Sauvignonismysaviour
Personally I wouldn't request to go back part-time until I'm actually on maternity leave. With my first baby I didn't put in the request until a few months before I was due to come back.
I don't always subscribe to her corporate way of thinking but here's some advice from Sheryl Sandberg who is a top exec at Facebook and basically the goddess of women's career advice - she has a mantra "don't leave before you leave"
“…when it comes to integrating career and family, planning too far in advance can close doors rather than open them. … Of all the ways women hold themselves back, perhaps the most pervasive is that they leave before they leave.”
Another one of her quotes:
"What I am arguing is that the time to scale back is when a break is needed or when a child arrives—not before, and certainly not years in advance. The months and years leading up to having children are not the time to lean back, but the critical time to lean in.”
I do agree with her on this, because you don't know for sure how you're going to feel not just immediately after the baby arrives, but say 8 months later. At first you might not be able to imagine leaving your baby even for a day. After a few months you may decide mothering a baby is incredibly boring and actually, you prefer working full time/3 or 4 days a week/etc. I think it's best to wait and see how you feel as you settle into life with your new baby.
Plus, if everyone at work knows well over a year in advance that you're planning on coming back part time, you may find you get less opportunities coming your way. Even just being pregnant I can sense that I'm probably going to be on the bottom of my managers mental list of people when it comes to offering training opportunities and so on.