From what I've heard and read, how well you relax into the 2nd stage of the labour has more to do with it than the size of the baby. My friend had a 2nd degree tear when birthing her preemie at 26 weeks because she was totally in denial that she was already in active labour yet my cousin pushed out an 11½ pounder and needed only 3 stitches because she was mentally in a much better place. My mum (a tiny, little woman!) grunted out my 10+ pound brother and was getting on with life as normal 2 days later.

I'm sure you (and I) will do fine.

BTW, to answer your question, I've put on 12 kgs in 6½ months and plan to not put on more than another 3 kgs for the rest of the pregnancy if I can avoid it!