Sonographic values in obstetrics and gynecology

Obstetric and gynecological ultrasound is rampant with numerous cut-off values. Some of these get revised over the years. The following list is a useful aid to refer to and revise.

• 1 mm

  • rate of increase of a mean sac diameter per day in early pregnancy

• 2 mm

  • generally accepted value for a thickness for a significant fetal pericardial effusion

  • generally accepted thickness for a decidual reaction for an early pregnancy

  • generally accepted thickness of an intertwin membrane in order to differentiate an MCDA from a DCDA pregnancy

• 2.5 mm

  • considered by some as the upper limit for a normal nuchal translucency (others take it as 3 mm)

  • traditional single cut off lower limit value for a nasal bone length below which it is considered as a hypoplastic nasal bone

• 3 mm

  • considered by some as the upper limit for a normal nuchal translucency (others take is as 2.5 mm)

  • considered by many as the upper limit of separation between the choroid and the medial wall of the ventricles in or to support the diagnosis of a mild fetal ventriculomegaly

• 4 mm

  • considered by some as the lower limit for an endometrial thickness below which is interpreted as endometrial atrophy

  • considered by many as the upper limit for a fetal renal pelvic diameter in the second trimester beyond which it is taken as fetal renal pelvic dilatation

• 5 mm

  • depth of invasive component for cervical cancer to be upgraded from stage Ia to stage Ib

  • accepted upper limit for an endometrial thickness in a postmenopausal patient

  • considered by many as the upper of the thickness of fetal subcutaneous tissues beyond which it is interpreted as fetal anasarca

  • upper size limit of microcysts in a type III CPAM

  • accepted upper limit for an endometrial thickness following a DC/STOP procedure beyond which the diagnosis of retained products of conception should be considered 

  • considered by some as a minimum size difference between a gestational sac over the size of the fetal pole for a healthy pregnancy

• 6 mm

  • considered by many as the upper limit for a nuchal thickness

  • maximal CRL beyond which a fetal heart rate should be seen for a viable pregnancy under ASUM criteria

• 7 mm

  • revised RCOG recommendation for CRL beyond which a fetal heartbeat should be seen (ASUM criteria in process of revision).

  • width of invasive component for cervical cancer to be upgraded from stage Ia to stage Ib

• 8 mm

  • maximal gestational sac diameter (MSD) on a transvaginal scan beyond which a yolk sac should be seen for a viable pregnancy

• 10 mm (1 cm)

  • upper limit for the accepted width of normal fetal ventricles beyond which it is considered as mild fetal ventriculomegaly

  • considered by some as an upper limit for a simple appearing anechoic ovarian cyst beyond which sonographic follow up required in a postmenopausal patient

  • upper limit for the diameter of the cisterna magna beyond which it is considered as a mega cisterna magna

• 12 mm (1.2 cm)

  • accepted by many as the upper limit for a junctional zone in MRI above which is concerning for adenomyosis

• 15 mm (1.5 cm)

  • upper limit for the accepted width of normal fetal ventricles in mild fetal ventriculomegaly beyond which it is considered as fetal hydrocephalus

• 16 mm (1.6 cm)

  • considered by some as the upper limit of normal for gestational sac diameter (MSD) on a transvaginal scan beyond which a fetal pole should be seen for viable pregnancy: some consider this as 20 mm

• 20 mm (2 cm)

  • considered by some as the upper limit of normal for gestational sac diameter (MSD) on a transvaginal scan beyond which a fetal pole should be seen for viable pregnancy (ASUM criteria)

  • considered by some as the upper limit of size for a gestational sac (MSD) on a transabdominal scan beyond which a yolk sac should be visible 

  • minimum depth of amniotic fluid pocket below which it is reported as oligohydramnios

  • accepted by many as the minimum distance between the placental edge and the internal cervical os below which it is classified as a low lying placenta

  • size of peritoneal deposits in ovarian cancer which differentiates stage IIIb from stage IIIc

• 25 mm (2.5 cm)

  • upper limit for mean sac diameter on a trans-abdominal scan beyond which a yolk sac should be seen for viable pregnancy

  • revised RCOG recommendation for upper limit MSD beyond which a fetal pole should be seen (ASUM criteria in process of revision).

  • accepted lower limit for a cervical length up to ~24 weeks below cervical incompetence is considered

• 30 mm (3 cm)

  • considered by many as the upper limit for an ovarian cyst or paraovarian cyst in a premenopausal patient beyond which it should be mentioned on a radiology report

• 35 mm (3.5 cm)

  • considered by some as the lower limit length of the umbilical cord to be considered as a short umbilical cord

• 40 mm (4 cm)

  • considered by many as the maximal placental thickness at any gestation beyond which is it taken as placentomegaly

• 45 mm (4.5 cm)

  • often accepted as the lower limit for a CRL in order for a nuchal translucency to be valid

• 50 mm (5 cm)

  • considered by some as the upper limit for an ovarian cyst or paraovarian cyst in a premenopausal patient beyond which sonographic follow is recommended 

• 70 mm (7 cm)

  • considered by many as upper limit for a simple appearing ovarian cyst or paraovarian cyst beyond which MRI is recommended for full evaluation

  • considered by many as upper limit for the umbilical cord length beyond which it is taken as a long umbilical cord

• 80 mm (8 cm)

  • maximum depth of amniotic fluid pocket. >8 cm is indicative of polyhydramnios

• 84 mm (8.4 cm)

  • often accepted as the upper limit for a CRL in order for a nuchal translucency to be valid

See also

• normal reference values in radiology