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The cells of the zygote divide repeatedly as the zygote moves down the fallopian tube. In the uterus, the cells continue to divide, becoming a hollow ball of cells called a blastocyst.

If fertilization does not occur, the egg moves down the fallopian tube to the uterus, where it degenerates, and passes through the uterus with the next menstrual period.

Blood vessels continue to develop in the embryo and placenta.

Almost all organs are completely formed by about 10 weeks after fertilization (which equals 12 weeks of pregnancy).

The placenta forms tiny hairlike projections (villi) that extend into the wall of the uterus.This stage is characterized by the formation of most internal organs and external body structures.Most organs begin to form about 3 weeks after fertilization, which equals 5 weeks of pregnancy (because doctors date pregnancy from the first day of the woman's last menstrual period, which is typically 2 weeks before fertilization).If more than one egg is released and fertilized, the pregnancy involves more than one fetus, usually two (twins).Because the genetic material in each egg and in each sperm is slightly different, each fertilized egg is different. Identical twins result when one fertilized egg separates into two embryos after it has begun to divide.During this period, the embryo is most vulnerable to the effects of drugs, radiation, and viruses.

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