Why do we calculate the length of a pregnancy from the first day of a women’s last period?
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Becuase that’s the start of her last cycle and it’s a known date. It’s important to note that they still base your due date as if you ovulated on day 14, which many women don’t. I always did around day 18. That’s also why when you have an early ultrasound they then change the due date based on how it’s measuring. Probably ovulated at a different time and then conceived at a slightly different time
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