Any time - if you see something you like, I reckon you should just go for it.
Personally, I've tried to spread out the process of buying baby gear throughout my pregnancy, as time just seems to drag by if you buy absolutely everything in the early days... But perhaps don't leave too much to the last minute though, as you might not be able to walk around shops as comfortably in a few weeks than you will be able to do now lol.
I bought little things, clothes, nappies and toys gradually throughout my pregnancy. The bigger stuff i left till later on. I got the nursery furniture the other week, only because we didn't have the space and it takes up a lot of room!
I tried to get a lot in the sales too. Nothing worse than seeing something you bought full price get reduced!
i've bought all the nursery interior like the bumper, mobile, sheets, changing mat, curtains etc but leaving the bigger things like the cot + changer until later, purely cos we've got no room to store any of it as our nursery's in the 'removing christmas decorations and other poo' process (it's literally a store room!)
my cousin went out and bought hers the day after her 20 week scan though it's whenever you feel ready i guess, and how prepared you are with your nursery!
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