Whether the baby will be a boy or girl is determined approximately eight and one-half months before birth, at the critical second of fertilization. The mother’s egg has nothing whatever to do with determining the sex of her offspring; in this respect the ovum is neuter. The father ejaculates two types of spermatozoa in apparently equal numbers. The two types differ from each other to a minor degree, yet this small variation makes a major difference—the sex of the individual. Every spermatozoon contains the same twenty-two chromosomes (autosomes) which affect the inheritance of all bodily structures and functions except the sex of theand some associated characteristics. The twenty-third chromosome, the sex chromosome, is very different in the two kinds of spermatozoa. In half, the sex chromosome of the sperm’ cell is relatively large, resembling the twenty-third or sex chromosome of the egg, and is called the ‘X chromosome.’ In half of the sperm cells the sex chromosome is much smaller and very different in appearance; it is called the ‘Y chromosome.’ When an ovum is fertilized by an X-bearing sperm cell, a female fetus is created; when it is fertilized by a Y-bearing sperm cell, a male results.
From the practical viewpoint, the husband alone determines the sex of the offspring—not by any process within his control, to be sure. However, no wife need feel that it is she who failed her husband by not bearing a male to carry on his name.
Is Sex Inherited?
There is a popular fallacy that the tendency to produce a preponderance of males runs in some families and an equally strong tendency toward the creation of girls in other families.’ This concept has been repeatedly investigated by geneticists and statisticians, and each study leaves no doubt that the production of a child of one sex or the other is not affected by heredity; the sex of an infant is purely a matter of chance. The problem goes back to the old coin-tossing experiment. Once in 128 tosses there will be seven tails or seven heads in succession.
Your chances for having a boy in the next pregnancy are precisely the same whether you have already had five daughters in five pregnancies, or five sons.
Can We Control the Sex?
As I have just stated, apparently chance determines sex, and we cannot affect chance. The multiple prescriptions recommended at one time or another, such as intercourse at a particular phase of the menstrual cycle, or coitus preceded by an alkaline or acid douche, work correctly half the time and incorrectly half the time.
We shall not be able to control the sex of the offspring until by one means or another we can either sort the X-bear- ing from the Y-bearing spermatozoa, or render the ovum refractory to fertilization by one variety or the other of sperm cell.
Dr. Landrum Shettles of the College of Physicians and Surgeons claims that he can prepare dried, stained, microscopic slides of semen which clearly demonstrate spermata-zoa of two varieties, the difference being in the size and shape of the heads. In the larger he believes he can identify the extra X chromosome. Of course, such sperm cells are dead, and separating them in such condition has no practical value.
Can the Sex Be Diagnosed before Birth?
Since the time of the Pharaohs men have promulgated first this method and then that for predicting the sex of the unborn. They have used urine, blood, and saliva as the test substances; they have depended on the color of the mother’s skin, the relative size of the right and left breasts, or the way the infant was ‘carried,’ to uncover the answer. Each method in its time and place was hailed as the key to the solutionVand brought temporary fame and sometimes fortune to its innovator. Each in turn was soon challenged, and each was soon discarded.
Since 1949 Dr. M. L. Barr, a Canadian anatomist, and his associates have published a series of articles demonstrating that if human or animal tissue cells from any part of the body are properly prepared, stained, and highly magnified under the microscope, a recognizable difference exists between the cells obtained from a female and those obtained from a male. The majority of the cells from a female show a dark-staining, bean-shaped body lying along the outer border of the nucleus. The cells from a male lack this specific body, thought to be the two X chromosomes, side by side. Epithelial or skin-type cells show the difference particularly well. Skin cells from both human and animal fetuses, even very early ones, showed the same sex difference found in the adult animal.
While the fetus occupies the uterus it constantly sheds skin cells into the amniotic fluid surrounding it. Making use of this fact, Israeli investigators put a fine hollow needle through the abdominal wall and uterus of a pregnant woman and aspirated a small amount of fluid from the amniotic sac. They recovered fetal skin cells from the amniotic fluid and were able by the Barr technique to prediagnose accurately the sex of the unborn and, according to the report, pregnancy progressed undisturbed. One questions wheth- er the simple satisfaction of curiosity justifies so complex a medical procedure—one that is not without theoretical danger.
Just recently an investigation has been made of amniotic fluid recovered vaginally from patients in whom the membranes ruptured before the onset of labor or early in its course. The Barr technique, as would be expected, again foretold the sex. The only drawback is that the child usually was discharged from the nursery before the cell preparations were readied for study.
Actually, as an obstetrician of much experience, I secretly hope that no simple, practical method of diagnosing the sex before birth will be found. If this happens it will rob the birth act of much of its excitement and drama. As things now stand, I can hardly wait for the rear end of the infant to appear to discover what the Browns got. Knowledge of the sex before birth would certainly rob the doctor’s announcement to the expectant father of much of its punch. All the obstetrician could say would be, ‘You have a baby.’ It would be a trifle anticlimactic to grin from ear to ear and say, ‘It’s a boy!’ when the man had already known this for six months.
In breech presentations a vaginal examination late in labor will occasionally permit the doctor to determine the sex before birth by feeling the genitals. Also in most breech cases the genitals are visible through the vaginal orifice in the last minutes before actual birth.
Sex ratio, by definition, is the number of males to every hundred females. There are several types of sex ratio: primary, the number of males conceived per hundred females conceived; secondary, the number of males born per hundred females; and tertiary, the number of males living at any given postnatal period per hundred females.
The sex ratio at birth, the secondary sex ratio, is uniformly between 105 and 106 throughout the world. What accounts for this universal and consistent preponderance of male births? The first assumption would be that there are more male-producing sperms than female-producing sperms. But this is not the case. As we stated before, X- and Y-bear-ing spermatozoa have been proved to be in equal numbers. One might suspect, then, that more female fetuses die before birth. Actually, the reverse is true. More male fetuses die than female; in every five miscarriages, and in every five babies born dead, there are approximately three males and two females. The primary, or conception, sex ratio is postulated to be 130, twenty-five higher than the sex ratio at birth. Perhaps this high conception sex ratio is due to some inherent advantage that Y-bearing sperms possess over X-bearing sperms. It could be that Y-bearing sperm cells are more active and vigorous; or it may be that a greater chemical affinity exists between the unfertilized egg and the Y sperm. Whatever the reason, man in his ignorance cannot as yet wrest the secret from nature.
Certain small ethnic groups are reported to have a higher or lower secondary sex ratio than the general population. In this country the Jews are said to have a high sex ratio and Negroes a low sex ratio. Also, the sex ratio is said to rise a point or two after wars which involve most of the population. The obvious variable is the incidence ofin a given sample; when its frequency rises the sex ratio declines, and when it falls the sex ratio is elevated. It is believed that among the Jews there is a lower miscarriage frequency than among Negroes. After a war impregnation occurs in a population whose women have had enforced reproductive rest, which may reduce the incidence of miscarriage.