eli5: Why do hormonal IUDs take 7 days to start working whereas Copper IUDs start working instantly? And why is it 7 days? Can’t it be less or more? How is that calculated?
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VThink of copper IUDs like a pesticide. The sperm enter, but the copper kills the sperm before they can even reach the egg. The second the pesticide is present, the sperm will be killed.
Hormonal IUDs work a different way. Some of them prevent an egg from being released from the ovaries. This is super great for preventing pregnancy; however, if the egg has already left the ovary, the hormones aren’t going to do anything.
Some hormonal IUDs thicken the mucus around the cervix, essentially blocking the sperm. This sort of thing can’t happen immediately. It takes time to thicken enough to block sperm.
Some also thin the lining of the uterus, so the egg can’t implant. This prevents a pregnancy from beginning.
All of the biological processes mentioned take time. Depending on where you are in the menstrual cycle, you might be able to get pregnant prior to the effects kicking in. That’s just how the body works.
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