Fetal Biometry in Second & Third Trimester

Second & Third Trimester Fetal Biometry and Growth

• Fetal cardiac activity needs checking at the beginning of each ultrasound study to confirm the fetus is alive.
• If fetal demise or obvious anomaly is initially recognized, the sonographer should involve the physician immediately.

Second and Third Trimester Protocol

• Check for fetal cardiac motion and heart rate.
• Cervical length should be greater than 3 cm.
• Measure biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur length (FL), humerus length (HL), cerebellum/head circumference (CM), lateral ventricles, and amniotic fluid index (AFI).
• Understand and document presentation and lie.
• Utilize ADD (Additional Data Display) as appropriate.

Basic Protocol

• Document cardiac activity.
• Determine the number of fetuses.
• Estimate fetal age.
• Document measurements (BPD, HC, AC, FL, HL).
• Note fetal position.

Basic Protocol (Additional Information)

• Record placental cord insertion.
• Count the number of vessels in the cord.
• Examine the kidneys.
• Note umbilical cord insertion.
• Assess the bladder, stomach, and four-chamber heart/outflow tracts.
• Document placental position.

2nd & 3rd Trimester Ultrasound

• Measure cerebral ventricle width, cerebellum size, cisterna magna size, spine, diaphragm and heart situs.
• Measure 12 long bones, hands, and feet.

Cervical Length

• Measure cervical length from the internal to the external os.
• Avoid transabdominal measurements if the cervix cannot be clearly visualized, use transvaginal or translabial approaches instead.
• Cervical length should be 3 cm or longer.

Fetal Presentation

• Describe fetal lie in relation to the maternal long axis.
• If fetal lie is oblique, describe the quadrant of the uterus containing the fetal head and direction/position of the spine.

Fetal Presentation Types

• Vertex/Cephalic: Head down; relationship of fetal occiput to the maternal pelvis.
• Breech: Buttocks first; Frank breech may require C-section; complete breech may be turned; relationship of fetal sacrum to the maternal pelvis.
• Transverse: Lying sideways, look for placenta previa or a mass in late pregnancy, and prevent movement into vertex/cephalic presentation.

Type of Breech

• Complete breech: Fetal hips and knees flexed with feet pressed against chest.
• Incomplete breech: Buttocks present but one or more legs are extended.
• Frank breech: Buttocks are presented, but the legs are extended, with knees and feet near the head.

Situs

• Situs Solitus: Normal organ arrangement (right side gallbladder, apex of heart pointing to left, fetal aorta left of midline).
• Fetal aorta slightly to left of midline, anterior to spine; inferior vena cava is to right of midline and slightly more anterior to aorta.
• Document the right and left sides of the fetus to ensure normal situs.

Fetal Anatomy - Cranium

• Ossifies by 12 weeks.
• Check skull contour at highest levels; oval shape.
• Pathology or fetal death may distort skull shape.
• Brain structures more hypoechoic due to water content.
• Sulcus and gyrus are more echogenic.
• Becomes more complex as pregnancy progresses.

Cranial Anatomy

• Examine the cerebellum, choroid plexus, cisterna magna, lateral ventricles, midline falx, and cavum septum pellucidum (CSP).

The Cranium

• Midline falx is an important landmark because its presence indicates separation of the cerebrum.

The Cranium - Major Scan Planes

• Three major planes (axial (transverse) is used for most scans except the cerebellar view, which is axial/oblique) accommodate three views:
• Thalamic view (taken at the level of BPD and HC measurements).
• Ventricular view (obtained through the atrium of the lateral ventricle).
• Cerebellar view (angled through the posterior fossa, cisterna magna).

Ventricular View

• Fetal ventricular system consists of two paired lateral ventricles, a midline third ventricle, and a fourth ventricle adjacent to the cerebellum.
• Contains cerebrospinal fluid (CSF).
• Choroid plexus tissue produces CSF within the lateral ventricles.
• Located within the roof of each ventricle, except at the frontal ventricular horns.

The Cranium: Lateral Ventricles

• Measure the posterior horn of a lateral ventricle (atrium) directly across the posterior portion, perpendicular to the long axis of the ventricle not the falx.
• Calipers positioned at the junction of the ventricular wall and lumen/cavity.
• Normal atrium measures 6.5 mm.
• Measurements of >10 mm warrant serial imaging and further evaluation.

The Cranium: Choroid Plexus

• This should be assessed but not measured.

Transthalamic Plane

• Identifying cavum septum pellucidum, thalami, cerebral peduncles, and third ventricle.

Thalamic View

• Identify thalami and third ventricle.
• Identify the calvaria.

The Transcerebellar Plane

• Locate the cerebellum, cerebellar vermis, and cisterna magna.
• Include measurement of cavum septi pellucidi.

Nuchal Skin Fold

• Measure nuchal skin fold thickness in the plane containing cavum septi pellucidum, cerebellum, and cisterna magna.
• 5 mm or less up to 20 weeks' gestational age is normal.

The Thorax

• Evaluate fetal breathing, size, texture, and location of masses.
• Note bordering structures such as the heart (medially) and rib cage (laterally).
• Observe the diaphragm (inferiorly).

The Thorax (Homogeneous Lungs)

• Lungs are lateral to the heart, superior to the diaphragm.
• Include stomach, lungs, and placenta in the view.

The Heart

• The heart is more transverse in the fetus compared to an adult.
• The apex of the heart points to the left anterior chest.
• Observe the right and left ventricles in a four-chamber view.
• Observe the four chamber of the heart taken with a beam perpendicular to the heart or in a view with a perpendicular beam.

The Heart (Four-Chamber View)

• Locate the fetal stomach and angle the transducer towards the head to visualize the four chambers (locate the fetal heart with the beam perpendicular to the heart, or a perpendicular view).
• Document position, situs, axis, and if there are equal-sized right and left ventricles/atria.
• Check for an interventricular septum and normal placement of tricuspid and mitral valves.
• Document normal rhythm and rate.

The Heart - Echogenic Foci

• A bright echogenic structure within the heart chamber, which persists despite transducer position changes, is considered an echogenic intracardiac focus (EIF).
• Normal variation within many normal pregnancies.
• May be associated with increased risk of aneuploidy and cardiac defects.

Fetal Cardiology

• AIUM/ACR standards include four-chamber views, fetal heart position, and LVOT and RVOT if applicable.
• With outflow tracts, 83-85% sensitive for detecting anomalies.

Four-Chamber View

• Label each chamber (RV, LV, RA, LA).
• Measure each chamber.
• Check for symmetry in the ventricular septum.
• Observe the atrial septum.

Left Ventricular Outflow Tract

• Identify the left ventricle (LV), right ventricle (RV), interventricular septum (IV septum), aorta.
• Document the aorta's normal caliber,+/- LA, +/- RA.
• Note medial wall ascending aorta, merges with top IV septum.
• Document possible pathology (VSD, tetralogy of Fallot, transposition, truncus arteriosus).

Right Ventricular Outflow Tract

• Identify the branching of the main PA into the right PA and ductus arteriosus.
• Ascending aorta should be seen in cross section with the descending aorta to the left of the spine.
• Confirm the PA crosses the anterior to the ascending aorta.
• Note possible pathology (transposition, truncus arteriosus).

Thoracic Vessels

• Assess the fetal aortic arch, pulmonary artery, trachea, and esophagus.

Ductal Arch/ductus arteriosus

• Confirm correct orientation and communication between the aorta and pulmonary trunk.
• Look for a hockey-stick appearance.
• Note the placement of the ductus arteriosus (anteriorly).

The Diaphragm

• Note the diaphragm's appearance, which differs from that of an adult.

The Hepatobiliary System and Upper Abdomen

• The liver is the largest organ in the upper abdomen; the left lobe is usually larger than the right lobe.
• The liver appears as pebble-gray and is discernible by its corresponding portal and hepatic vessels.

The Hepatobiliary System and Upper Abdomen (Liver Image)

• Observe the liver, diaphragm, and bowel.

The Gastrointestinal System

• Fetal bowel is typically larger than the fetal liver, and can be hyperechoic (similar in echogenicity to bone).
• Assess the presence of an echogenic bowel (small bowel as fluid-filled rings).
• Fetal bowel with hyperechogenicity is associated with risk of aneuploidy and neonatal/childhood pathology.

The Hepatobiliary System and Upper Abdomen (abnormal fluid around liver)

• Check for fluid collections around liver margins.
• Ascitic fluid, fluid retention, anemia, or congenital anomalies can indicate fluid collection.
• Assess for liver masses; they are not common but may be present.

The Hepatobiliary System and Upper Abdomen (Gallbladder)

• Examine the fetal gallbladder, which appears as a cone- or teardrop-shaped cystic structure located below the left portal vein in the right upper abdomen, to distinguish it from the left portal vein.

Gallstones and Sludge

• Evaluate for gallstones and sludge.

The Gastrointestinal System (Stomach)

• The fetal stomach is present early, filling as a result of swallowing amniotic fluid;
• full stomach should be seen in all fetuses beyond 16 weeks.

The Urinary System

• Evaluate the kidneys and bladder.
• Adrenal glands are usually more prominent in the fetus than in adults and positioned adjacent to the kidneys.
• Kidneys appear as ovoid retroperitoneal structures with indistinct borders during the second trimester.
• Renal pelvis measurement is in sagittal view when fluid is present; it is considered abnormal above certain measurements related to gestational age.

The Urinary System (Adrenals)

• Assess the adrenals, usually seen by the 20th week and taking a transverse view above the kidneys; they should appear as rounded, bean-shaped structures.

Kidneys vs Adrenals (Image)

• Differentiate between kidneys and adrenals in a transverse plane.

The Urinary System (Bladder)

• The bladder may not always be completely full due to decreased fluid ingestion.
• The bladder refills normally.

The Genitalia

• Assess for gender identification to identify possible risk for gender-linked disorders, such as aqueductal stenosis or hemophilia, or for determining chorionicity in multiple pregnancies.

The Upper and Lower Extremities

• Check for the presence of legs and arms;
• Short femur and short humerus signify increased risk for aneuploidy.
• Assess for unusual anatomic configurations of fetal limb bones such as bowing, fractures, or demineralization (common in skeletal dysplasia).
• Evaluate hand and finger movement within a sagittal plane or evaluate hand clench throughout examination.
• Individual fingers can often be counted in the first trimester of pregnancy.

The Upper and Lower Extremities (Hand Observation)

• Note if hands are clenched in examination or if finger movements are normal.

The Upper and Lower Extremities (Measurement of Bones)

• Evaluate bone length as an important part of assessing fetal growth.
• Femur is measured from the neck of the femur to its distal end, not including the epiphyses, to assess the ossified portion.
• Measure all long bones if a significant difference is noted in timing compared to other parameter measurements.

The Upper and Lower Extremities (Humerus)

• Assess the humerus for any abnormalities in ossification or length in comparison to femur length.

The Upper and Lower Extremities (Tibia and Fibula)

• Measure tibia and fibula length.
• Tibia will be longer and fibula will be thinner and lateral to tibia.

The Upper and Lower Extremities (Radius and Ulna)

• Measure radius and ulna.
• Ulna will be longer than radius on proximal and will be the same on distal views.

Ocular Measurements

• Measure interocular distance (ICD), binocular distance, and ocular diameter.
• Measure these parameters if fetal position prohibits more complete fetal assessment.

Fetal Weight

• Sonography is not completely precise for assessing fetal weight, and accuracy decreases with increasing gestational age and particularly if assessing post-term fetuses.
• Serial sonograms are helpful for monitoring weight changes and documenting fetal growth.

Extra Fetal Obstetric Evaluation

• Evaluate the placenta, amniotic fluid, and pelvis.

Umbilical Cord

• Examine the umbilical cord, noting the presence of two arteries and one vein.
• Cord twists may be considered associated with poor pregnancy outcomes.
• The umbilical cord is easily imaged on both sagittal and transverse sections.
• Umbilical vein diameter increases throughout gestation and reaches a maximal diameter of 0.9 cm by 30 weeks.
• Assess for placental insertion and location; velamentous insertion is atypical.

Umbilical Cord (Insertion, etc.)

• Document placental insertion, important for procedures like amniocentesis.
• Report atypical insertion as velamentous insertion.

Placenta (Image)

• View the placenta for assessment of size, position, and other features.

Amniotic Fluid

• Amniotic fluid permits fetal movement, maintains intrauterine pressure, protects the fetus from injury, and results from production by the cord and membranes as well as the lungs, skin, and kidneys.
• Oligohydramnios is low amniotic fluid; polyhydramnios is high amniotic fluid.

Amniotic Fluid (Images)

• Locate and assess the amniotic fluid.

Membranes

• Assess the amnion and inner/outer membranes, typically not visualized during the second and third trimesters.
• The amnion is contiguous with the membrane lining the umbilical cord; at the umbilical cord's insertion point, the amnion expands over the surface of the chorionic part of the placenta.

Estimation of Fetal Age & Measurements

• Use Naegele's rule (LMP + 3 months + 7 days) for due date estimations.
• Utilize CRL, BPD, HC, AC, FL, and HL for more accurate estimations.

Estimation of Fetal Age: Second Trimester Measurements

• Measure BPD, HC, AC, FL, and HL.

Additional Measurements

• Measure cerebellum size/occipitofrontal diameter (OFD); lateral ventricles/cephalic index (CI).

Biparietal Diameter (BPD)

• Assess the BPD, including the falx, thalamus, third ventricle, cavum septum pellucidum.
• Calipers are placed outer-to-inner, and the angle should be 90 degrees between 10- and 12-weeks gestation, decreasing in reliability after the 26th week.

BPD and HC

• Assess the BPD/calvaria, third ventricle, thalami, and CSP.
• Ensure that the calvaria is smooth and symmetrical;
• Assess the presence of CSP within the brain's anterior portion.
• Position calipers circumferentially around the calvarial wall without including the calvarial wall.

Occipitofrontal Diameter (OFD)

• Measure OFD.
• Use image containing BPD for better accuracy.
• Ensure 90-degree perpendicularity to the fetal BPD.

Cephalic Index

• Calculate cephalic index (CI) using the formula C.I. = BPD / OFD x 100.
• A CI of 0.7 to 0.86 is normal; below 0.7 is dolichocephalic; above 0.86 is brachycephalic.

Head Circumference (HC)

• Measure HC on the same image as BPD.
• Don't include the scalp/calvaria.
• HC is more accurate than BPD for dolichocephalic or brachycephalic heads.
• Don’t consider cerebellum in measurement area.

Additional Measurement Techniques

• Document the lateral ventricles in an axial view.
• Measure across the ventricle, near the posterior tip of the choroid plexus, perpendicular to the axis of the lateral ventricle in the axial view.

Cerebellum, Cisterna Magna

• Assess the cerebellum and cisterna magna in an axial view of the posterior fossa.
• Useful for gestational age determination when other measurements aren't possible.

Cerebellum/Cisterna Magna Measurements

• Assess cerebellum size and shape, along with vermis connection.
• Measure cerebellar width.
• Measure cisterna magna depth: normal 2-11 mm, average 5-6 mm.

Cerebellum/Cisterna Magna Measurements (Transverse Plane)

• Assess the cerebellum in a transverse plane, measuring across the cerebellar hemispheres at the level of the cerebellar hemispheres.
• Measure from the vermis to the inner skull.

Abdominal Circumference (AC)

• Assess the liver, which is the largest abdominal organ.
• AC reflects growth aberrations, such as growth restriction and macrosomia.
• Measure AC at the point of the liver's greatest transverse diameter.

Abdominal Circumference (Measurement Plane)

• Measure AC in a plane positioned with the umbilical portion of the left portal vein positioned midline within the liver, with the curve towards the right.
• Measure across skin line to encompass soft tissues and subcutaneous fat, and note fetal spine, umbilical vein, portal sinus, and adrenal glands.

Abdominal Circumference (Observation Checklist)

• Verify position with fetal spine, fetal stomach, umbilical vein and portal sinus, and adrenal glands for appropriate placement and symmetry.

Femur Length (FL)

• Measure length from the neck of the femur to the distal end of the femoral condyle, excluding epiphyseal parts.
• FL remains less affected than other measures by IUGR.
• Combination of FL, BPD, and AC are used to determine fetal weight; FL is less reliable in establishing gestational age.

Femur Measurement

• Place calipers on the ossified part of the femoral diaphysis, measuring from end to end.

Humerus Length (HL)

• Measure HL less affected by parental height than FL and is a stronger marker of Down Syndrome than short femur.
• Measure is from the ossified part of the diaphysis.
• Identify fetal spine, humerus, and scapula for proper placement.

Humerus Measurement (Procedure)

• Obtain an image of the fetal spine in the upper thoracic/lower cervical region and identify the scapula.
• Rotate transducer until the long axis of the humerus is clearly visible. Measure the humeral shaft only (diaphysis); do not include any proximal humeral points.

Tibia and Fibula Measurements

• Notice that the tibia is longer than the fibula and the fibula is thinner, lateral to the tibia.
• Measure the length of tibia and fibula from point to point on image.

Radius and Ulna Measurements

• Locate the ulna and radius.
• Ulna is longer than radius on the proximal view and both are of equal length on the distal view.

Ocular Measurements (Procedure)

• Locate the lateral orbital rim.
• Perform measurements if necessary.

Fetal Weight (Important Points)

• Document weight changes from serial sonograms to assess changes.