"A good deal of the confusion on this point seems to have arisen from considering forty weeks and nine calendar months as one and the same quantity of time, whereas, in fact, they differ by from five to eight days. Nine calendar months make 275 days, or if February be included, only 272 or 273 days, that is thirty-nine weeks only instead of forty. Yet we constantly find in books on law, and on medical jurisprudence, the expression "nine months or forty weeks." Another source of confusion has evidently had its origin in the indiscriminate use of lunar and solar months, as the basis of computation in certain writings of authority."[59] It is owing to this uncertainty that a considerable latitude has been allowed by the codes of law in different countries for the duration of pregnancy, in order to prevent the risk of deciding where the data are so uncertain. Experience has shown that the ordinary term of human pregnancy, wherever it has been capable of being determined with any degree of accuracy, is 280 days or forty weeks; and this period seems to have been generally allowed even from the remotest ages. As, however, it is so difficult to fix the precise moment of conception, it has been customary in different countries to allow a certain number of days beyond the usual time; thus the Code Napoléon ordains 300 days as the extreme duration of pregnancy, allowing twenty days over to make up for inaccuracy of reckoning. In Prussia it is 301 days, or three weeks beyond the usual time. In this country the limit of gestation is not so accurately determined by law, and therefore gives rise occasionally to much discrepancy of opinion. The grand question which this subject involves, is, whether a woman can really go beyond the common period of gestation. A great number of authors have considered that the _partus serotinus_, or over-term pregnancy, is perfectly possible; but by far the majority use such an uncertain mode of reckoning that little confidence can be placed in them. Two questions here arise, the determining of which will greatly assist us in forming a correct view of this intricate subject, viz. _first_, what has been the duration of those cases of pregnancy where the moment of conception has been satisfactorily ascertained? _secondly_, what are the causes which determine the period at which labour usually comes on?