Ultrasound can give them an idea - my midwife gave me a guess based on measurement and palpation of my abdomen plus years of experience - it's just a guess though. They can be off by even a couple pounds even with ultrasound measurements which is huge... my midwife said "about eight pounds" at 39 weeks and when he arrived a week later he weighed nine pounds even...so she was close!
They do mine by ultrasound
Okk...m due for an ultrasound in mid may..hope to know it :)
The ultrasound can be off +/-1lb. But it will give you a good idea. I had my 20 week ultrasound and baby is 310.9 grams! (10.9 oz) -/+46.51grams. So really baby can be 276-356grams. I had an ultrasound with my daughter the week before she was do and it was pretty much right on.