get bigger then if u have to buy more 1 or 2 when lo is here they're cheaper than the bigger sizes anyway. it's not just about the size/weight of lo either, some babies pee more than others and saturate a smaller size. hth
Also ignore the weights they say on the packets. I stocked up on a few packs of size 1's as they went up to 10Ib (i think) and when my son was about 8Ib they gave him a rash on his tummy as they were abit tight.
Dd was in size 1 for 12 weeks and she was 8lb when she was born, it really is hard to tell....she was in size 2 for ages and at 11 months has just gone onto size 4....she is 50th centile for her actual age and 75th for corrected (she was 4 weeks early)