Ultrasound Criteria for the Diagnosis of Incompetent Cervix
	Normal Pregnancy	Cervical incompetence
Cervical Length	>3 cm	<3 cm
Cervical Width	<2 cm in second trimester.	>2 cm in second trimester.
Cervical Canal Width	<8 mm	>8 mm
Cervical funneling	None or < 25%	25-50% or > 50%
Bulging of Membranes into cervical canal	None	Present ± fetal parts
Twin pregnancy: Membranes of both twins in incompetent cervix

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References

1. Ludmir J. Sonographic detection of cervical incompetence. Obstet Gynecol 1988;31:101-109.