Cervix change...

Jolanda • Sidney Henry IV 💙 10/10/10 Quentin Amaru Henry 💙11/23/11 Kobe Navorro Henry💙 4/26/16 Baby #4 due 9/3/18

I’m 20 weeks and had my cervix length checked today and I have been every two weeks since I was 16 weeks. Any way today I noticed it was different last time it said I was 4.6 may 4.7 cm and today I’m 3.3 was the lowest one I saw. Is that normal for 20 weeks?