determination. For example, the length of the child is practically
constant for each of the ten lunar months into which the whole
gestation period is divided; if, therefore, the length of the newborn
infant is known, the stage of its development can always be inferred.
From the fifth month the calculation is especially simple, since the
length measured in centimeters divided by the figure 5 gives the
month to which pregnancy has advanced. Similarly, we can infer the
period of development from the weight, though the calculation is more
intricate and the method less reliable, inasmuch as the size of the
child in the latter months varies somewhat according to the weight of
its mother.

At the end of the fifth month, the weight of the fetus is from nine
to ten ounces; whereas an average infant when born at the expiration
of the full term of pregnancy, that is, with the completion of the
tenth month, weighs about seven pounds. The fetus, therefore,
acquires roundly ninety per cent, of its weight during the second
half of pregnancy, which clearly indicates that Nature reserves this
period of gestation for the fetus to increase in size, a phenomenon
less mysterious but no less important than the evolution of the
embryo.

Nothing is more valuable than the weight in affording an indication
as to whether a prematurely born infant may be reared. It is unusual
to raise a child weighing less than four pounds, which corresponds
approximately to the end of the eighth lunar month of development (a
trifle more than the seventh calendar month). After this time, the
prospect of living becomes greater in proportion to the nearness with
which the infant has approached maturity. No truth exists in the
widespread belief that the seventh-month child is favored above that