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The general rule about organ development is that all of your organs are formed in utero and that's the set you live with, but you can think of the placenta as a kind of an organ transplant from your fetus to you that naturally forms inside you.
After conception, the fertilized egg (now called the blastocyst) needs to find a way to use mom's nutrients. It meanders to the end of your Fallopian tube and enters your uterus searching for the uterine wall. This trip is the most dangerous trip any human being ever takes, because if the blastocyst does not implant successfully within 7 days of conception, the uterine lining will shed and you'll never even know you were pregnant. If it does, the placenta will begin to form at the point of attachment. Generally, the placenta starts to develop about a week after conception.
After conception, the fertilized egg meanders to the end of your fallopian tube and enters your uterus, searching for the uterine wall. If it successfully implants itself there, the placenta will begin to form at that very spot about a week later. By the end of the first trimester, the placenta is fully grown and weighs about 1 1/4 pounds.
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