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The uterine lining builds-up in preparation for fertilization so that the zygote/embryo can implant itself into the uterine lining and then develop into a baby. Menstruation only takes place when fertilization doesn't take place, this occurs so the uterine lining is shed and can start a fresh next cycle so it can be ready for pregnancy again. If the uterine lining were to break down after fertilization a zygote/embryo wouldn't be able to implant itself or it would result in miscarriage.

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10y ago

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