Thanks for your question. It can be very confusing to figure out how your due date is calculated. It can be especially confusing when it is changed after having an ultrasound.
Based on your 18 week ultrasound, your estimated due date (EDD) would be July 1, 2012. This may be a different date than the one that was calculated from your last menstrual period (LMP). An ultrasound can alter a due date by a week or so.
Generally speaking, if your due date is changed based on an ultrasound during the first trimester (12 weeks or before), that new date is considered to be more accurate than the one based on your LMP. However, if your EDD is different based on a second or third trimester ultrasound, the original date from your LMP is considered to be most accurate.
Remember though that your EDD is only as estimate and your baby is considered to be “full-term” anytime after 37 weeks. Statistically, most babies are born between 39 and 41 weeks gestation.
I hope this didn’t add further to your confusion!
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