In a broad sense, the period of gestation for each variety of mammal is determined by the time required for embryonic development to reach the point where the young may live independently of the mother. This point is reached more quickly with small animals than with large. The mouse, for example, generally brings forth its young in three weeks, whereas the pregnancy of the elephant lasts two years. In human beings, counting from the time of conception to the time of delivery, pregnancy continues approximately 273 days. This number is merely an estimate calculated from hundreds of cases in which there was no question as to the underlying facts. Individual cases vary notably, and indicate that two women may become pregnant on the same day and yet not necessarily be delivered at the same date. Irregularities in the duration of pregnancy are not limited to man. Thus, while the mean period of gestation in the rabbit is thirty-one days, it may be either shorter or longer by as many as eight days. Similar variations occur in the pregnancies of all animals, and are, moreover, notably greater among larger animals, since for such animals the period of gestation is relatively long. For instance, the accurate observations of veterinarians indicate that the mean period of pregnancy in the cow is 285 days from the time of conception. This fact notwithstanding, a competent observer found that, of 160 cows, 67 were delivered before the 280th day; 68 between the 280th and the 290th day; and 25 after the 290th day. Although nothing unnatural was observed in any instance, the first animal was delivered 67 days before the last, and in 5 instances gestation continued 308 days. In ancient times it was believed that the duration of pregnancy was of even more uncertain length in man than in the lower animals; but since the eighteenth century thirty-nine weeks have been accepted as the average duration of the human pregnancy when reckoned from the day of conception. As this date is seldom known, it is most convenient to reckon from the first day of the last menstrual period. Estimated in this way its average duration is 280 days. As this period corresponds to ten menstrual cycles, physicians prefer to describe pregnancy as lasting 10 lunar months of four weeks each. This is equivalent to 9 calendar months, in terms of which its duration is popularly stated.