hi anna. i was still co-sleeping with ds1 and bfing all night long when i fell pg with ds2 so it can defintely happen without you changing anything. we sort of said we'd just 'see what happens' and i fell pg straight away. i went on to bf ds1 until i was 25wks pg with ds2 when he self-weaned and it was all very gentle with no tears or sadness- he just happily stopped- giving me a couple of months off before i started bfing ds2. i would say just keep doing what you're doing, you can do this without pushing your dd's needs to the side iykwim? we've given ds1 the best present in the world- a younger sibling who he absolutely loves.