Eli5: Why is pregnancy counted from the first day of your last period?
In: Biology
Because every woman has a different period cycle so there’s no way for know exactly when she ovulated, let alone when the sperm intercepted the egg. The day of the last menstrual period (LMP) is the only thing people can definitively say. Later as the pregnancy progresses, things such as hormone levels or dating ultrasounds (different organs develop in different weeks) can be used to guess-timate hold old the fetus is. This can be helpful if the woman forgot the date of the LMP or has irregular or unusually short/long menstrual cycles
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