Conception occurs the moment the sperm fertilizes the ovum, which forms the zygote. At conception the sperm adds its 23 single chromosomes to the 23 of the egg. Conception ususally takes place in one of the fallopian tubes, and within the next 2 weeks the zygote travels to the uterus and attaches itself to the uterine wall. During this 2 week period rapid cell division occurs. At the end of this first state of prenatel development, the zygote is only about the size of the period at the end of this sentence.

A zygote the full complement of 46 chromosomes, which contain about 30,000 genes. These genes carry all of the genetic information needed to make a human being.

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