SON 2121 Obstetrical Sonography I Lecture 7 Ultrasound evaluation of Fetal Biometry and Normal and Abnormal Fetal Growth Holdorf SON 2121 Obstetrical Sonography I

Things to consider in this Lecture: Biparietal Diameter Head circumference Abdominal Circumference Femur Length Epiphyseal Appearance Fetal weight Other Parameters Fetal position

A multitude of techniques exist for determining gestational age in the second and third trimesters. The most accurate sonographic method is measurement of the crown-rump length in the first trimester. This has a plus or minus range of 4 days. In the second and third trimesters, all of the following techniques have a range of approx. plus or minus 2 weeks. At least two separate measurements should be made of each parameter used. When a significant disparity in those two measurements occur, additional measurements should be taken.

Some rules of thumb for estimating gestational age in the second and third trimesters include: Multiple measurements should be used: BPD, HC, FL and AC If the AC falls more than 2 SD from the mean for GA, look for additional signs of IUGR If the FL falls short more than 2 SD, look for other signs of skeletal dysplasia If the head measurements and or AC fall short, calculate the ratio of the HC/AC

Standard Deviation Each band has a width of one standard deviation.

What is Standard Deviation? A measure that is used to quantify the amount of variation or dispersion of a set of data values.  A standard deviation close to 0 indicates that the data points tend to be very close to the mean (also called the expected value) of the set, while a high standard deviation indicates that the data points are spread out over a wider range of values.

What is the MODE and Range? To find the median, numbers must be listed in numerical order. The "mode" is the value that occurs most often. If no number is repeated, then there is no mode for the list. The "range" is just the difference between the largest and smallest values.

What is the MEAN? The mean is the average of the numbers. It is easy to calculate: add up all the numbers, then divide by how many numbers there are. In other words it is the sum divided by the count.

What is the Median? To find the Median, place the numbers given in value order and find the middle number. Example: find the Median of 13, 23, 11, 16, 15, 10, 26. The middle number is 15, so the median is 15. (If there are two middle numbers, you average them.)

Who is Dr. Hadlock? Dr. Hadlock- investigated the role of ultrasound in assigning gestational age and defining abnormal fetal growth patterns. (His formula for weight prediction is still being used in most ultrasound scanning equipment.)

What does the following Statement Mean? “The greatest accuracy per medical dollar spent is achieved in the first trimester and the greatest value per medical dollar spent is achieved in the early second trimester”

The most accurate measurement of a fetus is made during the 1st trimester- the CRL would be the greatest accuracy per medical dollar spent being achieved. The greatest value per medical dollar spent is achieved in the early 2nd trimester because that is when the most measurements are done, the HC, BPD, AC, FL NOTE: The least accurate dating method is the AC.

Cephalic Index The CI can be used to determine whether head shape is appropriate. In cases of dolicocephaly, CI<75% or Brachycephaly CI>81% the BPD is eliminated from gestational age determination.

Cephalic Index

Scaphocephaly/dolicocephaly =describes a head with a long, narrow shape.

BPD: Biparietal Diameter Axial section through fetal head at the level of the thalami and cavum septum Pellucidum. Measure outer surface of near parietal bone to inner surface of far parietal bone (leading edge to leading edge)

BPD Cont… First measurement between 10-12 weeks. Angle of asynclitism: derived from clinical obstetrics. Refers to the presentation of the fetal head with the axis oriented obliquely to the axial planes of the pelvis. Sonographically, it refers to the relationships between the fetal cranial sagittal suture and the central portion of the us beam. For an exact BPD, the angle should be 90 degrees.

BPD

BPD

What is the Cavum Septum Pellucidum?

CSP CSP is a marker for fetal neural maldevelopment. The septum Pellucidum is a thin, triangular, vertical membrane separating the anterior horns of the left and right lateral ventricles of the brain

The Lateral Ventricles of the Brain

What is the Thalamus? The thalamus  is a midline symmetrical structure of two halves, within the brain, situated between the cerebral cortex and the midbrain. Some of its functions are the relaying of sensory and motor signals to the cerebral cortex, and the regulation of consciousness, sleep, and alertness. The two parts of the thalamus surround the third ventricle. It is the main product of the embryonic diencephalon.

What is the Flax Cerebri? The falx cerebri is also known as the cerebral falx, named from its sickle-like form. It is a large, crescent-shaped fold of meningeal layer of dura mater that descends vertically in the longitudinal fissure between the cerebral hemispheres. 

What are the choroids? Network of blood vessels in each ventricle of the brain. They derive from the pia mater and produces the cerebrospinal fluid.

One of the nicest BPDs you will ever see…?

HC Head Circumference Measured at the same level as BPD Circumference should not include scalp echoes when using electronic ellipse software. When occipito-frontal OFD measurements are used in conjunction with BPD, the OFD should be measured Outer edge to outer edge. More accurate than BPD when the fetal head is dolichocephalic or brachycephalic

Head circumference

Head Circumference

AC Abdominal Circumference Measured at the level of the portal vein and stomach Circumference measurements –both ellipse and linear- should include soft tissue. The least reliable in establishing gestational are due to significant genetic and physiologic variations in size After 25 weeks.

Abdominal Circumference

Anatomy that must be seen in the Abdominal Circumference The Abdominal Circumference must include the portal section from the umbilical  vein, the stomach and a true cross section of the spine with 3 ossification centers.  It should be circular in shape.

Abdominal Circumference

FL Femur length Best obtained with a linear array transducer (eliminates artifactural bowing due to sector beam geometry. Includes only ossified diaphysis, excludes Epiphyseal cartilage. If femur length falls below 2 SD of the mean, short limb dysplasia may be present, and other long bones should be measured.

Femur Length

Femur Length

Short femur length

Upper femur length

Epiphyseal Appearance Gestational age can be estimated by identifying the presence of ossification centers within various epiphyses. The identification of ossified epiphyses can be helpful in cases where the other parameters are not concordant or when dates are uncertain. Distal femoral Epiphyseal appearance (DFE) can be seen at 33 weeks Proximal tibial Epiphyseal appearance (PTE) can be seen at 35 weeks Proximal humeral Epiphyseal appearance (PHE) can be seen at 38 weeks

Epiphyseal Appearance

Fetal Weight Sonography is an inaccurate predictor of absolute weight, and is less accurate the greater the GA as well as in term and post- term fetuses. Changes in weight from previous sonograms may be used as an indicator of growth Significant variation in normal fetal weight is common, especially in the 3rd trimester.

Fetal Weight One pound = 453.592 grams.

Other Parameters A variety of other biometric techniques have been established to estimate fetal age: Long bone length (radius, ulna, tibia, fibula Humeral length Clavicular length Foot length

Humeral length

Fetal foot length- close to a femur length

Binocular distance: measured from lateral orbital rim to lateral orbital rim. May be useful in assessing Gestational age in growth restricted fetuses, or when obtaining other head measurements is impossible Due to fetal position / cannot obtain a proper BPD

Biocular Distance

Biocular Distance

Transcerebellar distance: Measured from lateral aspects of cerebellum in an axial plane of section. May also be useful in assessing gestational age in growth restricted fetuses, or when obtaining other head Measurements are not possible to obtain.

Transcerebellar distance

Measuring the Cerebellum/posterior fossa

Just what is the cerebellum? The cerebellum is the area of the hindbrain that controls motor movement coordination, balance, equilibrium and muscle tone.

Fetal Position/Presentation Presentation relates to the fetal part closet to the internal cervical os/birth canal.

Cephalic: Head presenting Most common of all types of presentation. Various subdivisions exist determined by the degree of flexion of the fetal head: Vertex-most common of the cephalic presentations at term. Parietal bones presenting. Sinciput-deflected vertex (Sinciput refers to the forepart of the skull) Brow-cannot deliver vaginally if fetus is term

Vertex presentation/Cephalic

(A) vertex, (B) Sinciput, (C) brow, and (D) face presentations

Breech: Fetal head in fundus of the uterus Types include: Footling-hips extended, feet presenting Frank-hips flexed, knees extended

Breech

Breech-footling/feet presenting

Transverse fetal lie