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Prenatal development, the process encompassing the period from the formation of an embryo, through the development of a fetus, to birth. This process can be divided into three distinct stages: the pre-embryonic stage, the embryonic period, and the fetal period.
Prenatal development - Fetal Growth, Nutrition, Environment: In the seventh and eighth weeks of development, the head becomes more erect, and the previously curved trunk becomes straighter. The heart and liver, which earlier dominated the shape of the ventral body, yield to a more evenly rounded chest-abdomen region.
fetus, the unborn young of any vertebrate animal, particularly of a mammal, after it has attained the basic form and structure typical of its kind. A brief treatment of the fetus follows. For more information on the human fetus, see pregnancy.
zygote, fertilized egg cell that results from the union of a female gamete (egg, or ovum) with a male gamete (sperm). In the embryonic development of humans and other animals, the zygote stage is brief and is followed by cleavage, when the single cell becomes subdivided into smaller cells.
Pregnancy - Abortion, Fetal Development, Complications: Abortion is the termination of a pregnancy before the infant can survive outside the uterus. The age at which a fetus is considered viable has not been completely agreed upon.
human development, the process of growth and change that takes place between birth and maturity. Human growth is far from being a simple and uniform process of becoming taller or larger. As a child gets bigger, there are changes in shape and in tissue composition and distribution.
pregnancy, process and series of changes that take place in a woman’s organs and tissues as a result of a developing fetus. The entire process from fertilization to birth takes an average of 266–270 days, or about nine months. (For pregnancies other than those in humans, see gestation.)
Prenatal development - Growth, Differentiation, Fetal: Growth is an increase in size, or bulk. Cell multiplication is fundamental to an increase in bulk but does not, by itself, result in growth. It merely produces more units to participate in subsequent growing.
In humans the embryo begins to develop about four days after an egg is fertilized. Appearing initially as a tiny mass of cells, it eventually gives rise to the fetus, an obvious human form. In the first days of the embryo’s existence, it journeys along the fallopian tube.
Prenatal development - Implantation, Placentation: Six to 10 days after fertilization, the naked sticky blastocyst comes into contact with the uterine lining and adheres to it. The site of attachment is variable and not predetermined.