If the difference between the two is less than a week apart, she’ll generally go with the date calculated from your last period.
She may notice that the size of your uterus doesn’t synch up with the standard growth charts (you measure big).
If she estimates your measurements are off by three weeks or more, in many cases she may move the due date to reflect this.
Between week 14 and week 22 of pregnancy, your doc likely ordered a blood test to screen for AFP — short for Alpha fetoprotein, a protein that’s normally produced by the liver and yolk sac of your still-developing bambino; levels vary widely throughout your pregnancy.
(This is often part of the triple screen.) While high levels of AFP sometimes indicate a potential genetic abnormality, the number one reason is simply your due date was miscalculated.
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After all, unless you went the IVF-route, your physician has no way of knowing the exact time of conception.
Because of this, pregnancy is calculated from the first day of your last menstrual period. It can be pretty difficult to determine a due date using the date-of-last-period method for women with irregular cycles — which is why an ultrasound exam is often required to determine gestational age.