Get the larger sizes because they are on clearance(they can always grow into them). Closer to your due date, buy a blanket sleeper and a pack of onesies in newborn size. This way, if they don't fit, you can exchange them right away, and also have something that fits.
I was told by my midwife that I was going to have about an eight pound baby, but he surprised us all by coming five weeks early, and being 5lb 11oz and 19 in.
My Mother had me, nearly 9 lbs, my sister, just shy of eight, and my brother, nearly eight as well, so you can't really judge past pregnancies.
Everyone got me newborn sizes for my son, and he ended up being a preemie, and nothing fit him(until my awesome friend rushed home and grabbed some of her son's old baby clothes).
He literally fell out of the newborn sizes. I was glad for them too, as he wore them until he was about seven months(he is going to be eight in two days, and is four foot tall, but really skinny, so it's all custom tailored clothes for him, as six is too baggy for him).
I always thank everyone for larger sizes, as they will grow into them.
Just my 2cp.