The ideal cervix length at 17 weeks of pregnancy is typically around 30-40 mm. A measurement of 28 mm may be slightly shorter than average, but it does not necessarily indicate a problem. It's important to remember that individual variations in cervix length can occur. However, if you are concerned or have a history of preterm labor or cervical insufficiency, it would be best to consult with your healthcare provider for further evaluation and guidance. They may recommend additional tests or interventions to ensure the health and safety of both you and your baby.