The life of a human being begins at fertilization (or conception), when a sperm cell fuses with an oocyte (egg) to produce a new human organism. This individual is called a zygote at the one-cell stage of development, an embryo through the first eight weeks, and a fetus from eight weeks until birth. Each of us was once a zygote, embryo, and fetus, just as we were once infants, toddlers, and adolescents. All of these terms refer to stages in the life of a member of the species Homo sapiens.
Here are some of the milestones of human prenatal development. These dates are measured from the time of fertilization rather than from the last menstrual period (the dating method typically used during pregnancy), which occurs about two weeks earlier.
- Conception: A new member of the human species begins. The zygote has a complete and unique set of 46 chromosomes (23 from each parent), the entire genetic blueprint. He or she needs only a suitable environment and nutrition in order to develop himself or herself through the different stages of human life.
- 6 days: The developing embryo, called a blastocyst at this stage, begins attaching to the wall of the mother’s uterus.
- 17 days: Blood cells have developed.
- 19 days: The eyes start to develop.
- 20 days: The foundation of the nervous system has been laid.
- 18-21 days: The heart begins to beat. […]
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