ARDMS OB/GYN
2nd/3rd Trimester
questions and answers
twins have a faster or slower growth rate in the
3rd trimester compared to a singleton? - answer
slower
Biparietal Diameter (BD) - answer - axial view
- obtained at the level of the third ventricle, cavum
septum pellucidum and thalamus
- falx cerebri should be seen separating brain into
2 equal hemispheres
- measurement performed from leading edge to
leading edge of the skull or outer to inner borders
- can be measured as early as 10-11 weeks
-measurement most accurate for dating before 20
weeks
- less accurate than HC for gest age
- Corrected BPD-OFD is also used with the BPD to
take into account the shape of the fetal head
- BPDc= square root (BPD x OFD/1.265)
,Head circumference (HC) - answer - performed at
the same level as the BPD (thalamic view)
- more accurate for dating than standard BPD
- corrected BPD and HC are the most accurate
biometric measurements for dating before 20wks
- measure the skull circumference around the outer
edge of skull
- caliper placement is around the calvarial wall and
should not include scalp thickness
- very important to exclude skin thickness when
scalp edema is present
- reliable measurement that does not vary in
accuracy with fetal head shape
Occipital-Frontal Diameter (OFD) - answer -
performed at the same level as BPD
- measure from anterior skull to post skull on outer
edges of cranium
- corrected BPD calculated with OFD
- BPDc= square root (BPD X OFD/1.265)
Cepahlic Index (CI) - answer helps describe the
SHAPE of the head
-ratio of the BPD/OFD
- 0.70-0.85 WNL
,- can be described as a %, BPD/OFD X 100%
- dolichocephaly
brachycephaly
dolichocephaly - answer - CI <.70
- elongated head, BPD shortened
- seen with breech fetuses, holoprosencephaly and
oligohydraminos
brachycephaly - answer - CI >.85
- rounded head, BPD elongated more equal to OFD
- more commonly a normal variant than an
abnormality
- when abnormal can be associated with Trisomy 21
and spina bifida
Lateral Ventricles - answer - axial view of the
cranium at the level of the choroid plexus
- best visualized when the ventricular walls are
perpendicular to the US beam
- measure inner diameter of the posterior
ventricular horns; AKA atria measurement
- < 10mm NL
- measurement usually consistent throughout the
pregnancy
, Cerebellum - answer - transverse axial view of the
cranium at the level of the posterior fossa
- grows 1mm per week and normally measures the
same distance in millimeters as the number of
weeks in gestational age + or - 3 to 5mm; EXP.
25mm= 25 weeks gestation
cisterna magna - answer - transverse axial view of
the cerebellum and posterior fossa
- measure the largest AP diameter of the cavity
posterior to the cerebellum
- less than 10mm considered normal
Nuchal Skin Fold - answer - usually measured at
the base of the skull in the transverse oblique axial
plane demonstrating the CSP, 3rd ventricle,
cerebellum and CM
- measure the thickness of the skin at the base of
the skull from the outer edge of the occipital bone
to the outer edge of the skin
- normal <6mm at 15-21 weeks
- always be sure that the fetal chin is in a neutral
position and the neck is not hyperextended when
performing the measurement
2nd/3rd Trimester
questions and answers
twins have a faster or slower growth rate in the
3rd trimester compared to a singleton? - answer
slower
Biparietal Diameter (BD) - answer - axial view
- obtained at the level of the third ventricle, cavum
septum pellucidum and thalamus
- falx cerebri should be seen separating brain into
2 equal hemispheres
- measurement performed from leading edge to
leading edge of the skull or outer to inner borders
- can be measured as early as 10-11 weeks
-measurement most accurate for dating before 20
weeks
- less accurate than HC for gest age
- Corrected BPD-OFD is also used with the BPD to
take into account the shape of the fetal head
- BPDc= square root (BPD x OFD/1.265)
,Head circumference (HC) - answer - performed at
the same level as the BPD (thalamic view)
- more accurate for dating than standard BPD
- corrected BPD and HC are the most accurate
biometric measurements for dating before 20wks
- measure the skull circumference around the outer
edge of skull
- caliper placement is around the calvarial wall and
should not include scalp thickness
- very important to exclude skin thickness when
scalp edema is present
- reliable measurement that does not vary in
accuracy with fetal head shape
Occipital-Frontal Diameter (OFD) - answer -
performed at the same level as BPD
- measure from anterior skull to post skull on outer
edges of cranium
- corrected BPD calculated with OFD
- BPDc= square root (BPD X OFD/1.265)
Cepahlic Index (CI) - answer helps describe the
SHAPE of the head
-ratio of the BPD/OFD
- 0.70-0.85 WNL
,- can be described as a %, BPD/OFD X 100%
- dolichocephaly
brachycephaly
dolichocephaly - answer - CI <.70
- elongated head, BPD shortened
- seen with breech fetuses, holoprosencephaly and
oligohydraminos
brachycephaly - answer - CI >.85
- rounded head, BPD elongated more equal to OFD
- more commonly a normal variant than an
abnormality
- when abnormal can be associated with Trisomy 21
and spina bifida
Lateral Ventricles - answer - axial view of the
cranium at the level of the choroid plexus
- best visualized when the ventricular walls are
perpendicular to the US beam
- measure inner diameter of the posterior
ventricular horns; AKA atria measurement
- < 10mm NL
- measurement usually consistent throughout the
pregnancy
, Cerebellum - answer - transverse axial view of the
cranium at the level of the posterior fossa
- grows 1mm per week and normally measures the
same distance in millimeters as the number of
weeks in gestational age + or - 3 to 5mm; EXP.
25mm= 25 weeks gestation
cisterna magna - answer - transverse axial view of
the cerebellum and posterior fossa
- measure the largest AP diameter of the cavity
posterior to the cerebellum
- less than 10mm considered normal
Nuchal Skin Fold - answer - usually measured at
the base of the skull in the transverse oblique axial
plane demonstrating the CSP, 3rd ventricle,
cerebellum and CM
- measure the thickness of the skin at the base of
the skull from the outer edge of the occipital bone
to the outer edge of the skin
- normal <6mm at 15-21 weeks
- always be sure that the fetal chin is in a neutral
position and the neck is not hyperextended when
performing the measurement
