When would IVF twins turn into triplets if they were going to?

Would have already happened although they can’t always find them all until 7-8 weeks.

It is actually the egg itself that splits not the sac. Identical twins may share the same amniotic sac or have separate sacs depending on when the egg splits. And this would happen within a very few days of fertilization. To make it a little clearer, at 5 weeks along, you would not have a sac spontaneously split in half. But, you could have identical twins sharing one sac and perhaps, your doctor was unable to detect two embryos at this early stage. Probably better if this is not the case, as it would be extremely high risk to carry monozygotic, monoamniotic twins along with a third baby. I hope I didn’t make is sound too confusing.

Congratulations and best of luck!