I would go first by the mama's instinct, then palpating by an experienced m/w. I would NEVER go by u/s or fundal height.....Sure sometimes they are right but more often than not they are wrong.
My OB was wrong on all counts (u/s and palpating and going by fundal height) with dd1 (they were saying she was LGA, she ended up being a bit over 8lbs.)..... my m/w was just about right on with dd2 and I had a feeling that she wasn't that big (she was almost 8lbs.)....with my last p/g my m/w was guessing he would be about 8 1/2lbs. and eventhough I had a feeling he was going to be big (I had horrible pubic symphysis pain and my belly just felt soooo heavy) I just kept telling myself he wasn't going to be big...(he was a 10lber)......
And maybe I am the exception to the rule but his birth was by far the hardest! My labor with him was about the same but pushing was (and I hate saying this) awful. But in my defense he did have a head that was 1 1/2" bigger than my dd's.
I don't think it's anymore than a scare tactic. Plus all they worry about most of the time is the weight of the baby.....Any other mama out there with a big headed newborn will tell you that whether that baby was 7lbs. or 11lbs., that head is what matters! Fat squishes!
