On the other hand, where impregnation has taken place shortly before a menstrual period, the uterus, especially if the patient has already had several children, will probably not have attained such a volume and development as to prevent its passing the ninth period without expelling its contents, but may even go on to the next before this process takes place: it is in this way that we would explain the cases related by Dr. Dewees and Dr. Montgomery. We are aware that, under such a view of the subject, the duration of time between the catamenial periods of each individual should be taken into account, some women menstruating at very short, and others at very long, intervals; but although this will affect the number of periods during which the pregnancy will last, it will not influence the actual duration of time, as this will more immediately depend upon the size and weight of contents which the uterus has attained. The valuable facts collected by M. Tessier respecting the variable duration of pregnancy in animals, which have been quoted by some authors in proof of the partus serotinus, are scarcely applicable to this question in the human subject; the absence of menstruation, and the different structure of the uterus, prevent our making any close comparison. CHAPTER VIII. PREMATURE EXPULSION OF THE FOETUS. _Abortion.--Miscarriage.--Premature labour.--Causes.--Symptoms.-- Prophylactic measures.--Effects of repeated abortion.--Treatment._ The uterus does not always carry the ovum to the full term of pregnancy, but expels it prematurely. This expulsion of its contents may occur at different periods, and is characterized accordingly: thus, among most of the Continental authors, it has been divided under three heads; those cases which occur during the first sixteen weeks coming under the head of _abortion_; those which occur between this period and the twenty-eighth week are called _miscarriages_; and when they take place at the latter period, until the full term of utero-gestation, they receive the name of _premature labours_.