There's an urban legend that a girl can jump up and down after sex to prevent pregnancy. That old wives tale is centuries old and is completely untrue.
The theory behind this myth is that the girl, who now has sperm inside her because of the sexual act, is "shaking out" all those sperm so they don't get in to the egg. Unfortunately, it is NOT gravity that drives the sperm towards the egg. They don't swim "up" or "down". It is the actual activity of the sperm that actively drives them, with their little tadpole-like tails, inwards towards the egg. If the egg is there waiting, the sperm will find it.
Sperm fluid is very thick and doesn't just 'shake out'. Since humans walk in an upright position, it would be extremely hard for the human race to continue if the sperm just 'fell out' every time a woman stood back up again after sex! The whole system is designed to work even when the couple stands up again. So the important thing is to prevent the sperm from getting IN to the vagina beforehand, with proper protection. Once it's in there, there's no way to just "get it out".
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