Hi would a size s fit my 10 week old her head is 39 and a half cm

Hi Samantha, thank you for your enquiry, the Babu Organic Cotton Hat might not be suitable for your little girl as they dont come in a bigger size and the 3-6 mth is for 35-40cm. We hope this helps. ?For further information on this or any of our other products available, if you follow the link https://www.babyonthemove.co.nz/store-locations/ this will take you to our online store finder. Here you can find contact details for your nearest Baby On The Move Store where one of our team experts will be happy to assist you in your product search and help answer any further questions you may have. We hope this has helped with your query.