In Shetlands it depends IMHO on the background of the foal. If she is 37" at a full age 2, she could only go a couple more inches, or she could have late growth spurt and hit 41 or 42".
I have a mare that foals, she generally throws smaller on the fillies and taller on the colts. And it doesn't matter which stallion I throw at her.
Her first foal is 45" (not my stallion), second foal 42", third foal 44", fourth foal is around 40" and is a filly at one. This years foal looks like he could go his sire's height, but then he is the same size as his full sister from last year.
As far as a sure fire method - I don' t know of one for predictions in shetlands.