It is now ten years ago since we first surmised that "the reason why labour usually terminates pregnancy at the 40th week is from the recurrence of a menstrual period at a time during pregnancy when the uterus, from its distension and weight of contents, is no longer able to bear that increase of irritability which accompanies these periods without being excited to throw off the ovum." Under the head of PREMATURE EXPULSION, we shall have occasion to notice the disposition to abortion which the uterus evinces at what, in the unimpregnated state, would have been a menstrual period: for some months after the commencement of pregnancy, a careful observer may distinctly trace the periodical symptoms of uterine excitement coming on at certain intervals, and it may be easily supposed that many causes for abortion act with increased effect at these times. Where the patient has suffered from dysmenorrhoea before pregnancy, these periods continue to be marked with such an increase of uterine irritability as to render them for some time exceedingly dangerous to the safety of the ovum. Even to a late period of gestation, the uterus continues to indicate a slight increase of irritability at these periods, although much more indistinctly; thus, in cases of hæmorrhage before labour, especially where it arises from the attachment of the placenta to the os uteri, it is usually observed to come on, and to return, at what in the unimpregnated state would have been a menstrual period. We mention these facts as illustrating what we presume are the laws on which the duration of pregnancy depends, and also as being capable of affording a satisfactory explanation of those seeming over-term cases which are occasionally met with. From this view of the subject it will be evident, that the period of the menstrual interval at which conception takes place, will in great measure influence the duration of the pregnancy afterwards; that where it has occurred immediately after an appearance of the menses, the uterus will have attained such a dilatation and weight of contents by the time the ninth period has arrived, that it will not be able to pass through this state of catamenial excitement without contraction, or, in other words, labour coming on: hence it is that we find a considerable number of labours fall short of the usual time, so much so that some authors have even considered the natural term of human gestation to be 273 days or 39 weeks: for a somewhat similar reason we can explain why primiparæ seldom go quite to the full term of gestation, the uterus being less capable of undergoing the necessary increase of volume in a first pregnancy than it is in succeeding ones.