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,

5were bornin 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.

The question therefore of the partus serotinus; as far as these data are concerned, remains still undecided: of 10 cases which have occurred under our own immediate notice, where the patients determined the commencement of their pregnancy from other data than the last appearance of the catamenia, a similar variation was observed, viz. that nearly one-third went beyond 280 days, six of these individuals reckoned from their marriage, and four from peculiar sensations connected with sexual intercourse, which convinced them that impregnation had taken place: of these, seven did not go beyond the 280th day, two having been delivered upon that day, and three went beyond it, viz. to the 285th, 288th, and 291st days: the two former reckoned from their respective marriages; the latter, who went 291 days, from her peculiar sensations.

The calculation from the date of marriage is liable to the same objections as that taken from the last appearance of the catamenia; for if it had been solemnized (as is usually the case where it is possible) shortly after a menstrual period, and if conception did not take place until a fortnight or three weeks afterwards, the patient’s pregnancy would thus have appeared to have lasted so much longer than the natural term. The case, however, which is stated to have gone 291 days, does not come under this head, for here the pregnancy really appears to have lasted 10 or 11 days beyond the full period, which cannot be accounted for in the way above mentioned: we should not have ventured to quote this, if a similar instance had not been recorded by Dr. Dewees. “The husband of a lady, who was obliged to absent himself many months, in consequence of the embarrassment of his affairs, returned, however, one night clandestinely, and his visit was only known to his wife, her mother, and ourselves. The consequence of this visit was the impregnation of his wife; and she was delivered of a healthy child in 9 months and 13 days after this nocternal visit. The lady was within a week of her menstrual period, which was not interrupted, and which led her to hope she had suffered nothing from her intercourse; but the interruption of the succeeding period gave rise to the suspicion she was not safe, and which was afterwards realized by the birth of a child.”[60]

Although it is to be regretted that this case has been calculated in the ordinary vague manner of calendar months, yet it is perfectly evident that the pregnancy was longer than the ordinary duration. We shall, therefore, endeavour to investigate the possibility of over-term pregnancy still more closely by a consideration of the second question, viz. what are the causes which determine the period at which labour usually comes on?

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 dysmenorrhœa 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.