What Serafina said.
I had PTL last year and delivered my baby one day before 18 weeks. They are watching me like a HAWK! I don't get it checked every visit. I've only had it checked once by ultrasound since I was hospitalized. My next official u/s is at 26 weeks. I may ask that it be checked manually next week though at my 23 week visit (I see my OB every 2 weeks, I get my progesterone shot every week). She may not do it and wait until my u/s.