The Fetus

The origin and growth of the fetus was a simple thing to our medical forefathers. In 1548 all embryology could be put on a single page; today it cannot be crammed into a library of hundreds of volumes. I venture the guess that more pages have been written about the obscure fetus than about the illustrious Shakespeare.


The life of the fetus begins at the moment of fertilization. At that moment the precursor of the child is of almost microscopic size, a speck of tissue so very tiny that it is just barely visible to the naked eye of the expert—a mass so light that its weight cannot be expressed in even thousandths of an ounce. Within nine months this minute dot of tissue develops into a twenty-inch, seven-and-a-half-pound screaming infant. The ovum implants itself into the substance of the uterus, excavating the permanent home which it will occupy for more than eight months by digesting its way into the interior lining of the uterus. In the process it taps very, very small maternal blood vessels and soon finds itself surrounded by a veritable lake of its mother’s blood into which it dips vigorous, hungry cells. These cells grow out like streamers from the surface of the blastocyst—the term for an ovum of this age—and absorb minerals, vitamins, carbohydrates, proteins, and fats essential to growth. With absorption of nourishment the ovum increases rapidly in size. At a certain region on the inside of the covering a thickened mass of cells now appears; this mass is called the inner cell mass or embryonic area, and it is from these cells that the embryo itself develops. The cover of the ball-like egg continues to grow, and from the surface which serves as the base for this ball of life the placenta (from the Latin for ‘cake,’ which is descriptive of its shape) or afterbirth is formed.


We can now summarize the development of the fetus, designating weeks and months since the onset of the last menses. Let us assume that fertilization takes places on the thirteenth day of the cycle; however, it ordinarily occurs at varying times between the tenth and seventeenth days.

End of second week: Fertilization (13th day). End of Third week: Ovum traveling down tube and floating in uterus (13th-22nd day).

Beginning of fourth week: Implantation (22nd day). Egg still barely visible to naked eye.

Beginning of fifth week: Embryo a minute piece of uniform, gray-white flesh. End of fifth week: Backbone forming, 5 to 8 vertebrae laid down. Spinal canal forming. Embryo 1/12 inch long and about 1/6 inch wide.

Beginning of sixth week: Head forming, heart visible. End of sixth week: All of backbone laid down and spinal canal closed over. Tail end of embryo distinct. Beginnings of arms and legs visible. Depressions beneath skin where eyes are to appear. Length, !4 inch.

Seventh week: Chest and abdomen completely formed. Beginnings of fingers and toes. Clearly perceptible eyes. Length, Vz inch.

Eighth week: Face and features forming. Ears forming. Length, ½ inch. Weight, 1/30 ounce (1 gram). Ninth week (end of 2nd month): Face completely formed. Arms, legs, hands, and feet partially formed. Stubby toes and fingers. Length, 1 1/5 inch. Weight 1/15 ounce (2 grams).’

From this time on, looks very much like a miniature infant. End of third month (13 ½ weeks): Arms, legs, hands, feet, fingers, and toes fully formed. Nails appear. Ears completely formed. External genital organs begin to show differences, and about 11th week trained observer can determine sex in a fetus of this age. Eyelids closed. If fetus removed (at operation) and placed in warm fluid, movements of arms and legs. Head extremely large in proportion to body. Length, 3 inches. Weight, 1 ounce (30 grams).

End of fourth month (18 weeks): Casual observer could now distinguish sex in infants delivered at this phase of development. At end of month sometimes fetal movements felt and heart heard. Fine, downlike hair all over skin, which is less transparent and pinker. Eyebrows and eyelashes appear. Length, 8 ¼ inches. Weight, 6 ounces (180 grams). End of fifth month (22 ½ weeks): Hair appears on head. Fat being deposited under skin, although fetus very lean. If born, may live a few minutes, though never survives. Length, 12 inches. Weight, 1 pound (450 grams).

End of sixth month (27 weeks): Fetus covered with cheeselike secretion, vernix caseosa. Skin wrinkled. Hair on head fairly well developed. Eyes open. If born, most live for several hours or even days. When given expert premature infant care, approximately one in ten born in the 27th week survives. Length, 14 inches. Weight, 2 pounds (920 grams). End of seventh month (31 ½ weeks): In male fetus, testicles usually descend into scrotum. Child born alive during this month has slightly better than a 50 per cent chance for survival. The age-old superstition that a baby born in the seventh will do better than one born in the eighth is entirely fallacious. Each day nearer term makes the child’s chances for survival that much better. Length, 16 inches. Weight, 3 pounds, 12 ounces (1700 grams),

End of eighth month (353A weeks): Child has better than 90 per cent likelihood for survival. Length, 18 inches. Weight, 514 pounds (2735 grams).

End of ninth month (40 weeks): Full term. Skin smooth, polished-looking, and still covered by cheeselike secretion. No downlike hair except over shoulders and arms. Head hair 1 inch long. Nails protrude beyond ends of fingers and toes. Eyes usually a slate color; impossible to predict their final tone. Average circumference of head equals circumference of shoulders (13 inches). Length, 20 inches. Weight, 7 pounds, 6 ounces (3350 grams). When born alive, 99 out of 100 survive.