Baby Clothes

I have decided that baby clothes sizing is worse than grown up clothes sizing. I started buying fall/winter clothes for them back in August (when my favorite consignment shops started getting them in). I bought both 6-12 mo. and 12-18 mo. sizes since the girls turned 9 mo. earlier this month. One would assume that they would be the perfect size for the 6-12 mo. pants, but that assumption would be entirely incorrect. The pants fall right off as soon as they begin crawling or pulling themselves up. I tried the 3-6 mo. pants from last winter, but those pants are entirely too short. I might as well have them wear their shorts from the summertime. Are my daughters so strangely built or do others have troubles like these. This morning I went out and bought the girls a few pairs of overalls, figuring that overalls cannot fall down like pants do. Its becoming more and more chilly out, and they need to be wearing pants. Then the question arises, when might my daughters be able to wear these 6-12 mo. pants? Hopefully before June!