FAQ about growth ultrasounds

I've gotten a few questions about how the growth ultrasounds work so I'll try to answer them here. Please note I did not do any actual research to find out these answers - anything I know I got from the doctors.

1. Are the percentiles for single babies or multiples?
The percentiles are for single babies. Apparently there are no uniform stats for multiples, which I find hard to believe given the explosion of multiples over the past many years. However, I'm always reassured to know that my babies are of average size compared with singletons!

2. How do they estimate the weights?
They measure head diameter and circumference, abdominal circumference, femur length, and humerus length. Then they plug all the numbers into the computer and some sort of formula is used to spit out their weights and percentiles. There's probably always a degree of measurement error, but I prefer to believe these good weights are extremely accurate!

3. What else do they look at during the ultrasound?
For each baby, they look at the level of amniotic fluid, the condition of the placenta, organs such as the bladder and stomach (apparently if there is fluid in them it's a good thing, because it means they're swallowing fluid), heart rate, and the ventricles in the brain. They also like to make sure they're moving around enough.

4. How long does the ultrasound take?
Thankfully, I have cooperative babies and a skilled ultrasound technician. This last one took just over 30 minutes to get all the measurements needed for the 3 babies. Still, it's a long time to be on my back (even elevated), and I got really hot at one point. The tech said that's the first sign of restricted blood flow, so she had me lay on my side and gave me a cool washcloth for my face. That really helped.

Of course, then we waited almost another 30 minutes for the doctor to come in to review the results with us. Why do doctors always take so long, and what are they doing while we're waiting??

5. How often do the ultrasounds happen?
They usually like to do the growth ultrasounds every 4 weeks so they can really see the changes and trends in growth. Any earlier than that and there would be too much chance of measurement error and little change between scans. It's hard to wait that long but we're trying to be compliant. :)

Ok, I think that's it. If anyone has more questions feel free to ask!