I have one every 4 weeks. They predict a big baby, his measurements are all above the 95th percentile on his chart for his abdomen, femur length and head circumference. However, I'm assuming nothing until he is born
A growth scan isn't necessarily to predict the birth weight as such (they're notoriously inaccurate!), but to check that the other parameters of growth of the baby such as the head and tummy circumference are progressing as they should. They can look at how things are developing, the amount of fluid around the baby, how the placenta is coping. They can be an attempt to predict any problems and avert later issues, eg by perhaps going for an earlier delivery if the baby is struggling or not growing as fast as it should.
They're also hugely reassuring, personally I think that going from 20-40 weeks without a scan is a long time.
If you're being offered one, I would definitely have it and view it as a good thing. I had them with all of mine for various reasons and found them very reassuring. No one was in the least bit interested in the actual predicted birth weight, just that all the measurements were progressing well along the expected growth curves.
Mine is more to predict the birth weight as I've had a previous 4th degree tear with my son who was 9lb 6 1/2. They obviously don't want this happening to me again & neither do I:-( but I've heard there so inaccurate I almost don't see the point of it.
I'll be having growth scans this time round because my DD was 5lb 4oz, which is below a threshold weight. There's nothing wrong with her, she's just petite, and always has been (always followed her centile line). I'm 5'1", and while my husband is 5'11", he's quite slim so I figured she got my height and his slimness.
I was told they're to check baby is growing ok and at right rate etc, so I understood this as making sure there's not a problem with placenta etc. I mean, if I have another wee baby, how can they make it put on weight etc? That's what I'm confused about.