The main reason for the absence of menstruation during lactation is related to suction and blocking of ovarian function.
When the baby suckles, the function of the hypothalamus-pituitary axis is inhibited and therefore no hormones are produced that control ovulation. In this way, the maturation of the egg in the ovary does not take place, nor is progesterone generated. The result of these effects is the absence of menstruation in the woman.
In addition, the levels of the hormone prolactin in lactation are high, which also leads to a maladjustment in the hypothalamus-pituitary axis and causes amenorrhea.
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