Why did my o.b. do an ultrasound to date my pregnancy when i know exact date of last period?

im 16 weeks pregnant and when i went in for my first prenatal, about 9 weeks along, she sent me to get an ultrasound even though i knew the first day of my last missed period - and when the ultrasound results came back, i wasn't as far along as i would have been based on my period. i thought all pregnancies were dated from first day of last period if that information was known. So even though i know the date, do i calculate my pregnancy by what she says, or what i know?|||im in the uk and ultra sounds are used to date pregnancies rather than your last period as women can ovulate later.


my dating scan and my period were 3 weeks out i had ovulated later than i thought|||It's just something doctors do. Not all babies grow at the same rate, and just because you know the date of your last menstrual doesn't mean that you know the date that you ovulated. Go by what the doctor says. |||That's standard procedure.





LMP is not the absolute accurate way to date a pregnancy.





Early ultrasounds can give more realiable due-dates.


But, this is only true in the beginning.





What she did was normal.





You should always trust the 1st trimester "dating" ultrasound more than your LMP|||They do this to make sure because you could have ovulated sooner or later in the cycle and this is just to exact the measurements. It can be crucial that they have the right timing when it comes to doing certain prenatal tests during only certain periods during your pregnancy. Use the date the doctor gives you. A Period is how they estimate a due date but isnt always correct, the ultrasound due date can be more accurate.|||Because going by LMP leaves 2-3 weeks of guessing for the due date, an ultrasound is more accurate, my doctor didn't do one when I was pregnant with my daughter and I ended up spending a extra week in the hospital because she was late according to my LMP, just go with what your doctor is telling you to do|||What she says. Ultrasounds in the first trimester are the most accurate for dating. A lot of women find out they are either further or earlier then they thought they were because nobody knows for a fact when they ovulated and the time the concieved. I'd go buy the doctors dates. |||it is the most accurate because when I went for my ultrasound according to my LMP I should of been 8 weeks come to find out I was only 6 wks 4 days.