Why does pregnancy make me so emotional?

At times during pregnancy you can feel ecstatic, elated, like a fertile winged mythical love goddess (well, okay, maybe not), contented, confident, optimistic and relaxed. But you can also feel depressed, terrified, worried, tense, crabby, moody and like a blundering hippo complete with giant yawns. Tears are almost inevitable.
Sometimes it can seem that you’re on a hair-trigger. Anything can suddenly set you off: sad movies, or news stories about things that happen to babies or mothers. The rest of the world goes on despite your pregnancy, and with a bit of bad luck yours might coincide with a relationship break-up, other personal complications, or a death in the family. Even without extra stress you can feel miserable, especially during the first trimester when your hormones seem to be stuck on the spin cycle. Maybe you recognise some of the symptoms from PMS (premenstrual syndrome).
Your moodiness might be manifested by you being cranky and overcritical, or flying into a rage or panic about something that isn’t really so important, or crying for no specific reason. Or when you see a puppy.
There’s a list of common worries (fears about the baby, body image, being a good parent …) and how to feel better about them in the book Up the Duff: The Real Guide to Pregnancy.