You are here
I usually wait until I'm around 6 or 7 months. I start buying big things I don't expect to receive at my baby shower, and then I buy any little things I need that I didn't get at the baby shower. I like to be prepared early, but I'm sort of paranoid and don't want to do it too early so that I jinx things. I also have to wait until I find out the gender of my baby so that I can buy pink or blue stuff.
Whenever you feel comfortable buying things. There's a Jewish superstition that you don't buy anything before the baby is born (kind of like counting your chickens before they hatch), but obviously most people don't follow that rule. Some people have absolutely everything they'd need purchased and arranged several months before the baby arrives, which definitely has its benefits (you can shop sales, compare prices, and not have any last-minute things to buy). We adopted, so we didn't have a single thing before we got the call that our son was born - and that had its advantages too, because if something had gone wrong with the adoption, we wouldn't have had the reminders around the house.
I started buying diapers every time I went to walmart. Sometimes just a small package and other times a big box! I was so thankful after my baby was born and I didn't have to make trips to walmart.
The only problem with buying diapers far in advance is that you can't predict how quickly your baby is going to grow. You may end up with many unopened packages of newborn or other smaller sized diapers and sometimes things like that can be a hassle to return or exchange. I still have half a pack of newborn diapers from when my 17 month old son was born. He grew so fast we couldn't use the 4 packs we got.