The circumstances under which it happens that we are able to ascertain the precise date of impregnation occur so rarely, that it is nearly impossible to collect any considerable number of such cases. Three have occurred under our own notice, in which there could be little doubt as to the accuracy of the information given, and in each of these the patient went a few days short of the full period. One, a case of rape, was delivered on the 260th day; in the two others, sexual intercourse had only occurred once; in one case she went 264, in the other, 276 days. We could have mentioned several others, but where even the slightest shadow of doubt as to their accuracy has existed, we have rejected them as inconclusive. The mode of calculating the duration of pregnancy, which is ordinarily adopted, viz. by reckoning from the last appearance of the catamenia, although the chief means which is afforded us for so doing, is nevertheless much too vague and uncertain to ensure a decided result; for although it is a well-known fact, that conception very frequently takes place shortly after a menstrual period, there can be no doubt that it is liable to occur at any part of the catamenial interval, and particularly so shortly before the next appearance: hence, by this mode of reckoning, we are not more justified in expecting labour in nine months time from the last appearance of the catamenia, than at any part of the interval between this and what would have been the next appearance. Dr. Merriman, who has devoted much attention to this intricate but important subject, says, "When I have been requested to calculate the time at which the accession of labour might be expected, I have been very exact in ascertaining the _last day_ on which any appearance of the catamenia was distinguishable, and having reckoned 40 weeks from this day, assuming that the _two hundred and eightieth_ day from the last period was to be considered as the legitimate day of parturition" (_Synopsis of Difficult Parturition_, p. xxiii. ed. 1838;) and gives a valuable table of "one hundred and fifty mature children, calculated from, but not including, the day on which the catamenia were last distinguishable." Of these, 5 were born in the 37th week, 16 --- in the 38th, 21 --- in the 39th, 46 --- in the 40th, 28 --- in the 41st, 18 --- in the 42nd, 11 --- in the 43rd; so that about one-third were born three weeks after the 280 days from the last appearance of the catamenia; a circumstance which is perfectly easy of explanation, from what we have just observed, without the pregnancy having overstepped its usual duration: in other words, it would appear that 28 of these cases had conceived one week, 18 two weeks, and 11 three weeks after the last appearance of the catamenia.