7 weeks short cervix

A little TMI. I had my first ultrasound today, at 7 weeks, and our little bean is doing fine, right on track. But my midwife just called and let me know my cervix is measuring at 2.6 cm and it should be 3-4.5. She wants to start me on progesterone suppositories because apparently I'm at risk for a pre-term birth. Does anyone know what causes a short cervix or has anyone gone through this before? FTM and pretty scared.