What size coat?

[EG2018]

So, my little one is 5 months old and is just getting in to 0-3 (little baby). I have bought a coat and love it so I whilst it is the sale I wanted to get it for next winter also. I'm wandering whether to get her 12-18 or 18-24. What do you think?

12-18. Based on the fact that she will need it from October at least which is 10 months away so clothes size 12isb months based on current size.