertilization age
- The date of implantation (about one week after conception).
As measured on a reference group of women with a menstrual cycle of exactly 28-days prior to pregnancy, and who had spontaneous
onset of labor, the mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the last
menstrual period as recalled by the mother, and 280.6 days when the gestational age was retrospectively estimated by obstetric ultrasound
measurement of the fetal biparietal diameter (BPD) in the second trimester.Other algorithms take into account a variety of other variables,
such as whether this is the first or subsequent child (i.e., pregnant woman is a primipara or a multipara, respectively), the mother's race,
parental age, length of menstrual cycle, and menstrual regularity), but these are rarely used by healthcare professionals. In order to have a
standard reference point, the normal pregnancy duration is generally assumed to be 280 days (or 40 weeks) of gestational age. There is a
standard deviation of 8–9 days surrounding due dates calculated with even the most accurate methods. This means that fewer than 5
percent of births occur on the day of being 40 weeks of gestational age; 50 percent of births are within a week of this duration, and about
80 percent are within 2 weeks. It is much more useful and accurate, therefore, to consider a range of due dates, rather than one specific
day, with some online due date calculators providing this information.
