I'm not a mom yet. However here's been a bit of our timeline and why...
We ordered the crib at 20 weeks. Why? Because the store said it would take 14 weeks to get in and I freaked! I didn't want to be just getting the feel of a crib in the nursery that late...and what if it took longer than that? And what if there were damaged parts?
I started an online Amazon registry at about 14 weeks. I kept it private so no one could view it but it was a great way to get a feel for some of the products out there and read reviews. I added things we MIGHT like/need and it made actually registering easier. I think the more time you have looking and reviewing products the better. It's less stressful if you take a few items a week to look into rather than a whole store.
We registered at a store at 22 weeks. By then we knew the gender and could make better choices as far as colors.
As for buying... I like to spread out some of the pricier purchases. We bought the car seat at around 16-17 weeks (because I had a coupon). I've also ran into a few fabulous clearance deals (bedding and travel swing). But really keep in mind that you'll get a completion coupon that might make some of those larger items easier to buy and then if you're planning to have a shower who knows what you'll end up with. I've been watching my amazon registery (still set to private) and have picked up some super good deals there too. If it's super cheap, grab it. If you have a coupon, grab it. Otherwise maybe wait it out a bit...
The weekend after our big ultrasound (20 weeks). But I had already been researching with the Baby Bargains Book, reading online reviews, and polling my mommy friends for suggestions. Don't register for clothing early (if at all) because it will be out of stock by the time your shower rolls around.
Good luck! So fun. If you haven't already. Check out the many, many pinterest.com boards on mommy registry suggestions.
I registered right at the end of 1st trimester/beginning of 2nd trimester.Just put down everything I could possibly think I would need, didn't tell anybody I was registered though.
When we found out the gender (22 weeks), I went back to my registry, changed some things around, tweaked it, added/deleted things since I had done more research at this point, and then shared it with people when they asked.
I think we, personally, started buying baby items ourselves around 20 weeks. And I mean big items like crib, dresser etc. Baby clothes and little trinkets started being bought almost immediately after we found out haha
Registered after our 20 week u/s. We bought almost nothing until after all showers were done.