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