Conception occurs when sperm fertilizes the egg. This is the first event in the first trimester of pregnancy. Once fertilized, the egg begins its journey through the fallopian tube to the uterus. About a week later, the fertilized egg has become a group of around 100 cells called a blastocyst. It attaches itself to the uterine wall and begins to form the embryo and placenta. The released hormones signal to the uterus that there is a pregnancy. Instead of shedding its lining, the uterus keeps it, which prevents the monthly menstrual period. This is one of the first external signs of pregnancy.
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