Thus it is plain that the period of gestation is more variable than some have supposed.

The same thing is also observed in animals, where, in general, a much more true record can be kept, than is possible in regard to the human species. M. Tessier, in France, caused a set of observations to be made on the duration of pregnancy in 160 cows, which commonly go nine months with young. His table is as follows:

3brought forth on the270th day.
50„270th to the 280thday.
68„280th to the 290th„
20„300thday.
5„308th„
14„241st to the 260th„

There is, as will be seen in this table, a variation of sixty-seven days in the time a cow may carry her young.

A similar variation has been found in mares as cows, respecting the time of gestation. The same author—M. Tessier—made observations which were extended to 102 mares. The shortest period at which they brought forth was two hundred and forty-one days, the longest at three hundred and eight days.

Where there is no knowledge as to the precise time when an impregnation took place—and such is the fact in almost all cases with the human subject—a very good mode of reckoning is this: first ascertain the precise time when the menses last appeared, then reckon two weeks; at which time consider pregnancy as beginning. Nine calendar months, or forty weeks, which may vary somewhat from that period, will be about the time at which labor will ordinarily come on.

Another method is as follows: Quickening takes place at about the twentieth week; add twenty weeks more to the previous time, and we have an approximation to the truth. But, as I have elsewhere shown, the time of quickening is subject to a good deal of variation, namely, from the twelfth to perhaps the thirtieth week.

I think it will seldom be found to happen that labor occurs short of forty weeks, or 280 days, after the last appearance of the menses. But it is not out of place for me to inform you that medical men have varied considerably in regard to the duration of pregnancy. Thus, Dr. Guy has quoted five authorities, of greater or less eminence, to wit:

Sir Charles Clarke; 40 weeks—280 days.

Dr. Blegborough; 39, usually 40 weeks—273–280 days.