Understanding Your Baby's Gestational Age: Ultrasound vs. Period Dates

Question
hi, I just got my first ultrasound today 1/30/08. the doctor told me the baby looks 7 weeks 6 days old. the first day of my last period is nov 20th or 23rd I think. I thought I should be 10 weeks pregnant. Is it possible to determine how many weeks my baby is not according to my menstrual period. should I be worried that my baby is not developing properly?

Answer
In fact, it is more accurate at this stage to determine the age of your baby using ultrasound than it is to calculate using your last menstrual period.  The method of calculating from LMP comes from a time before women and doctors knew about and were able to determine when ovulation occurred, and it assumes ovulation at day 14.  Since most women actually do not ovulate right on day 14, and some women can even be weeks off from this, due dates based solely on LMP are more likely to be wrong than right.

Something else to consider is that since calculations based on LMP assume ovulation at 2 weeks, your baby is actually 2 weeks old by this method on the very day of conception!  If your doctor was actually telling you your baby was 7 weeks 6 days old SINCE CONCEPTION, and you ovulated on day 14, then this would line up with your LMP perfectly.