Unless the same person is measuring you the result will be differing measurements. My second daughter I did not put on any weight for 9 weeks and was ,measuring quite behind, I had a scan to check everything, she was perfect and so was the placenta. She came out small, with a small placenta, and is still tiny but HEALTHY!
My third baby I did not step on the scales once, she measured behind but because the same midwife measured me each time, she knew that although behind, I was still growing each time. Baby came out my smallest yet with another tiny placenta, now she is two, very tall and robust.

TRy not to worry too much, each baby is so different as they are when they arrive x