I've bought new mattresses each time, but that's probably more because DD1 and DD2 were both in cots at the same time, and then I had nothing left anyway when DD3 came along. When I was reading your OP I was thinking oh no, she should get a new one just to be safe but then I thought how ridiculous that is when each and every night and most day sleeps I put DD3 down to sleep on our mattress which has been used heavily obviously by us for years now. So, I would say that so long as you used a mattress protector on it with Bub No. 1 then go for it.