Sounds like she's a bit smaller than Ciara at 16 weeks, who was about 28 pounds at the time. Can you see all of her ribs? If not she may just be on the small side.
Ciara at almost 1 1/2 years is about 64 pounds and that appears to be the perfect weight for her.
Caleb (aka Caleb-Moose)
Ciara(aka Ciara Belle, Black Devil)
RIP Aodhán, Rica, Max, Kelly - gone but never forgotten - forever in my heart