Cervix question

whit🌹 • •••1 toddler and twins •••

Had ultrasound other day doc said cervix is like 3.12 long what does that mean? Like when I check last time with the doc it was 4. Something long idk just thought I should ask plus 17+2 pregnant with twins

Anyways pls explain to me about the cervix anything about it thanks this my second pregnancy