Stages of Formation of a Human Baby

Stages of Development

From the moment when a sperm cell penetrates and fertilizes an egg cell, the resulting fertilized egg goes through several stages of growth. Initially no more than a self-replicating packet of genetic material, the egg changes and grows through the zygotic, blastocystic, embryonic and fetal stages before birth.
Initially a woman produces an egg cell, which is typically fertilized two weeks after it is formed. Consequently, the age of a developing human in utero can be counted from the day the egg was released (gestational age) or from the moment of fertilization. The gestational age will typically be two weeks greater than the time since fertilization. In this article weeks will be counted from the gestation date--the time the egg is released--unless otherwise noted.

Zygotic

At fertilization, a single sperm penetrates the woman's egg cell membrane. The resulting cell is called a zygote. It contains 50 percent of its DNA from the mother's egg and 50 percent from the father's sperm. The zygote then moves down the fallopian tube. As it travels it divides repeatedly to form a ball of cells. Eventually, the cells have divided enough times to form an inner group protected by a shell. At this point the cell mass is called a blastocyst. The inner group of cells will continue to develop into an embryo.

Embryonic

The blastocyst will reach the uterus five days after fertilization, or nearly three weeks after gestation. When in the uterus, it attaches itself to the uterine wall. The surrounding layer of cells will develop and specialize into the systems which connect the developing human to its mother. When attached to the uterine wall, the blastocyst becomes known as an embryo.

The cells which compose the embryo begin to take on specific roles and functions, moving closer to the organs they will grow into. In the first week of embryonic development the brain, heart, and spine can be distinguished. A week later the heart begins to beat, arm and leg buds grow, and the brain begins to divide into recognizable areas. Six weeks after gestation webbed fingers and toes develop. The lungs and facial features (including ear shape and eyelids) develop nine weeks after gestation--just six weeks after the development of the embryo. By that time, here is also detectably increased sophistication in brain structure and organ differentiation.

At some point within the next two weeks the embryo has grown sufficiently and become organized to the point that the eyelids close, the digestive tract rotates within the abdomen, and genitals are visibly distinguishable. The embryo then enters the fetal stage.

Fetal

The fetus is defined as the unborn human ("fetus" may refer to any vertebrate which carries its developing young within its body, but for our purposes we will be discussing humans exclusively) during the fetal stage of development. Before the fetal stage, the fetus is referred to as an embryo. This stage begins nine weeks after fertilization and 11 weeks after the initial release of the egg (gestation). The fetal stage continues until birth. During this time the fetus develops organs and grows to 425 times its starting size.

The fetal stage is reached during the 11th week of gestation. The fetus begins at about 1.2 inches long and weighs about 8 grams (.28 ounces). Many of the organs are already present, including the heart, brain, hands, feet and lungs, but they exist in rudimentary forms and do not function as they do when fully developed. Movement, including the appearance of twitches and breathing, can occur as muscles and nerves develop, but volitional movement and pain sensitivity does not occur until the third trimester.

16 to 25 weeks

During this time, ending around five months after fertilization, the fetus grows to eight inches in length. Organ development continues. The mother will be able to detect fetal movement at around 21 weeks post-fertilization.

26 to 35

The fetus grows rapidly, gaining full-length fingernails, hair, and increasing body fat. By 35 weeks the bones of the fetus are fully developed but still soft. At 35 weeks the fetus is developed to the point that it could exist outside the uterus and is called "full-term." Most pregnancies last closer to 40 weeks, however.

35 to 40 weeks
The fetus will have grown to 19 to 21 inches in length before birth. Though the thalamic neural connections which regulate sensory input and voluntary movement are by now relatively well-developed, purposeful movement and coordination are still crude and will continue to develop through puberty.

References

• Introductory Maternity Nursing
• Preterm Birth: Causes, Consequences, and Prevention
• Journal of the American Medical Association: Fetal Pain, A Systematic Multidisciplinary Review of the Evidence Susan J. Lee, JD; Henry J. Peter Ralston, MD; Eleanor A. Drey, MD, EdM; John Colin Partridge, MD, MPH; Mark A. Rosen, MD.