Its a good idea too. Even when there brand new. I did and I'm almost 35 weeks.
Everytime I got a new pair of clothes or blankets or anything I washed it immediately to put them in the dressers. Anything new I get I wash.
I started at 35 wks, im now 39 wks.
I washed anything that will come in contact with their skin to get the chemicals out but just used our normal washing detergent as we already use a hyperallergenic one and that way it will also smell like us. We washed everything over a coyple of weeks around 26-29wks as we're not going past 36wks.
I washed it as i got it and packed it away the more work u get done earlier on the less you have to do when all you want to do is relax
im not washing mine until she is here. my first born the docs told me i was having a girl and i ended up having a boy. i dont trust the docs so im waiting. i went thru hell returning everything amd trying to sell the clothes i washed. my kids father will wash them while im in the hospital