don't take any risk just save few hundred dollars!! 6-18mth is definitely too big for new born bb and just buy the szie fitted to your bb la!!
my bb also born in Feb this year and he is using 2.5tog until last mth(my bb is M-L szie). now the new one is 6-18mth and also 2.5t coz bb is always in a air-con room. i just give him nearly naked and take only one thin short sleeves pajamas inside in the summer and long sleeves thick one in winter time. just refer for more info. http://www.grobag.com.au/sizes.aspx#whattowear