I am going to attempt to explain centime charts, or at least my understanding of them. Centime charts are like a plot of the normal distribution of height, weight or whatever for a particular population. It is not that you are only 'normal' or healthy if you are on a particular centile as clearly some of us are short or tall, have a slightly bigger or smaller head, but can all be normal. At 5'2'', I am on the 25th centile for adult women, so not surprisingly my baby girl is on the 25th. Similarly a woman could be 6 inches taller than me but we would both be within the normal spectrum. Another way to look at it is that in a room of 100 women, 75 will be taller than me, 25 will be shorter. Same with babies. The important thing with babies is that they grow along a centile, or don't deviate too much so that you can see their growth is steady. And that if they are on a particular centile for one variable (eg weight), they are on a comparable centile for others (eg height) so that they are in proportion. My baby has a relatively big head (so do I) on the 50th centile. So her head is the average size for baby girls of her age but because the rest of her is smaller than average, her head looks a bit big. So, back to your scan, the AC has dropped from one centile to a lower one. Doesn't mean that his abdomen has got smaller but that the growth has slowed so they need to keep an eye on things. The other thing to remember is that scans and measurements and centile charts are for screening, they are not always diagnostic. Unfortunately this means that some people will be put through unnecessary worry to enable problems to be detected in most cases. Please try not to worry too much, it sounds like they are being cautious. Finally, I know KCH had its problems, but I found their antenatal care and scanning excellent. Had a private scan too which was nowhere near as good - less time, less measurements, much less qualified sonographer. Hope some of this makes sense.