Prenatal depression is a recognised thing (though not by many people), and there is no sign of it being linked with PND (thankfully).
I suffered terribly with DD (first pregnancy), but was going through various work sh*t etc, and put it down to that. Having had "normal" depression in the past, I knew what it was, but didn't realise it could be pregnancy related. I found something online about it later and it all sank into place.
The second time round, it happened again, and I could see it coming, but just couldn't stop myself from falling. The main thing I found was that I needed to be honest with both myself, and my family and friends. I wasn't feeling a bit blue and tearful, I was feeling Depressed with a capital D. By using the dreaded "d" word, people took me more seriously and suddenly realised how bad it was and that I needed support.
I cried on my midwife too, explained to her the situation (with my past history), and she was sooo supportive, telling me to let her know if things got worse, that they could sort out pregnancy safe anti depressants if I felt it was needed, and was just generally really supportive.
I was lucky - I know that rhubarb had it much much worse, and that her story is related to her feelings towards the baby too. For me it was just myself and my life - I did get very low, and scared that one day I would suddenly get so low that I could do something stupid, but the life inside me was what kept me going.
With both pregnancies, I gave birth, depression went, and I was like you - basic baby blues.
Email me if you want to chat at all - sometimes just saying what you are feeling helps a lot.
flamesparrow at googlemail dot com.