I only have a sound monitor. I never even had one of those until DD was 6 months (i just left the bedroom door open, her cot was right against my side of the bed and she often co-slept with me until 6 months) then i got one when she went into her own room.

TBH i don't know anyone in the UK who has one unless bubs was premmie or ill. The hospitals lend them to parents who've suffered a loss to SIDS, but it's not the norm to buy them as far as i know.

I'd say go with your gut. If you'd feel better having one and you can afford it, then why not?

Bec