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The cells lining the fallopian tube facilitate fertilization.If a sperm penetrates the egg, fertilization results.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).
Because one egg was fertilized by one sperm, the genetic material in the two embryos is the same.
The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta.
The placenta produces several hormones that help maintain the pregnancy.
The heart begins to pump fluid through blood vessels by day 20, and the first red blood cells appear the next day.
Blood vessels continue to develop in the embryo and placenta.
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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.