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Prenatal development is divided into three periods: The germinal period, the embryonic period, and the fetal period. The following is an overview of some of the changes that take place during each period.
3D computer animation depicting prenatal development from fertilization through the fetal stage.
The Germinal Period
The germinal period (about 14 days in length) lasts from conception to implantation of the fertilized egg in the lining of the uterus. At ejaculation millions of sperm are released into the vagina, but only a few reach the egg and typically only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. It is typically fertilized in the top section of the fallopian tube and continues its journey to the uterus. As a result, a new cell is formed. This cell, containing the combined genetic information from both parents, is referred to as a zygote.
During this time, the organism begins cell division through mitosis. After five days of mitosis there are 100 cells, which is now called a blastocyst. The blastocyst consists of both an inner and outer group of cells. The inner group of cells, or embryonic disk will become the embryo, while the outer group of cells, or trophoblast, becomes the support system which nourishes the developing organism. This stage ends when the blastocyst fully implants into the uterine wall (U.S. National Library of Medicine, 2015).
Mitosis is a fragile process and fewer than one half of all zygotes survive beyond the first two weeks (Hall, 2004). Some of the reasons for this include: the egg and sperm do not join properly, thus their genetic material does not combine, there is too little or damaged genetic material, the zygote does not replicate, or the blastocyst does not implant into the uterine wall. The failure rate is higher for in vitro conceptions. The figure below illustrates the journey of the ova from its release to its fertilization, cell duplication, and implantation into the uterine lining.
The Embryonic Period
Starting the third week, the blastocyst has implanted in the uterine wall. Upon implantation this multi-cellular organism is called an embryo. Now blood vessels grow forming the placenta. The placenta is a structure connected to the uterus that provides nourishment and oxygen from the mother to the developing embryo via the umbilical cord.
During this period, cells continue to differentiate. Growth during prenatal development occurs in two major directions: from head to tail called cephalocaudal development and from the midline outward referred to as proximodistal development. This means that those structures nearest the head develop before those nearest the feet and those structures nearest the torso develop before those away