I started pregnancy at 4.3cms cervix now at 29 weeks i am 3.1cms is that ok?

Yes: A cervical length of 2.5cm or longer is strongly reassuring against a preterm delivery. It's good news. The decrease from 4.3 to 3.1 cm isn't necessarily worrisome. Cervical lengths can hangs from 1 reading to the next (ex: it could be >3.1cm at next measurement). If you r cramping or bleeding then you need to have a new eval for that, but 3.1cm is very good.