Others have covered some facets of this such as puberty/menarche starting earlier now than in the past on average for various reasons, but also worth noting that the first few periods a girl has are usually anovulatory. Meaning no egg is actually released. Ovulation (egg release) requires certain hormones to reach a threshold in order to happen and while those hormones are fluctuating enough to cause bleeding (progesterone withdrawal) when a girl gets her first period, they are not high enough (LH/FSH) to release an egg. This could be the case for just one or two periods in some girls, or periods could be entirely anovulatory for up to a few years in others.
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