DURATION OF PREGNANCY.

There are few questions of great importance and interest respecting a subject under our daily observation, about which such uncertainty and so much diversity of opinion exists, as the duration of human pregnancy; and yet, as is the case with the diagnosis of pregnancy, upon a correct decision frequently depend happiness, character, legitimacy, and fortune. In like manner it frequently happens, that the data upon which we have to found our opinion are exceedingly doubtful and obscure; and to increase the difficulties of the investigation still farther, we have not uncommonly to contend with wilful deception and determined concealment.

The duration of pregnancy must ever remain a question of considerable uncertainty so long as the data and modes of calculation vary so exceedingly. “Some persons date from the time at which the monthly period intermits; others begin to calculate from a fortnight after the intermission; some reckon from the day on which the succeeding appearance ought to have become manifest; some are inclined to include in their calculation the entire last period of being regular; and others only date from the day at which they were first sensible of the motions of the infant.”[58]

“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?

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,

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 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.

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.