i started off with 41kg at 163cm, so im even more underweight than you.
im in my 37 wks and my baby weighs 2.4 kg, so she's not underweight. it might be cos i drink those maternity milk powder every single day since i knew i was preg. if you're not doing that now it might be good to start, cos it's very nutritious.
as for now dont get too worried. after all you're only at 28 wks so your baby still has lots of time to grow! =)