I think 18lbs at 8 months is not bad. My boy was 18.4lbs at 8 month and down to 18lbs at 9 months. Scale at his pediatrician and his family doctor office are different too. I was worried about my boy too when he's about 11-12 months old that he didn't gain weight or get taller. Now he is 14 months and I kind of stop measuring his weight because he doesn't seem to gain weight each month but he's getting taller again and doesn't look slimmer. So I'm not sure what's going on with measuring his weight. I think if your boy looks healthy, develops according to each milestone, you should not be worried much about it.