One of the first attempts to assess the gestational age at term in the human was made by Franz Naegele (1778-1851) in 1812. He held that pregnancy lasted ten lunar months from the last menstrual period. Known as Naegele’s rule, and not based on empirical data, pregnancy duration is 288 days (or 40 weeks) from the first day of the mother’s last menstrual period (LMP), which assumes that ovulation occurs regularly on day 14 of 28 day menstrual cycle. According to this rule, the due date is calculated as follows:
LMP + 7 days) – 3 months = due date
During the 1980s, it was noticed by Robert Mittendorf that due dates for women in his Chicago practice (mainly second-generation Irish-Americans) were on average seven days beyond this date. After reviewing records of 17,000 births, he determined the average healthy, white, private-care, primiparous woman had a mean of 288 days from LMP to birth, this being eight days longer than that derived by means of Naegele’s rule. He also determined a number of influences that determine term gestational age, including ethnicity, parity, nutrition, substance use, mother’s age, and maternal size. Accordingly, a more appropriate way of estimating this age is held to be:
(LMP – 3 months) + 15 Days = due date
with the qualification that 10, rather than 15, is added if mother is not Caucasian, or multiparous. About 80% of all pregnant women with a reliable last menstrual period will deliver within 14 days either side of 280 days from the first day of their last menstrual period. However, for other women, their menstrual cycles are not regular, and in such cases alternative methods of estimating gestational age have been devised based on measures of neuromotor and physical maturity.
See Corrected age, Chronological or postnatal age, Dubowitz scale, Ductus Botalli (or ductus arteriosus), Fetal breathing, Fetal movements, Fetus, Neuronal migration disorders, Olfaction, Oligohydramnios, Perinatal