Why do we calculate the length of a pregnancy from the first day of a women’s last period?
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We really don’t. It is a starting rough estimate. On your first ultrasound they measure multiple parts of the fetus and give a more accurate estimate of your term. after the second visit the current length of pregnancy and due date is basically solidified unless something changes throughout the pregnancy. currently going through this process with my wife. So far developmentally it is spot on accurate.
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