How accurate are ultrasounds in taking measurements and estimating gestational age?

Answer 1

In the first trimester, the gestational age is determined by measuring the CRL (crown-rump length). This is the most accurate time to get the fetal age, because, this early, nearly all babies grow at the same rate. We all start out at the same size and we grow at a predictable rate. Later in pregnancy, the measurements are less accurate, as babies will grow at different rates in the 2nd and 3rd trimester.

"The CRL is highly reproducible and is the single most accurate measure of gestational age."

http://www.obgyn.ufl.edu/obgyn101/Ultrasound/1st_trimester_ultrasound_scannin.htm

"Early in pregnancy (1st trimester), fetuses of the same gestational age all are nearly identical in size. At this time in pregnancy, there is very little difference in size between those fetuses who ultrimately will be large, average size, or small. As pregnancy continues, these genetic and constitutional differences in size become more important clinically. It is not possible, for example to separate a 20 week fetus who is large from a 21 week fetus of normal size, or a 22 week fetus who is small for its age. All three will measure as though they were 21 weeks....The accuracy of ultrasound in predicting gestational age gets worse as the pregnancy advances. By 20 weeks, ultrasound is accurate only to within plus or minus two weeks, and by the third trimester, its accuracy falls to plus or minus 3 weeks."

http://www.obgyn.ufl.edu/obgyn101/Ultrasound/2nd%20and%203rd%20Trimester%20Ultrasound%20Scanning.htm

Answer 2

Gestational Age Ultrasound

Answer 3

Depends on when the ultrasound is taken. The closer it is to conception - the closer it is to getting an accurate reading for age. This is because after the initial formation of the fetus, all babies grow at different rates. If it's just a big baby they might assume that it's older...or if it's small younger. And they usually tell you that they can be off by two weeks (forward or back). But in the first 2 months they can generally gauge what stage it is in the formation process since all babies form basically at the same rate.

As for the accuracy of the measurements...it's not a science and it's really just an estimate. Things like shadow, position and quality of equipment come into play...and a lot of times they misjudge it. I've known women who have been told that they would deliver 10lb babies only to go through a c section and find a healthy 7lb...and vice versa. There is no way they can tell you how big the baby is until it slides out and they use a scale.

Answer 4

they can get pretty close but are never 100% accurate. Usually when they are measuring, especially in early pregnancy, they will probably only be off by a millimeter or two which might translate to 2 or 3 days off in gestational age.

I had an ultrasound at what I thought was 7w4d and ultrasound said only 7weeks. Then I had another yesterday at what I thought was exactly 12weeks and ultrasound said 12w3d. So they are never spot on but get pretty darn close.

The later the ultrasound in pregnancy the greater risk for discrepancies but you can still trust them to be pretty accurate within a week or so of the estimated due date.

best of luck

Answer 5

They're pretty spot on as far as measurements go but as for the gestational age, that can be off depending on whether you have large or small babies.
My 3rd child was deemed to be X weeks along based on size meaning I got pregnant smack bang in the middle of my hubby being away for 4 months. They failed to take into consideration that his 2 siblings were really big babies. Given that he had to be born by c section 2 weeks early - I ended up with a baby in a humidicrib unable to breathe unassisted.

Answer 6

estimation of age is usually correct, but even the ultrasound tech will tell you that the weight can be off by a pound or so due to cramped quarters. my son was estimated to be 6lbs 13oz a week before delivery and he was 8lbs 6oz at birth.

Answer 7

Ultra sounds are pretty much right on the dot now a days. I would also recommend directing your concerns to your doc. But ultra sounds do not lie!

Answer 8

Ultra sounds are more accurate the further along you are. Very often due dates are adjusted later in pregnancies beacuse of this.

Answer 9

the sonographers have gone to uni to specialise in that area so they definatly know what they are talking about, they are very accurate and they tend to go by what the u/s says for your due date over your lmp

Answer 10

It was right about me