sonographic values in obstetrics and gynaecology

Obstetric and gynaecological 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 differentiate a 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 2.5mm)

    • considered by many as the upper limit of seperation 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 post menopausal patient

    • considered by many as the upper of the thickness of fetal subcutanous 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 mininum 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 current ASUM criteria

• 7 mm

  • • recently revised RCOG recommendation for CRL beyond which a fetal heart beat 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 a upper limit for a simple appearing anechoic ovarian cyst beyond which sonographic follow up required in a post menopausal 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 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 scan beyond which a fetal pole should be seen for viable pregnancy (current 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

    • recently 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 a 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 a 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 below which it is reported as polyhydramnios

• 84 mm: 8.4 cm

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

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