18 weeks pregnant cervix 27mm

Hello, i am 18 weeks pregnant. I see the consultant on Wednesday due to previous LLETZ procedures. My cervix was measuring 27mm so he has put me progesterone and I am going back Wednesday for another scan. If it has got any smaller he said I will have the stitch that day but if not I will carry on with progesterone and be checked every week up until 24 weeks. Can anyone tell me what happens after 24 weeks? I know you’re not meant to have the stitch after this time is it less likely for your cervix to change at this point? Thank you x