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.
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1 mm
- rate of increase of a mean sac diameter per day in early pregnancy
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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
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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
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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
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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
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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
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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
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7 mm
- recently 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
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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
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12 mm (1.2 cm)
- accepted by many as the upper limit for a junctional zone in MRI above which is concerning for adenomyosis
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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
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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
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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 (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
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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
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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
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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
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40 mm (4 cm)
- considered by many as the maximal placental thickness at any gestation beyond which is it taken as placentomegaly
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45 mm (4.5 cm)
- often accepted as the lower limit for a CRL in order for a nuchal translucency to be valid
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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
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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
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80 mm (8 cm)
- maximum depth of amniotic fluid pocket. >8 cm is indicative of polyhydramnios
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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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