Over the last decades much research has been done on the etiology of premature deliveries, especially the relationship between cervical length and premature births. There is a reverse direct relationship between cervical length and the risk of premature births.

Prematurity Risks by Cervix Ultrasound

The Positive Predictive Value of Cervical Length Measurements by Transvaginal Ultrasound and the Risk of Delivery <35 weeks

Prematurity Risks by Cervix Ultrasound at 24 weeks

• Cx<30mm: 9.3%;
• Cx<25mm: 17.8%;
• Cx<20mm: 25.7%
• Bishop score>5, 38.5%
• Bishop score>3, 12.1%
• Funneling: 17.3%

Prematurity Risks by Cervix Ultrasound at 28 weeks

• Cx<30mm: 7.0%;
• Cx<25mm: 11.3
• Cx<20mm: 16.7%
• Bishop score>5, 25.6%
• Bishop score>3, 9.9%
• Funneling: 11.6%

Source: Iams NEJM 334, 1976