Development of The Fetus

The increasing prevalence of sex-education courses notwithstanding, most American adults do not fully understand the mechanism whereby a pregnancy evolves and develops. It is a fascinating story. Let me tell it to you.


With rare exceptions, every woman is endowed with a vagina, a uterus (or womb), two fallopian tubes, and two ovaries. The lower part of the uterus is called the cervix (sometimes referred to as the “neck” of the womb for cervix is Latin for neck). The cervix is composed mainly of tough fibrous tissue and it contains a tiny canal through which the vagina communicates with the cavity of the uterus. The fallopian tubes, in turn, open at one end into the uterine cavity and at the other end directly into the abdomen. Thus in the female there is a direct pathway from the outside world by way of the vagina, the cervix, the uterus, and the tubes to the abdominal cavity itself. In their natural state, of course all of these organs, including the vagina, are collapsed. Air and water cannot enter the vagina and ascend into the uterus, for the cervical canal is ordinarily filled with a plug of impermeable mucus. But this uninterrupted chain of canals and cavities can, of course, be traversed by the spermatozoa.


The ovaries, which are located near the abdominal openings of the tubes, have two very important functions: to manufacture the eggs and to produce the female hormones. The ovaries of every female infant at birth contain thousands of tiny individual cells, the “primordial germ cells,” some of which are destined to become ova or eggs, which may or may not become fertilized. Every month, during the reproductive years of a woman (during the years that she menstruates), several of these primordial ova, which have lain dormant in her ovaries since she was born, develop into fertilizable eggs, around each of which a small fluid-filled follicle forms. These follicles reach the size of about half an inch in diameter and then, approximately halfway between two menstrual periods, one of the eggs bursts through the paper-thin wall of its follicle and escapes into the abdominal cavity. This would seem a rather foolish thing for it to do, for the egg, which consists of only one cell and is just barely visible to the naked eye, then finds itself free in the relatively enormous space which contains the woman's bowels, her liver, stomach, and so on. But Nature has prevented the egg from becoming lost or going astray by endowing the tubes with the power to ensnare it, almost in the manner of a frog catching a fly. As a matter of fact, the tube's ability to accomplish this feat is developed to such a fine point that if one tube and the opposite ovary have been removed the remaining tube will still be able to “catch” the egg which has been expelled from the ovary on the other side.

It is the follicle, which held the egg, that is responsible for the production of estrogen, one of the most important female hormones. After the egg has been extruded from it (this phenomenon being known as ovulation), the follicle continues to secrete estrogen and now a second hormone known as progesterone. These two hormones are secreted into the blood stream, through which they reach the uterus and prepare its lining (the endometrium) for the reception of a fertilized egg. It is this endometrium which is shed every month and comprises the principle constituent of the menstrual flow.

If the egg is fertilized, the ovary will continue to secrete estrogen and progesterone and menstruation will not occur. If the egg is not fertilized, it will die and two weeks later, the hormone secretion will cease menstruation will begin, and the whole cycle will be repeated. Since the human egg remains alive for only twelve hours, women are actually fertile, in the strict sense of the word, for only about six full days a year.


The male of our species displays a different type of inefficiency in his reproductive role. It is his job to provide spermatozoa, a different type of individual cell, which in turn is responsible for finding the egg penetrating it, and fertilizing it. Instead of being able to discharge these sperm only once a month, he is able to do so every time he has intercourse, and he will provide almost a quarter of a billion of them with every ejaculation. These sperm are much tinier than the eggs, and theirs is indeed a difficult task, for they must find the egg (if there happens to be one available at the time) and fertilize it at the far end of the tube, about eight inches away from the cervix. But sperm are capable of locomotion, and some of them reach their destination in as little as one hour.


In contrast to the sperm cells, eggs are incapable of locomotion and must depend for their transportation through the tubes and into the uterus upon the action of the tubes themselves. This is accomplished by the tubes through two ingenious devices. First, their interior lining is equipped with millions of infinitesimal hair-like projections (cilia) which are constantly swaying in unison toward the uterus, just like a field of wheat blown by the wind. And second, the muscular walls of the tubes are capable of peristalsis, similar to the peristalsis of the bowels, and this intermittent contracting and relaxing also helps to propel the egg towards its ultimate destination. If the tubes have ever been diseased, this mechanism may be interfered with to such an extent that the egg may become lodged in the tube itself and an ectopic (Greek for misplaced) pregnancy will result.

If all goes well, as usually happens, the fertilized egg will reach the uterine cavity in about thirty-six hours, and there the endometrium will have been properly primed and thickened by the ovarian hormones so that it will be able to provide the proper succulent bed to nourish this fragile little structure and enable it to grow from an almost weightless, formless object to a seven-pound infant which will be destined some day to share in this same miraculous process by which we perpetuate our race.


For about 270 days this egg is incubated by the human female. This may seem long in comparison with the hamster, whose incubation period is sixteen days, or it may seem unfair in comparison to the lady sea horse, whose mate carries the eggs; but, on the brighter side, it is a great deal shorter than the pregnancy of an elephant - 624 days.

The initial increase in the size of an egg is accomplished by simple cell division in the same manner by which an amoeba reproduces itself. This process takes place at a fairly rapid pace, so that within a day after the ovum has become imbedded in the endometrium it already consists of more than one hundred cells. It is at approximately this stage in development that identical twins are formed, by the splitting of the ovum into two identical halves. Non-identical or fraternal twins are formed by the simultaneous fertilization of two different eggs.

During these early days in its development the ovum depends for its nourishment upon the absorption of foodstuffs from the endometrium. After a few weeks a distinct organ is formed, the sole purpose of which is absorption of nutriments from and excretion of waste products into the maternal blood stream. Known as the placenta, or afterbirth, this structure is intimately adherent to the uterus on one side and connected to the fetus on the other by the umbilical cord. This cord contains blood vessels which communicate between the fetus and its placenta. These vessels do not communicate with the maternal blood stream at all, so that the blood of the mother and fetus never mix. Substances are passed back and forth in the placenta between the blood streams of mother and fetus by a simple process of diffusion.

Fanning out in all directions from the margin of the placenta and adherent to the inner surface of the uterine wall are the fetal membranes. These membranes comprise a thin, almost transparent sac, much like a balloon, which contains the fetus and the amniotic fluid. This fluid is clear, like water, and by the ninth month there is more than a pint of it. This fluid provides an ideal medium in which the fetus can grow and move about, and it helps to protect the fetus from injury. If you picture the fetus lying in a fluid-filled sac and surrounded by the inch-thick muscular wall of the uterus, which, in turn, is suspended from the bony pelvis by a web of strong but resilient ligaments, you will realize how magnificently it is protected from exterior harm.

The development of the fetus is a complicated process. Suffice it to say that all of its major systems (e.g., the cardiac, respiratory, intestinal, and urinary systems) are more or less formed and functioning by the end of the third month of gestation, by which time the fetus is just beginning to resemble !l human being. In many respects these first three months are the most crucial in its formation and development. Since most of the organs are formed at this time, the future normality of the baby is decided right here and now. And if anything does go awry at this early stage, Nature has a ready solution: miscarriage: Most of the fetuses which are destined to be deformed are miscarried. Largely for this reason, nine out of every ten miscarriages occur in the first three months of pregnancy. It is for this reason, too, that a fetus may be damaged if the mother contracts German measles during the first few months.


Now that its basic anatomical form has been established, the fetus has six more months in which to grow and mature before it can qualify for infancy. During this time it lies curled up in the most compact possible ball, completely surrounded by darkness, warmth, and water. It is hard for us to envision what such a totally aquatic life must be like, even though we all experienced it. We know of course that a fetus moves its arms and legs at irregular, not infrequent intervals. Occasionally it turns completely around, so that one day it may sit right side up and the next day it may stand on its head. It swallows and urinates, but it does not defecate. It wakes, it sleeps, it may have fits of hiccuping, and it may even suck its thumb. After rupture of the membranes, fetuses have actually been heard to cry out from the uterus. They must open their eyes occasionally, even though it's too dark to see, and there is no reason to believe they can't hear a loud noise from the outside world. They don't read their mothers' minds, however, so don't believe the many young wives' tales about a mother’s emotions and experiences affecting the unborn baby's personality.


The longer the fetus is incubated in the uterus, the bigger it becomes and the better are its chances for survival. At the end of the fifth month the fetus weighs about one pound; by the end of the six month, about two and a quarter pounds; the seventh, four pounds; the eighth, five and a half pounds; and the ninth, seven pounds. You can see that the rate of growth is much faster at the beginning of pregnancy than toward the end. The weight of the ovum is increased about 10,000 times in the first month and only 0.3 times in the ninth. But if even this latter, comparatively slow rate were continued after birth, the baby would weigh 160 pounds instead of twenty pounds on its first birthday.

Every phase of pregnancy - from ovulation to fertilization to the development of the fetus - is controlled by Nature. There is very little that you can do to improve upon this process and, contrariwise, there is little you can do to damage it. The role of the obstetrician is to promote the naturalness of a normal pregnancy, to prevent aberrations from the norm when possible, and to interfere only in the event of complications.

Health | Reproduction | Pregnancy

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