My twin one was head down at my 29 week checkup/usound. I went into preterm labour several days later and he had turned breech in those days. So i ended up with a c-section as he turned breech.
A trick to try and turn them is to sit in a warm bath - not too hot that its dangerous- with a bag of frozen peas on the top of your tummy. Encourages them to migrate to where its warm. Friend did this very late into her twin pg and it worked!
Good luck !
HTH