It could be pre-ejaculate, it's interesting that it came out like when he would ejaculate, but why not? It would not be an indicator of an STD (and shame on you for thinking he is cheating if that is what you are thinking, unless of course you already know he has cheated), and it doesn't sound like a negative indicator of his health. Try to relax. I know the last month of pregnancy is pretty emotional, but for heaven's sake.