Short cervix

Hello everyone. I am now 18 weeks pregnant. I just had my ultrasound to find out the gender. My doctor is concerned because my cervix is only 25mm long. He wants me to come back in two weeks to make sure it isn’t getting shorter and if it is I will need a stitch put on my cervix. He said the concern is pre term labor. I am so scared and don’t know what to think. Has anyone had any experience with this and everything has been ok? Any advice would help. Thank you!