Fetal Measurements

Chapter 10

Which measurements in pregnancy are of most importance

measurements taken early on

Naegele’s rule

is a formula used to estimate a due date by calculating the first day of the last menstrual period, adding one year, subtracting three months, and adding seven days.

Up until when is the first trimester

13 weeks and 6 days

What measurements are taken in the first trimester

MSD, YS, CRL, and NT

When is a GS usually seen first transvaginally

around 4 to 5 weeks gestation

When is the MSD done

when there is no fetal pole yet to be measured

How is the MSD measured

from inner to inner of the gestational sac walls.

What makes the GS vary in shape

fibroids, uterine shape, pelvic pathology, or an overfilled bladder

When is MSD best used

when a blighted ovum is suspected or if the MSD-CRL is <5mm

What is the normal YS measurement

<6mm

An enlarged or collapsed yolk sac indicates what?

that the pregnancy is unviable

The yolk sac is measured from

the inner to inner border is sag or transverse

What is the most accurate measurement in the first trimester

the crown-rump length (CRL)

When is the most optimal time to use CRL for dating

6-9 weeks

What position must the fetus be in to measure NT

in a midsagittal plane with the fetal neck in a neutral position

In order to measure NT

the CRL must be 45-84mm

What are the landmarks needed for a BPD

the CSP, thalamus, and falx cerebri

How is the BPD measurement taken

from the outer to inner skull, perpendicular to the falx

When is the BPD the most accurrate

at 14-28 weeks

Microcephaly

small head below 2 standard deviation with BPD/HC <3%

Macrocephaly

abnormally large head above 2 standard deviations, often indicated by increased BPD/HC ratio > 98%

Ventriculomegaly

a fetal condition where the fluid-filled ventricles in the brain are enlarged

Is HC dependent on the fetal head shape

no it is a shape-independent measurement

Closer to term, what is the more accurate measurement

of head circumference (HC) to assess fetal growth.

OFD- occipital frontal diameter

this measurement is taken from the outer edge of the frontal bone to the outer edge of the occipital bone

Cephalic index

used to assess the shape of the baby’s skull by calculating the ratio of the BPD and OFD to help identify fetal abnormalities

Dolichocephaly

a narrow fetal head that will have a short BPD and a CI below normal

Dolichocephaly is aka

scaphocephaly

Brachycephaly

a fetus with a short and round head that will have a long BPD and a CI above normal.

Abdominal circumference

the measurement of the circumference of the fetal abdomen, used to assess fetal growth and development.

What must be visible when taking the AC

the junction of the umbilical and portal vein facing away from the stomach, and the stomach

Where should the spine be when measuring AC

in a 3 or 9 o’clock position

What should not be visible when taking AC

the kidneys, heart, diaphragm and lungs

What is the most important measurement to determine fetal weight

the abdominal circumference

When there is an abnormality in the femur what should be measured

any other long bone not affected

How must the femur be on the screen for the measurement to be accurate

it must be perpendicular to the sound beam and straight across the screen

Which femur should be measured

the one closest to the transducer/ anterior

What should be included when measuring the femur

only the ossified diaphysis

How much will a fetus gain in the third trimester

1 pound every 2 weeks

When is the second trimester

14-28 weeks

When is the third trimester

28-40 weeks

Early anatomy scan

done between 16-18 weeks usually for high risk patients

Anatomy scan

done at around 20-24 weeks to ensure fetal anatomy and development is normal.

Growth scan

done from 32 weeks until delivery

Lateral ventricles

transverse view of the brain at the level of the choroid plexus

How are the lateral ventricles measured

inner to inner diameter at the atrium of the lateral ventricles

What is the normal lateral ventricles measurement

<1cm

When are the lateral ventricles most prominent

at 19-25 weeks

Which ventricle will be clearer

the posterior ventricle

Transcerebellar diameter

transverse view of the brain, it is the most opsterior and inferior view

How is the cerebellum measured

from top to bottom, including the echogenic borders

The cerebellum grows

1 mm per week until 24 weeks

Why is the cerebellum a good assessment of gestational age

because it is independent of cranial shape and not affected by growth disturbances

Until when can the cerebellum diameter be used to assess fetal age

up to 30 weeks

Cisterna magna

a cerebrospinal fluid-filled space that lies directly behind the cerebellum

How is the CM measured

the largest diameter of the cavity posterior to the cerebellum usually from the vermis to the inner skull

What is the normal cisterna magna measurement

< 10mm

What is a normal finding within the cisterna magna

linear septations

Nuchal fold

the skin behind the neck of the fetus

How should the nuchal fold be measured

from the outer edge of the occipital bone to the outer edge of skin

What is the normal nuchal fold measurement

< 6 mm

Up until when is the nuchal fold measured

21-22 weeks

What is an increased nuchal fold a soft marker for

chromosomal abnormalities such as Down syndrome.

Hypotelorism

when the eyes are too close together

Hypertelorism

when the eyes are too far apart

What measurements are done to measure the orbits

BOD and IOD

BOD- binocular distance

measures the distance from the outer edge to the outer edge of the orbits

BOD is aka

outer orbital distance

Interoccular distance IOD

measures the distance between the inner edges of the orbits.

Nasal bone

the length is measured during an anatomy scan to assess for a shortened or absent nasal bone that is associated with trisomy 21

Macrosomia

when a baby weighs more than 4500 grams at birth in non diabetic mothers and 4000 grams in diabetic mothers

LGA- large for gestational age

refers to a fetal size that is above the 90th percentile for gestational age, indicating potential growth issues.

What is the difference between macrosomia and LGA

macrosomia is with weight and LGA is percentiles