A re-look at the duration of human pregnancy

The duration of human pregnancy is arbitrarily taken as 280 days (40 weeks). Foetuses are considered to be at high risk once pregnancy goes beyond the expected date of confinement. This study was carried out with the aim of determining the mean gestation age of low-risk pregnancies that went into spontaneous labour and the incidence of adverse outcomes in relation to gestation.

Low-risk singleton pregnancies admitted in spontaneous labour at a single community hospital in the Udupi district of Karnataka in South India, from December 2002 to December 2003, were analysed for mean gestational age at the onset of spontaneous labour and rates of perinatal complications by gestational age.

Among the 1,094 women who went into spontaneous labour, the mean gestational age was 272.1 +/- 9 days. A significantly increased incidence of meconium-stained amniotic fluid beyond 39 weeks of gestation was observed. 783 of 1,094 women (80 percent) had delivered during the period of 261-280 days of pregnancy (period of one standard deviation around the mean gestational age at delivery). There was significant increase in perinatal morbidity indicators and mortality rates once the pregnancy carried beyond 280 days.

Mean gestational age at the onset of labour for women native to the area of study was 272 days (standard deviation 9 days). Pregnancies beyond a duration of 280 days showed significantly increased perinatal morbidity. It is suggested that there is a need for determining the length of gestation and to compile gestation-wise incidence of meconium-stained amniotic fluid as an indicator of foetal maturity or the undisclosed risk factor, in addition to other neonatal morbidity indicators for different populations.