Average Duration of Human Pregnancy
The average duration of human pregnancy is 280 days (40 weeks) from the first day of the last menstrual period, or 268 days (38 weeks and 2 days) from ovulation/conception. 1
Standard Obstetric Dating Convention
Gestational age (GA) is conventionally reported as completed weeks and days from the first day of the last menstrual period (LMP), not from conception. 1
The duration of pregnancy using this standard obstetric dating is 280 days or 40 weeks, which has been the accepted convention since Naegele's rule was established in the 19th century. 2, 3
When measured from the actual date of ovulation (conception), the median duration is 268 days (38 weeks, 2 days), which is approximately 2 weeks shorter than the LMP-based calculation. 4
The first trimester is defined as gestational age ≤13 weeks 6 days using standard obstetric dating from LMP. 1, 5
Natural Variation in Pregnancy Length
Even among uncomplicated pregnancies measured precisely from ovulation, the gestational length range spans 37 days, demonstrating substantial biological variation. 4
In one study of low-risk pregnancies in spontaneous labor, the mean gestational age was 272.1 ± 9 days (approximately 38 weeks and 6 days), with 80% of women delivering between 261-280 days. 6
The coefficient of variation is higher when measured by LMP (4.9%) than by ovulation (3.7%), reflecting the inherent variability in timing of ovulation among women. 4
Factors Influencing Pregnancy Duration
Maternal age, birth weight at the mother's own birth, and individual woman's previous pregnancy lengths are significant predictors of gestational duration. 4
Time to implantation and corpus luteum rescue pattern (reflecting ovarian response to implantation) predict pregnancy length, with conceptions taking longer to implant also taking longer from implantation to delivery. 4
Ethnicity, maternal height, menstrual cycle variations, parity, and maternal weight all significantly influence pregnancy length, challenging the notion that a single "280-day rule" applies universally. 2
Clinical Context and Limitations
The 280-day (40-week) convention remains the standard for clinical practice despite known variations, as it provides a consistent framework for prenatal care timing and risk assessment. 1
Naegele's rule should be considered a guideline for the expected date of confinement, not a definite date, as what may have been appropriate in 19th-century Germany deserves reconsideration with modern technology. 2
Perinatal morbidity indicators and mortality rates increase significantly once pregnancy extends beyond 280 days, justifying the clinical utility of this threshold despite individual variation. 6