The proposition that a pregnancy is 40 weeks or 280 days in duration is attributed to the German obstetrician Franz Naegele (1778–1851). His rule adds nine months and seven days to the first day of the last menstrual period. The expected date of confinement from this formula is approximately right in the majority of cases. However, the idea that this rule can apply to every pregnant female – young or old, nulliparous or multigravida, Caucasian, Asian, African, or Indigenous – stretches credulity. In addition, many women regard the 40‐week date as a deadline, which if crossed, may then place the baby under stress. Forty weeks is such a simple, round, convenient figure that it has proved difficult to challenge, despite criticism. Nonetheless, what might have been an appropriate formula in Germany in the 19th century deserves to be revisited in the 21st.

To review the length of pregnancy, in the light of current technology, in particular ultrasound scanning, and assisted reproductive techniques.

Material and Methods
A Medline search was performed for variables on the length of pregnancy, the expected date of confinement, and prolonged pregnancy.

A number of factors were found to significantly influence the length of a pregnancy, including ethnicity, height, variations in the menstrual cycle, the timing of ovulation, parity and maternal weight.

Naegele’s rule should be considered as a guideline for the expected date of confinement, and not a definite date.