Events which take place from prenatal development from zygote to newborn


Events:
The development of human being can be divided into prenatal (before birth) and postnatal (after birth) periods. During prenatal period the developing individual begins life as a zygote, then becomes a ball of cells called morulla and eventually becomes a blastocyst that implants in the endometrium. From two weeks after fertilization until the end of eighth week of its existence, the individual is called an embryo. From nine weeks until birth it is fetus. During or after birth it is called a newborn.

Pregnancy is divided into three month periods called trimesters.

First Trimester: After fertilization usually in the upper third of the uterine tube, the zygote goes through several cleavages as it moves down the tube. It eventually becomes a solid ball of cells called morulla and by fourth day, it develops into 50 to 120 cell blastula stage called a blastocyst.

The next stage of development occurs when the blastocyst adheres to the uterine wall and implants. During implantation outer cells of blastocyst called trophoblast, invade the endometrium. Implantation is usually completed 11 to 12 days after fertilization; from then on, the female is considered to be pregnant. One of the unique features of mammalian development is that most of the cells of early embryo make no contribution to the embryo’s body, giving rise instead to supportive and protective membranes. Only the inner cell mass gives rise to the embryonic body. Eventually these cells arrange in a flat sheet that undergoes a gastrulation similar to that of reptiles and birds. Once gastrulation is complete the rest of first trimester is devoted to organogenesis and growth. Regulatory events and inductive tissue interactions shape most of the organ systems.

By the middle of the first trimester all of the major body systems have begun to develop.

Second Trimester: In second trimester (fourth month), fetal growth is spectacular. By now, the pregnant mother is aware of fetal movements. The heart beat can be heard with a stethoscope. During the sixth month the upper and lower eye lids separate and the eye lashes form. During the seventh month, the eyes open, during this period the bones begin to ossify.

Third Trimester: Third trimester extends from seventh month until birth, during this time the fetus has developed sufficiently (with respect to circulatory and respiratory systems) to potentially survive it born prematurely. During last month fetal weight doubles.

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