newborn clothes

How old was your baby when they moved up a size? My baby is 7 weeks old and still fits in newborn, well sort of.... she is very long and thin. The 0-3 and fits her too wide but the newborn fits her kind of short from top to bottom. She is gaining weight but the 0-3 months is extremely lose on her still. Maybe it's the brands i buy?! I just don't know what to or on her right now!!