Before jenna was born I did a big round upof all my nasty old undies, and I used them in the hossy. I think I ended up throwing a few out. Another great suggestion (from Tanya) is to use DP/DH's old undies cause they are really wide across the gusset, and actually pretty comfy with pads!!
No point in buying new undies until you use up all the old grotty ones around the place, and then post baby you can treat yourself to some new undies (maybe to match new maternity bras if you can do the breastfeeding thing!)

As for post baby - I'm the sameweight I was pre baby #1 and nothing underwear wise fits like it used to. G strings are a little embarrassing cause my tum sort of rolls over them, and I usually need nice big knickers - specially when playing sports.

My best tip is, try a few different sorts, and when you find onethat works, buy it in bulk!!