The Prospective Mother | Page 7

been regular and then a period is missed, the patient has good reason to suspect she is pregnant; if the next period is also missed and meanwhile the breasts have enlarged, the nipples darkened, and the secretion of colostrum has begun, it is nearly certain that she is pregnant; whether morning sickness and the desire to pass the urine frequently are present is of no importance._ But the most characteristic evidence, we must remember, is not available until the eighteenth or twentieth week; then the signs of pregnancy are unmistakable.
THE DURATION OF PREGNANCY.--After the existence of pregnancy has become assured, perhaps the greatest interest centers about the date upon which the birth may be expected. Even to approach accuracy in this prediction the prospective mother must be familiar with certain facts which she will always observe, but which, unless she appreciates their importance early in pregnancy, she may fail to record or to remember. In a few cases, however, such exceptional information as knowing the date of conception does not lead to an absolutely accurate prediction. But the deviation from the rule will be understood only after we understand the rule itself, which is based upon what we accept as the average duration of human pregnancy, technically called the period of gestation.
In a broad sense, the period of gestation for each variety of mammal is determined by the time required for embryonic development to reach the point where the young may live independently of the mother. This point is reached more quickly with small animals than with large. The mouse, for example, generally brings forth its young in three weeks, whereas the pregnancy of the elephant lasts two years. In human beings, counting from the time of conception to the time of delivery, pregnancy continues approximately 273 days. This number is merely an estimate calculated from hundreds of cases in which there was no question as to the underlying facts. Individual cases vary notably, and indicate that two women may become pregnant on the same day and yet not necessarily be delivered at the same date.
Irregularities in the duration of pregnancy are not limited to man. Thus, while the mean period of gestation in the rabbit is thirty-one days, it may be either shorter or longer by as many as eight days. Similar variations occur in the pregnancies of all animals, and are, moreover, notably greater among larger animals, since for such animals the period of gestation is relatively long. For instance, the accurate observations of veterinarians indicate that the mean period of pregnancy in the cow is 285 days from the time of conception. This fact notwithstanding, a competent observer found that, of 160 cows, 67 were delivered before the 280th day; 68 between the 280th and the 290th day; and 25 after the 290th day. Although nothing unnatural was observed in any instance, the first animal was delivered 67 days before the last, and in 5 instances gestation continued 308 days.
In ancient times it was believed that the duration of pregnancy was of even more uncertain length in man than in the lower animals; but since the eighteenth century thirty-nine weeks have been accepted as the average duration of the human pregnancy when reckoned from the day of conception. As this date is seldom known, it is most convenient to reckon from the first day of the last menstrual period. Estimated in this way its average duration is 280 days. As this period corresponds to ten menstrual cycles, physicians prefer to describe pregnancy as lasting 10 lunar months of four weeks each. This is equivalent to 9 calendar months, in terms of which its duration is popularly stated.
THE ESTIMATION OF THE DATE OF CONFINEMENT.--Since pregnancy is not an absolutely fixed period, we possess no reliable means of predicting the exact day when it will end. The most satisfactory method of prediction consists in counting forward 280 days from the beginning of the last menstruation or, what gives the same result, counting backward eighty-five days from this date. _To make the calculation in the simplest way we count back three months and add seven days_; this addition is made because seven days generally represents the difference between three months and eighty-five days. If the last menstruation, for example, began on October 30th, we count back three months to July 30th and add seven days, which gives August 6th as the probable date of confinement.
A prospective mother should remember that this prediction is no more than approximate. The calculation does not give the exact date of delivery more than four or five times in a hundred cases. It is accurate within a week in half the cases and within two weeks in four-fifths. We also know that delivery is somewhat more likely to occur after the expected date than before