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Week 1 begins with fertilization of the ovum by sperm that have been deposited in the female reproductive tract around the time of ovulation. Only one sperm cell can fertilize an ovum.

After fertilization, the new individual is called a
zygote. it begins to divide, a process called cleavage and as it does so, it begins a week long journey down the oviduct to the uterus.

By the end of week 1, the zygote has become a hollow mass of cells called the
blastocyst and it is ready to invade the wall of the uterus to implant itself for development into the embryo and then the fetus.