Fetal development or conception begins in the fallopian tube, when a male sperm penetrates a female egg and begins the fertilization process. Joining of the egg and sperm produces a single cell called a zygote. Over the course of a few days the zygote will multiply the number of cells through division. This conglomerate of cells is called a morula. Additional cell division takes place and the morula becomes an outer shell with an attached group of cells. At this point in development the morula is called a blastocyte. The blastocyte will journey down the fallopian tube, between the 7th and 9th day after conception, and implant itself in the lining of the uterus (endometrium).
Around the 15th day after conception the embryonic phase begins. It will last until about the 8th week. During embryonic phase the cells of the embryo divide and take on specific functions. This process is called tissue differentiation. It is required for the different kinds of cell types that make up a human being (e.g., blood cells, kidney cells, nerve cells).
Weeks 3-8 of the embryonic phase. Rapid development of the embryo occurs from the 3rd week through the 8th week. In the 3rd week the embryo begins heart formation, brain and spinal cord formation, and the beginnings of the gastrointestinal tract.
Greater development follows during the 4th and 5th weeks, though the embryo is only about ⅛ inch long. In this period there will be formation of tissue that develops into the vertebra, the lower jaw, hyoid bone, and the cartilage of the larynx. There will also be formation of structures of the ears and eyes. Arms, legs, feet, and hands will show rudimentary development, along with rudimentary blood through the main vessels. The heart will develop further and present a regular rhythm. The brain develops into five areas and some cranial nerves will be visible.
The 6th week will bring formation of the nose, upper lip, trachea, two lung buds, and palate; and further development of the brain, upper and lower jaws, ears, arms, legs, hands, feet and blood circulation. The heart is nearly fully developed and the tail is receding.
During the 7th week all essential organs will have begun to form. The head is more rounded and eyes move forward on the face, as eyelids begin to form. The palate is nearing completion, the tongue begins to form, and the gastrointestinal tract separates.
By the 8th week the embryo is developed enough to be called a fetus. It now resembles a human being, though it is only a little over an inch long in length. All organs and structures are formed to some degree. Facial features continue to develop, the eyelids begin to fuse, the ears begin to take their final shape (though still set low on the head), external genitalia form, long bones begin to form, the anal passage opens, circulation through the umbilical cord is well developed, and muscles are able to contract.
Weeks 9-12 of the fetal phase. The fetus reaches a length of about 3.2 inches during this period. The head comprises almost half of the fetus’ size, but the neck is present. The face is well formed, the ears are more pronounced, the eyelids close, tooth buds form, digits are well formed, the urogenital tract completes development, genitals appear well differentiated, red blood cells are produced in the liver, and fetal heart tones may be heard with electronic devices.
Weeks 13-16 of the fetal phase. The fetus reaches a length of about 6 inches during this period. A fine hair, called lanugo, develops on the head. The skin of the fetus is nearly transparent. Bones become harder as more muscle tissue and bones develop. The fetus is able to swallow amniotic fluid, as well as make active movements. Meconium is filtered in the intestinal tract. Lungs are further developed, sweat glands develop, and the liver and pancreas produce fluid secretions.
Weeks 17-20 of the fetal phase. During this period the fetus reaches a length of 8 inches. Lanugo hair covers the entire body, while the skin becomes less transparent as fat begins to deposit. Nipples begin to appear, along with eyebrows and lashes. Nails appear on the fingers and toes. The heartbeat of the fetus can be heard with a stethoscope. The fetus is more active as muscle development increases. Quickening is evident (the mother can feel the fetus moving).
Weeks 21-24 of the fetal phase. The fetus reaches a length of 11.2 inches during this period and weighs about 1 lb. 10 oz. The entire body of the fetus is covered in vernix caseosa (a protective substance secreted by the fetus). The hair on the head is longer and eyebrows and eyelashes are well formed. Thicker skin develops on the hands and feet, and footprints and fingerprints form.
Weeks 25-28 of the fetal phase. During this time the fetus reaches a length of 15 inches, and weighs about 2 lbs. 11 oz. Rapid brain development occurs, as the nervous system develops enough to control some body functions. Eyelids open and close, while testicles begin descent into the scrotum if the fetus is male.
Weeks 29-32 of the fetal phase. The fetus will reach a length of about 15-17 inches during this phase and weigh about 4 lbs. 6 oz. There is a rapid increase in the amount of body fat, increased control over body functions, full development of bones (though soft and pliable), and body temperature is partially self-controlled. The fetus also begins storing iron, calcium, and phosphorus. The respiratory system is still immature, but it develops to the point where gas exchange is possible. The fetus may be born alive and survive at this point, though the possibility of death is high.
Weeks 33-36 of the fetal phase. During this time the fetus reaches a length of about 16-19 inches and weighs about 5 lbs. 12 oz. to 6 lbs. 12 oz. The lanugo hair begins to disappear. There is an increase in body fat. The fingernails grow to the end of the fingertips. There is increased central nervous system control over body functions. A fetus delivered during this time has a high possibility of surviving, but may need artificial support.
Weeks 37-40 of the fetal phase. Full-term delivery is considered 38 weeks and any delivery over that time is post-term. The fetus will fill the entire uterus now. During this period the fetus may be 17 to 22 inches in length, and typically weigh 7½ lbs. (males tend to weigh slightly more than females). The lanugo hair is gone except on the upper arms and shoulders. Fingernails extend beyond the fingertips and there are small breast buds present on both sexes. Hair on the head is coarse and thicker. See also Amniotic Sac; Female Reproductive System; Male Reproductive System; Placenta; Umbilical Cord
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