So if my baby is size 6m regular and 9/12m Jewish I should get a 0-1m size?!
I can never buy my newborns things from there because the smallest size is too big, so I think you'd have to try that and be prepared to return. Or buy 2 sizes.
I bought my 2.5 year old two dresses Chanukah time last year when she was about to turn 2 in the 18 m - 2t size. They'll be fitting her now for succos.