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.
After sexual intercourse, sperm move from the vagina through the cervix and uterus to the fallopian tubes, where one sperm fertilizes the egg.
The fertilized egg (zygote) divides repeatedly as it moves down the fallopian tube to the uterus. Then it becomes a hollow ball of cells called a blastocyst.
Other cells develop into an inner layer of membranes (amnion), which form the amniotic sac.
When the sac is formed (by about day 10 to 12), the blastocyst is considered an embryo.
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).
Once a month, an egg is released from an ovary into a fallopian tube.The amniotic sac fills with a clear liquid (amniotic fluid) and expands to envelop the developing embryo, which floats within it.The next stage in development is the embryo, which develops within the amniotic sac, under the lining of the uterus on one side.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).