IIRC from all my research, estimates of cervical length by vaginal exam aren't always very reliable. It seems that very few docs are being taught this anymore...so I'd make sure she was experienced at it, personally.
I also lost a pregnancy at 21 weeks, but the first sign of any problem was a bulging bag and then labor which couldn't be stopped.
Although I'm not an advocate of u/s scans, they are a good way to determine actual cervical length. I did have a cerclage placed with pg #3 and my cervical length was monitored closely, via u/s, from placement at 13 weeks, until about 22 weeks. The biggest cervical failure window being roughly 18-21 weeks.
At 28 weeks there's not as much direct pressure on your cervix as there was between 18-21 weeks, because the baby moves up out off the cervix at that point.
Anyway, this is what I remember. My loss was more than 4 years ago and my cerclage baby just turned 3. This pregnancy has been perfectly uneventful and a quick u/s at 19 weeks showed my cervix to be about 4.5cm in length...so no cerclage this time!