I agree that your body will grow a baby you can birth. And somebody has to be in the 90th percentiles, right? (That always blows my mind, that having a baby in the 90s is such a "deal.")
I've heard some stories of late ultrasounds being fairly accurate, too, so I don't rely SO much on the "they can be off either way" for comfort. (I have a friend whose baby was delivered electively at 37 weeks due to placenta previa. He was estimated at about 9 lbs, and he was born one day shy of 37 weeks, weighing 9 lbs 2 oz.) But even if the estimates are accurate, as they sometimes are, in my opinion there's no reason to freak at that information. (Meaning, I don't "need" to hang on to the idea that the estimates might be off.) However, I do keep that "+/- 10%" thing in mind if there is something disturbing or concerning....to keep things in perspective.
For example: I had a scan at 32 weeks that showed my first twin only gaining 9 oz in two weeks, while twin B had gained a more expected 1 lb 4 oz in the same time period. This widened their growth disparity to a more concerning percentage (25% disparity between them) than their usual consistent tracking at 18-20%, but all signs pointed to fetal well-being and no problem. The doppler cord flow was fine, the fluid levels were fine, both babies were doing the "breathing practice" and moving lots, and even the "small" twin was still tracking in the ~65th percentile ahead of dates, so not small for gestational age at all.
I was concerned just because I had an expectation that wasn't met, and we've had regular ultrasounds (monthly for awhile, then every 3 weeks) to monitor them because they share a placenta & some vascular connections, so even with the plus/minus of ultrasounds we had a pretty good idea of how they were growing, in general.
BUT, I was pretty sure it just was a matter of something being a little off in one or more of the measurements of that twin. And sure enough, we went in to a different facility for a biophysical profile a week later (wasn't supposed to have measurements done then) and the tech went ahead and started in measuring. I let her go (normally, I'd have told her it was just a straight BPP with no measurements, to limit the ultrasound exposure, but I was a little nervous about twin A so I figured since it was happening, it must have been meant to be.)
Based on that measurement, twin A either gained 17 oz in one week (while twin B gained 6 oz
) or the scan from the previous week had been off in its measurements. It really put my mind at ease, though I'd already chalked the "anomaly" up to a measurement error.
At my last scan, it was really challenging for the tech to get good images for measuring (particularly of the heads, for different reasons), and I suspect the estimates were a little off (probably on the low side, if I had to guess) but they were tracking "normally" with just a slightly slower gain at 35 weeks.
If it makes you feel better, my twin B has been tracking in the >97th percentile since about 20 weeks. I don't so much doubt the ultrasound's accuracy, as it's been pretty consistent along the way, but I doubt that it's a "big deal" to be measuring ahead. "Measuring ahead for dates" just means that the baby is not in the 50th percentile. Measuring "spot on" would put him in the 50th percentile. Is every baby going to be there?
And here's another parallel for you: My ultrasound was at 35 weeks 4 days (very close to yours) and my larger twin was estimated at 7 lb 5 oz. The other twin was estimated at 6 lb 6 oz. It really can happen.
Keep in mind +/- 10%, and have faith in your body's ability to birth whatever you grow.
(My last pregnancy, I had one late term u/s and they estimated on the small side. She was supposed to be under 7 lbs, and she was 8 lbs 12.5 oz. Of course, this kept the on-call midwife from pushing certain interventions on me, sort of the opposite of the flip-side, which I think I'd have been hearing if she suspected a "bigger" baby.)
We'll see on their birthdays!