Fetal Heart

• Heart


  A rounded structure that should be assessed from multiple angles and positions to ensure that even the smallest abnormality is not missed
• Heart size


  • More than doubles from 13 weeks to 19 weeks, therefore there are anomalies which may not be diagnosed at the early structural scan
• Heart beat


  Transvaginal ultrasound can detect a heart beat as early as 34 days of gestation
• Fetal heart rate


  • Before 6 weeks, averages between 100 and 115 beats per minute
  • Peaks at 8 weeks gestation between 144 and 159 BPM
  • After 9 weeks, plateaus at 137 to 144 BPM
  • Normal fetal heart rate is between 115 and 160 bpm
• Pulsed doppler


  May be used to assess the fetal heart rate in second trimester as long as the ALARA principle is adhered to
• Fetal rhythm abnormalities


  Occur in up to 2% of pregnancies and include fetal heart rates that are irregular, too fast or too slow
• Situs


  It is important to determine the left side of the fetus and the correct position of the heart axis and the stomach
• Right ventricle


  • Should be the most anterior part of the heart and can be determined by looking for the moderator band
• 4 chamber heart


  The transverse view should only contain 1 rib. If you can only get a 4 chamber view with more than this it could be a sign of cardiac pathology. The heart should occupy approximately 1/3 of the chest. The apex is orientated 45 degrees to the left. The right ventricle contains the moderator band.
• Atrioventricular valves


  The tricuspid valve is on the right between the right atrium and right ventricle, the bicuspid or mitral valve is the valve on the left between the left atrium and left ventricle
• Foramen ovale


  A shunt that bypasses the lungs and moves blood from the right atrium of the heart to the left atrium
• 3 vessel view


  Also known as the PAV (pulmonary artery, ascending aorta and superior vena cava) view. The 3 vessels should be positioned in this sequence of reducing size.
• Interventricular septum


  • Should be assessed when the fetus is in a decubitus position so the ultrasound beam is perpendicular to the septum, to avoid anisotropy and a false positive for septal defect. It should be assessed in both B-mode and Colour Doppler.
• Aortic arch


  Arises from the central heart and is a "walking stick" shape. The neck vessels can be seen arising from the transverse portion of the aorta.
• Ductal arch


  Has a "hockey stick" shape with the ductus making the superior portion appear flat, whereas the aortic arch is a "cane stick" shape.
• The role of ultrasound is to confirm normal anatomy to the best of our ability, and to progress or elaborate on known fetal heart pathology.
• Fetal circulation


  Circulation in the fetus before birth
• Structures associated with fetal circulation

  • Placenta
  • Umbilical vein
  • Ductus venosus
  • Foramen ovale
  • Ductus arteriosus
  • Umbilical arteries
• Fetal blood flow


  1. Blood rich in nutrients and oxygen flows through the umbilical vein to the ductus venosus

  2. Blood flows from the ductus venosus into the inferior vena cava up to the right atrium

  3. Blood mixes in the right atrium with blood from the superior and inferior vena cava

  4. Some blood flows to the right ventricle, some through the foramen ovale to the left atrium and left ventricle

  5. Blood pumped from the right ventricle flows into the pulmonary artery but mostly through the ductus arteriosus to the aorta

  6. Blood flows from the body back to the placenta through the umbilical arteries
• Fetal blood flow


  Oxygenated blood from the placenta reaches the coronary arteries and cerebral circulation
• Fetal blood flow


  Most blood bypasses the lungs through the ductus arteriosus due to high resistance in the pulmonary circulation
• Fetal blood flow


  Higher pressure in the right atrium promotes flow through the foramen ovale to the left atrium
• Only 8% of right ventricular output flows into the lungs in the fetus</b>
• Transition to postnatal circulation

  1. Umbilical cord clamped, placenta removed from circulation

  2. Systemic vascular resistance rises

  3. Lungs expand, pulmonary vessels dilate, pulmonary pressures decrease

  4. Foramen ovale and ductus arteriosus close

  5. Ductus venosus and umbilical vessels become ligaments
• Foramen ovale may not close completely in 15-25% of adults
• Ductus arteriosus closes completely in 4-10 days after birth
• Ductus venosus usually closes within 3-7 days after birth
• Umbilical vein and arteries become ligaments within a week after birth
• Tissue sealing the foramen


  Becomes the fossa ovalis
• Inability to correctly close the foramen ovale


  Can lead to multiple conditions
• Conditions caused by inability to close foramen ovale


  • Patent foramen ovale
  • Atrial septal defects
• Ductus venosus


  A shunt that allows oxygenated blood in the umbilical vein to bypass the liver and is essential for normal fetal circulation
• Fetal circulation


  1. Blood becomes oxygenated in the placenta

  2. Travels to the right atrium via umbilical veins through the ductus venosus

  3. Then to the inferior vena cava
• Umbilical arteries


  Carry deoxygenated fetal blood toward the placenta for replenishment
• Umbilical vein


  Carry newly oxygenated and nutrient-rich blood back to the fetus
• After birth


  1. Umbilical vein remnant forms the round ligament of the liver

  2. Round ligament of the liver contains paraumbilical veins in adults
• Development of the cardiovascular system

  1. Begins to develop toward the end of the third week

  2. Heart starts to beat at the beginning of the fourth week

  3. Critical period of heart development is from day 20 to day 50 after fertilization
• Many critical events occur during cardiac development, and any deviation from this normal pattern can cause congenital heart defects, if development of heart doesn't occur properly
• Fetal circulation


  • Three shunts: Ductus arteriosus, Ductus venosus, Foramen ovale
• Ductus arteriosus


  Protects lungs against circulatory overload, allows the right ventricle to strengthen, carries mostly medium oxygen saturated blood