My bub was 2.95kg or 6lb 8oz and DEF was 00000... She still is easily in 00000 and is almost 4kgs.
I would buy maybe 2-3 00000 jist in case and if your bubs is that small, then you can buy more after birth.

Target has massive baby sale, we got basic white suits and pink suits in 00000 for $3 each!