Bleeding can come because of hormonal changes and because of implantation spotting. Neither is indicative of a problem, though. It's fairly normal.
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In practical terms there aren't any. Bodies aren't clockwork perfect, 20% of the women using "safe periods" instead of contraceptives gets pregnant within one year. While it is uncommon, a girl can...
The menstrual cycle is the production of, then maintenence of, then finally expulsion of a female's egg from her uterus. The fluid and matter consists of blood, & soft tissue from uterus wall,...