While is true that scans to determine weight later in pregnancy can be off, if they're going by the length of a baby's body parts at a 20 week scan and the baby is measuring ahead that is a more reliable marker for a bigger baby than weight scans. However, having said that, just because baby is a little ahead now, that doesn't mean that it will be big at birth. They grow in growth spurts and it could be your baby had his a little early. I wouldn't start to worry just yet, though I can understand why you're concerned if your first birth was a bit tricky.
I make big babies. My husband and I are both tall (I'm only slightly tall at 5'8, but my husband is very tall at 6'4) and we were both massive babies - which I think is one of the few indicators of predicting whether your own babies will be large. Even from as early as the 12 week scan, my babies always measure ahead. Early scans will match up with my dates, but hen by 12 weeks, I always get moved forward, sometimes as much as 5 days (this baby is following its big brother and sister's footsteps already - my early scans confirmed my dates and it was measuring a day behind, then by 12 weeks my due date was moved four days forward). Then by the 20 week scan they're sometimes up to two weeks ahead and with later scans are always further ahead as well. My son was 9lbs and my daughter was 7lbs8oz, but she came at 36 weeks and would have been bigger had she been born later. With them, every single scan I had, they were very large in terms of the length of their bodies and limbs (and their 32 week growth scans had them both above 90th centile for weight - though these judgements are less reliable). That was the indicator for me. Maybe see if you can have a few more scans to check on baby's size over the next few months just to mke sure it wasn't a one-off and you caught him just after he had a growth spurt? And then discuss your concerns with your doctor? For now, try not to worry, one scan dating ahead is not a reliable indicator that the baby will be big. Good luck!
