how can they tell weight at ultrasound?

atownpreggo

Just curious about this. They told me Friday that Sarah weighs 3 pounds. How can they tell that from an ultrasound??

missjd07

I have no idea, but I'm thinking maybe based on measurements?

kmcrn08

Those goofy US machines somehow measure the surface area of baby on the screen, and figure it based on that?

leighannlange567

The us tech at my appointment said they measure the circumference of their head and abdomen, and then the length of their femur. It's an approx weight that can slide in either direction by a few ounces.

honey740

leighannlange567:

The us tech at my appointment said they measure the circumference of their head and abdomen, and then the length of their femur. It's an approx weight that can slide in either direction by a few ounces.

This is how they know. It's not an exact science and is why measurements are always off. 

izzourclue

Its just an estimate.  I think they base it on average weight for the length, circumference of head and belly, and other measurements they can take.  I've heard they can be off by whole pounds sometimes. 

pawcall

leighannlange567:

The us tech at my appointment said they measure the circumference of their head and abdomen, and then the length of their femur. It's an approx weight that can slide in either direction by a few ounces.

Or, you know, a few pounds.

Honestly, they really can't, it's just a guess based on size.

My little sister was estimated around 8lb3oz a week before they delivered her via c-section.   She must have been dieting in utero, because she was born a week later at 6lb8oz.

tfarabians

leighannlange567:

The us tech at my appointment said they measure the circumference of their head and abdomen, and then the length of their femur. It's an approx weight that can slide in either direction by a few ounces.

This.  I've been having lots of these because of small growth.  Remember that your baby does most of its weight gaining at the very end so it really depends on how close to delivery you have had a u/s.  The other problem is that the closer to delivery the more cramped the baby is and the more difficult it is to get correct measurements!  Catch 22 I tell ya!

wda519

cdobry01:

leighannlange567:

The us tech at my appointment said they measure the circumference of their head and abdomen, and then the length of their femur. It's an approx weight that can slide in either direction by a few ounces.

Or, you know, a few pounds.

Honestly, they really can't, it's just a guess based on size.

My little sister was estimated around 8lb3oz a week before they delivered her via c-section.   She must have been dieting in utero, because she was born a week later at 6lb8oz.

This. My Dr won't give a weight estimate because it can be very off. It just causes worry, 'is baby too small' or 'I'm scared to deliver because baby is giant'. My Dr just tell me that baby is healthy and a good size.

atownpreggo

Ok that's kinda what I figured. Thanks ladies!