An embryo is the early reproductive development state of complex living organisms. Embryos have similar characteristics across many species. A human baby is called an embryo for the first eight weeks it is in its mother's womb. After eight weeks, the developing human is called a fetus. During the time a baby is an embryo, major cell specialization and organ system development occur.

Embryonic development begins with the fertilization of the ovum by a single sperm cell. With the fertilization, the two cells merge to create a new cell called a zygote. The zygote begins to divide until, after three or four days it becomes a morula, named so because of its mulberry shape.http://www.visembryo.com/baby/index.html

The morula enters the uterus from one of the fallopian tubes and after another day or two the group of cells, now called a blastocyst, will implant itself into the uterine lining. The blastocyst produces human chorionic gonadotropin hormone, which starts a hormone chain reaction that prevents the mother's menstruation.http://www.visembryo.com/baby/index.html

After the 12th day, the blastocyst begins to differentiate, first into two separate groups, and then further into groups of cells that will develop into organ systems over the next two weeks. The embryo will develop from a small ball of cells into a tadpole-like organism. After the organ systems form, they begin to develop detail, with the heart and the brain dividing into sections and appendages forming visible fingers with webbing. At eight weeks past conception, the embryo is called a fetus.http://www.webmd.com/hw-popup/interactive-tool-from-embryo-to-baby-in-9-months http://www.visembryo.com/baby/index.html

During embryonic development, critical growth is going on. To prevent damage to embryonic cell growth, pregnant mothers should not drink alcohol, use unneeded drugs or sunbathe, especially during the early weeks of pregnancy.http://www.webmd.com/hw-popup/interactive-tool-from-embryo-to-baby-in-9-months

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