I'd say about 8 or 9 weeks. I started wearing a few maternity things early b/c they were alot more comfy than some of my pre-pg clothes. If you want to wear them, you should go to a B&M store and try a few things on. If nothing fits than you know you are too early. Warning though--sometimes maternity pants can be huge, as if they're made for someone in their 7th or 8th month! I can't fit into anything at Target yet and I'm 21 weeks! Mimi Maternity is good and so is Pea in the Pod (owned by the same company)--but I had to actually go a size down in maternity pants, so keep that in mind.
alb, no I haven't gotten the jeans yet. I tried on a few shirts at Old Navy Maternity this wkend and they are HUGE!! Sadly, most of my regular stuff is getting too tight in the bust, though. I'm frustrated b/c maternity stuff looks too big and regular stuff looks to small!!
When did you girls tell the people at work? I haven't told them yet. I was thinking of waiting until the beginning of March when I'll be into my 2nd trimester, but it may be too noticeable by then.