2. When Placenta Develops

The placenta begins to develop about a week after sperm fertilizes an egg, in the fourth week of pregnancy. Even though it develops early, the full function of the placenta does not come into play until the second trimester; a mass of cells, the corpus luteum, provides nutrients and hormones for the first trimester of the pregnancy. The placenta completely takes the place of the corpus luteum by the 12th week of pregnancy.

Placenta Develops Second Trimester Organ Richinpit / Getty Images


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