It depends on your situation and how your body is processing pregnancy. I had bad depression since like 20 weeks but ive had a smooth pregnancy for the most part. The depression was due to baby daddy and feeling insecure
I haven't hit one yet, but I'm going to say after 40 weeks and no baby might suck just a lot. :p
It's been on and off for me. One day I'll feel perfectly fine, the next I feel like crap. Guess I'm one of the lucky ones. (Hope I didn't just jinx myself)