baby measurements and ultrasound

I've noticed everyone seems to be getting growth scans and ultrasounds to find out how much baby weighs. My doctor doesn't do ultrasounds after the anatomy scan at 24 weeks. I did have a scan at 33 weeks to check fluid and when I asked the weight  the US tech said they won't check that after 24 weeks because it isn't accurate. Are there any other ways to estimate weight? If you had scans with previous babies were they accurate?