Stages of Prenatal
Development

1. Ovum - First 2 wks when zygote attaches to wall of uterus. Tendrils from the zygote penetrate the blood vessels in the uterine wall, and thus the zygote begins a physiologically dependent relationship with the mother.

2. Embryo. - 2-8 wks From the time of the zygote's attachment to the uterine wall until the first occurrence of ossification, the formation of solid bone, in the embryo. Lots of growth. Extremely susceptible to influence. Differentiation of everything --all organ systems. In three layers:

l) Ectoderm . Nerve system and Sensory cells - hair, nails, part of the teeth, the outer layer of the skin and skin glands, as well as the nervous system and sensory cells develop.

2) Mesoderm . Skeletal. Muscles, skeleton, excretory and circulatory systems, and inner skin layers develop.

3) Endoderm . Gastrointestinal organs - the gastrointestinal tract, eustachian tubes, glands, and other organs such as the lungs, pancreas, and liver develop.

By the end of the 4th week after conception, a small vessel, which is destined to become the heart, begins to pulse even though at this point the embryo is no larger than a human thumbnail. The Heart is beating at 200 heartbeats a minute at times (We at 80/min.).

By the end of this period, at eight weeks after conception, the embryo is approximately one inch long and begins to resemble a human being. It is somewhat out of proportion, because its head is about as large as the rest of its body. Its face and features are recognizable, and its fingers, toes, and external genitalia are present. The embryonic period is very hazardous. Not all embryos are properly attached to the wall of the uterus, thus most miscarriages occur at this time, as the embryo can become detached from the wall of the uterus and be expelled. It is estimated that close to 50% of all embryos are aborted during this period. During this embryo period, differentiation of important organs occurs. The inner mass of the zygote differentiates into three layers.

3. Fetus - 3 mos to birth. Increasing differentiation. During this period muscular development occurs at a rapid rate and various body parts become more differentiated. The central nervous system also develops very rapidly. However, the development of the central nervous system is not complete at birth but continues for six months thereafter (Tanner, 1978). There is increased vasculation (blood flow) to brain up to this time with temporary veins--lots of bleeding and the growing of brain cells. [Later, the brain cells die off. Less is more. Less synapsing. Sheathing. Efficiency.]

By the end of the 16 weeks after conception, the mother usually can feel the movement of the fetus. Is about 4 and 1/2 inches long at this point, its lips are well formed and can be moved, and its mouth can open and close.

By the end of 24 weeks, the eyes are fully developed and can open and close, and the fetus develops nails, sweat glands, a coarser skin, and hair (much of which is shed in utero although at times hair shedding continues after birth).

After that time, there is gradual improvement and organization of the nervous and sensory systems.

At the end of 26 to 28 weeks - a time known as the age of viability, fetal development is sufficiently advanced that if birth occurs the child will survive. If birth occurs before that time, the nervous and respiratory systems are usually not mature enough so the baby's survival outside the uterine environment is less likely.... Extremely premature infants may survive but they will have poorly developed motor skills and difficulties in feeding and sleeping for at least several months after birth.

Last 2-3 weeks = Fattening / toning up.