Normal Fetal Chest, Abdomen, and Pelvis

Fetal Thorax

• assess size, shape, and symmetry of the thorax

• echogenicity and symmetry of the lungs

• size, location, and axis of the heart

• presence of complete diaphragm

• presence/absence of fluid collections

Thorax

• always scan through the thorax in transverse an longitudinal planes

• assess the bony elements of thorax:

  • clavicles

  • ribs

  • scapula

  • vertebral bodies

  • sternum

Clavicles

• not a routinely imaged part of the body

• imaged in the presence of skeletal dysplasias and other rare suspected syndromes

• can be measured and used as a tool to determine GA

• Ossify as early as 8-9 weeks

Ribs

• Seen in transverse and longitudinal planes

• ossify around 10-12 weeks

• assess rib thickness, check for fractures, & symmetry

• not routinely imaged

Scapula

• not routinely imaged

• ossification around 21-27 weeks

Spine

• imaged in transverse and longitudinal planes

• identify skin line - looks for breaks in skin (spina bifida)

• curvature and normal alignment of vertebral bodies

Lungs

• Embryologic development of the lungs occurs around week 5

• at 16-24 weeks the normal number of bronchi are formed

• after 24 weeks the air spaces, blood vessels and capillaries change and increase in number to prepare for life outside the womb

In order for lung development to occur:

• Adequate space in the thorax

  • ↓ constriction = lung volume

• Fetal breathing motion

  • without ability to “practice” breathing → pulmonary hypoplasia

• Fluid within the lungs → distends airways

• Amniotic fluid

How does amnionic fluid aid in lung development?

• Amniotic fluid is vital

• Without fluid = pulmonary hypoplasia = usually lethal

Lungs U/S appearance:

• Symmetric & homogenous

• Early in fetal life lungs echogenicity ≤ to the liver

• Later in pregnancy lungs echogenicity ≥ to the liver

• by volume : right lung is slightly larger than the left

• Ultrasound cannot determine lung maturity

• Fetal breathing does not indicate lung maturity

Diaphragm

• Muscle that separates the chest from the abdomen

• Fetal heart and lungs should be visualized superior to the diaphragm

• Fetal stomach, liver, and bowel should be below the diaphragm

• Appears as hypoechoic band between heart and stomach in the longitudinal and coronal planes

Abdomen

• Superior border is the diaphragm

• Identify stomach, liver, kidneys, adrenal glands, gallbladder, bowel, cord insertion, bladder, and genitalia

• Spleen and pancreas are not routinely imaged and can be difficult to identify

Stomach

• Anechoic stomach “bubble” should be positioned on the left side of the abdomen

• Check for single “bubble”

• Non-visualization of the stomach, especially in the presence of polyhydramnios may indicate abnormality

• If the stomach is still not visualized at the end of the scan, the exam should be repeated in 30 minutes

• Echogenicities or debris may be visualized within the stomach and are normal - usually vernix

Bowel

• Bowel appears somewhat echogenic

• After 20 weeks the large and small intestines may be distinguished

• Colon can be seen most commonly in the 3rd trimester

• Small bowel should not exceed 6-7 mm

• Colon should not exceed 23 mm

Liver

• Large compared to other abdominal organs

  • 10% of fetal weight at 11 weeks

  • 5% at term

• Located in the RUQ

• Homogeneous echotexture

Gallbladder

• Should be visualized after 20 weeks

• Tear-drop shaped

• Anechoic

• Located in the RUQ

• Passive in fetal life but can develop stones

Pancreas

• Difficult to visualize due to lack of fatty tissue therefore it is not routinely imaged

• Echogenicity slightly more than the liver

Spleen

• DIfficult to image - Not routinely imaged, but can be visualized after 18 weeks

• Homogeneous texture similar to the kidneys, less echogenic than the liver

• best imaged in the transverse plane

• posterior and to the left of the fetal stomach

Adrenal Glands

• Elongated oval to triangle shaped structures superior to the kidneys

• Hypoechoic rim with echogenic center “stripe”

• Seen as a “cap” of the superior pole of the kidney

• Can be included in the abdominal circumference or APD / AD measurements

Kidneys

• Paired hypoechoic structures located on either side of the spine

• Can be imaged in sagittal plane as well - appear bean shaped

• Can be imaged as early as 12 weeks

• Renal pelvis can be visualized - anechoic

• Ureters not usually visualized unless abnormality is present

Renal to Abdominal Circumference ratio should be:

0.30 throughout pregnancy

Renal Pelvis

• Up to 20 weeks = 5mm or less

• 20-30 weeks =8 mm or less

• 30 weeks to term = 10 mm or less

Abdominal Cord Insertion

• Vessels can be visualized as they enter fetal abdomen

• assess w/ color and 2D

• 2 arteries and 1 vein (Mickey Mouse Sign)

• Be sure both arteries can be visualized coursing around fetal bladder (confirms 3vc)

• Be sure abdominal wall is intact at the area of the cord insertion

Bladder

• Visualized in lower pelvis

• Anechoic

• Fetus urinates approx. 1-2 times per hour

• If bladder is not visualized throughout exam, repeat in 20-30 min

• if bladder cannot be visualized and there is oligo/anhydramnios → most likely kidney anomaly

Male Anatomy

• “Turtle” sign

• Penis and scrotum can be visualized

• Testes descend by 32 weeks

• Hydroceles are common

Female Anatomy

• “Hamburger” sign

• 3 lines → labia majora with central labia minora

Thoracic Circumference

• Measured at the level of the 4 chamber heart

• Compared to Abdominal Circumference to evaluate fetus for pulmonary hypoplasia

• TC/AC ratio should = 0.89 throughout pregnancy

• Not a routine measurement

Abdominal Circumference (AC)

• Routine measurement to calculate EGA / EFW

• Transverse plane

• Landmarks:

  • Umbilical Vein

  • Stomach

  • Adrenal glands (may/may not be in image)

• DO NOT include lungs or kidneys in image

• Be on axis → round abdomen without “off axis” ribs

Anterior to Posterior Diameter (APD)

Measured from outer edge of skin to outer edge of skin from transverse spine to anterior abdominal wall through J-hook

Transverse Abdominal Distance (TAD)

Measured from outer edge of abdominal skin at 90° angle to APD line

AC can be calculated by:

AC = APD + TAD x 1.57