I had one placed at 21mm at 19 weeks. They of course can’t force you to get one and it’s completely your decision - but for me they said it was their advice to get one. I was so nervous but it wasn’t even half as bad as I thought jt would be. First couple days just felt like a period discomfort and then that quickly went away. They advised me that it’s still very much in research as they don’t have enough evidence to prove it works yet however, we lasted all the way to my planned section at 39 weeks despite bleeding and signs of early labour in my last 6 weeks.
I had one placed with 1.3cm length at 21 weeks