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In mammals, the interval between fertilization and birth. It covers the total period of development of the offspring, which consists of a preimplantation phase (from fertilization to implantation in the mother's womb), an embryonic phase (from implantation to the formation of recognizable organs), and a fetal phase (from organ formation to birth).
There is widespread confusion over the duration of the gestation period in humans because of the way in which it is defined medically. The time of ovulation, and hence the time of fertilization, is difficult to determine in humans, so for purely practical reasons doctors measure the duration of pregnancy as the interval between the last menstrual period and birth, which is typically about 40 weeks or 280 days. For comparison with other mammals, however, the true gestation period between fertilization and birth in humans is about 267 days.
The length of the gestation period in mammals depends primarily on body size and the state of development of the offspring at birth. Large-bodied mothers have big offspring that take longer to develop, and development is also prolonged for offspring that are born at an advanced stage of development. Compared to all other mammals, human beings are found to have one of the longest gestation periods relative to body size.
One remarkable feature of mammalian gestation periods is that they show very little variability within a species. After excluding exceptional cases, departures from the average usually lie in a range of no more than ±4%. This is one of the smallest degrees of variability found in any biological dimension. See Reproductive system