What is the ordinary duration of pregnancy? Almost every woman considers herself competent to make the answer--nine months. She may be surprised to learn, however, that such an answer is wanting in scientific precision. It is too indefinite, and is erroneous. There is a great difference between the calendar and the lunar month. Each lunar month having twenty-eight days, the period of nine lunar months is two hundred and fifty-two days. Nine calendar months, including February, represent, on the contrary, two hundred and seventy-three days. Now the average duration of pregnancy is two hundred and eighty days, that is forty weeks, or ten lunar months. While most extended observations have shown that as a general rule, forty weeks, or two hundred and eighty days, is the true period of pregnancy, are we justified in the conclusion that this is its invariable duration? This important question, upon the answer to which so often depend the honor of families, the rights of individuals, and sometimes the interests of nationalities, has been in all times the subject of careful research by physicians, philosophers, and legislators. On the one side, have been those who contend that the laws of nature are invariable, and that the term of pregnancy is fixed and immutable. On the other side, have been those who assert that the epoch of accouchement can be greatly advanced or retarded by various causes, some of which are known, and others not yet appreciated. Abundant and satisfactory testimony has proved that the prolongation of pregnancy beyond the ordinary period of two hundred and eighty days, or forty weeks, is possible. Nor is this contrary to what is observed in regard to other functions of the human body. There is no process depending upon the laws of life which is absolutely invariable either as to the period of its appearance or duration. It is known, as we have already pointed out, that puberty may be advanced or retarded; the time at which the change of life occurs in women, as we shall have occasion hereafter to show, is also subject to variation; and it is a matter of common observation with mothers, that the period of teething is sometimes strangely hurried or delayed. A certain degree of variability, therefore, being frequently observed, and entirely compatible with health, in the various other natural processes, why should that of pregnancy form an exception, and be invariably fixed in its duration? And observation upon the lower animals affords most convincing evidence that nature is not controlled by any uniform law in reference to the length of pregnancy. In the cow, the usual period of whose pregnancy is the same as in the human female, instances of calving six weeks beyond the ordinary term are not at all uncommon.