so last week i got my 20 week ultrasound but they said the baby was like 12 ounces and looked further along according to their measurements like 21 or 22 weeks so could i be further along by a week or two or were the earlier ultrasounds more accurate

basically which ultrasounds are more accurate 8-12 week ones or the 20 week ones? in determing age of fetus?

Hmm that's a good question. I looked it up on a few sites and they say that the earlier ultra sounds are more accurate:

Ultrasound: Ultrasound can be used to examine the baby’s development, and determine his/her age based on which systems are fully developed. If ultrasound is performed in the first trimester, it can indicate fetal age within a range of 3 – 5 days. Later in pregnancy, it is less accurate. Up to 20 weeks, the margin of error is 7 – 10 days. By the third trimester, an ultrasound date can be off by as much as 3 weeks in either direction. Therefore, a baby that appears “term” (40 weeks) may be anywhere from 37 – 43 weeks.
http://www.transitiontoparenthood.co...cy/duedate.htm


I would have thought that the more detailed 20 week scan would be more accurate but I guess not.

Sounds about right to me, because they are able to take a full measurement of the baby from crown to rump from early on. When the baby becomes bigger, they measure different body parts ie head width etc and not the whole body like they do in 1st trimester, and since every baby seems to grow at thier own rates, might be why. We do diagnostic testing at work, if I can remeber to check with the ultrasound tech I will.

When i had scans at 6wks & 8wks, i was told that they are more accurate than those that they do later on in pregnancy!

ya i heard that too about the earlier ones being more accurate i guess i was just hoping to be two weeks ahead lol---

Unfortunately the Ultrasounds can't show if the baby has just had a growth spurt or if its just coming up to one. My midwife always goes on my LMP date to determine my gestation, not my ultrasound, although my ultrasound always shows I'm about a week ahead.

Also some people have big babies others don't... My daughter was 8lb 9oz when she was born, so I am looking at a big baby this time round. That should be fun.

Don't worry...the baby will come when it comes, and it doesn't seem to matter how big the baby is.. Although it would be LOVELY to be put a week ahead...

I'm in kinda the same situation. By my lmp I would be 18 weeks pregnant, but when they did the ultrasound and measurments they said it looked like 15 and a half weeks. But there is no way this could be possible. Hopefully just a miscalculation?

alloyd519. Im i the same boat, when I had my US I was measureing one week smaller, but I know my dates because I had just come off BC and I know my date of conception, I was tracking it, when I talked to an US tech at work she says its very common to measure small or big because everyone is different!!

ya but my bellly measurement was also 3 weeks ahead i measured 25 at 22 weeks ---belly measurement went up 8 in just 4 weeks---ob said baby must have just gone through a growth spurt