I would try not to worry too much about the one week difference. The farther along you are the more ultrasounds can be off, sometimes up to two weeks, unlike earlier ones that are only off by a few days. As for the AC (abdominal circumference) percentage, it can be smaller if you and your husband are smaller people, which will mean that your baby may also be smaller. It's a little early now to determine, but if you look at it like this, a normal 7lb baby is the 50 percentile, and 6lb baby is 25 percentile and a 8lb baby is 75 percentile. You need to remember since you are only at 25 weeks, a LOT can change before your due date. The baby goes through many more growth spurts before then and all these numbers can change dramatically. Being only a week behind from the ultrasound is really nothing to worry about, so try and relax, I know, easier said than done. :)
U/S are not accurate for dating this far along. My little guy has been measuring ahead since end of first trimester as well. No cause for worry at all....you might find he stays a bit behind, or suddenly has a growth spurt and shoots ahead...or back and forth. He is normal!
I concur with everybody that the u/s measurements are not as spot-on as you might think. They are really estimates. In fact, 3rd trimester u/s are notorious for misjudging the size of the baby by up to TWO POUNDS in either direction!
Did they measure your uterus from the outside with a measuring tape? My midwife says that the measurements roughly correspond with 1 cm per week. So at 25 weeks, if you're between say 23cm and 27cm on that measurement, you're fine. :)
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