Category: 5. 2nd-3rd Trimester Obstetric

Normal Placenta

The placenta should be clearly visible by the beginning of 2nd trimester. Here are the characteristics of a normal placenta:

• Uniform echogenicity (shade of gray) and rounded margins
• Slight hypoechoic area behind the placenta
• Usually, the cord will be centrally inserted
• Average thickness of 2-4 cm.

To locate the placenta follow these steps:

• Starting at the superior end of the uterus, sweep from left to right in a sagittal plane.
• Follow the same pattern for more inferior sections of the uterus
• Identify the margins of the placenta.
• If the fetus is creating artifact and obscuring a posterior placenta, scan laterally.

Placenta Previa

Placenta previa refers to when the placenta is covering the internal os. This will commonly present as painless vaginal bleeding.

The earlier that placenta previa is detected, the less likely it is to be present at delivery since the placenta appears to migrate during pregnancy. However, if the placenta is covering the internal os at the onset of labor, a C-section is indicated.

Transvaginal ultrasound is preferred over transabdominal ultrasound for detecting low lying placenta and placenta previa.

Marginal placenta previa is when the placenta is extending to the edge of the internal os.

A low lying placenta is when the placenta is within 2 cm of the internal os.

Placenta Accreta Spectrum

Placenta accreta spectrum, also known as morbidly adherent placenta or just placenta accreta, is the excessive proliferation of the placenta into the myometrium. It is defined along a spectrum based on how deep into the myometrium the placenta invades (see figure below).

• Risk factors: Prior uterine surgery, placenta previa, assisted reproduction
• Placenta Accreta: Placenta is in contact with the myometrium
• Placenta Increta: Placenta invades the myometrium
• Placenta Percreta: Placenta invades past the myometrium into adjacent structures

Placenta accreta spectrum has these features on ultrasound:

• Discontinuous myometrium or thin (<1 mm) myometrium
• Absent retroplacental hypoechoic space that is seen in a normal placenta
• Increased retroplacental vascularity
• Placental lacunae – hypoechoic areas within the placenta
  • This is said to look like Swiss cheese
  • These will have turbulent blood flow on color doppler
• In placenta percreta: Abnormal or loss of interface between uterus and adjacent structures (e.g. bladder) with chaotic vascularity

Placental Abruption

Placental abruption occurs when the placenta prematurely separates from the uterine wall. For the fetus, this can lead to preterm birth or stillbirth. For the mother, abruption can lead to large blood loss, DIC, or even death.

Diagnosing placental abruption with ultrasound can be difficult due to the varying presentation and severity. The clinical and ultrasound appearance of abruption depends on several factors:

• The location of the placenta
• The location of the separation and bleeding (pre-placental, retroplacental, placental margin)
• How old the bleeding is

How do these factors influence ultrasound findings? Acute bleeding will appear as hypoechoic or isoechoic to the placenta. The blood will appear hyperechoic after 1-2 weeks. Therefore, a blood clot over the internal os can look similar to a placenta previa. Additionally, a retroplacental abruption can mimic the appearance of the hypoechoic space behind a normal placenta.

With those factors in mind, here are some ultrasound findings that may suggest the presence of an abruption:

• The placenta “jiggles” with quick transducer pressure. This is known as the jello sign.
• Retroplacental, preplacental, or placental margin collection of fluid — will have a lack of flow.
• Placental heterogeneity
• Varying placental thickness

In the 2nd and 3rd trimester, various measurements of the fetus, known as fetal biometrics, can be used to estimate the weight and gestational age. These aren’t as accurate as the 1st-trimester crown-rump length measurement, but they can be helpful nonetheless.

The ultrasound OB calculation package will automatically calculate the weight and gestational age for you.

Biparietal Diameter

• Acquire a transverse view of the fetal head and at the level of the thalami, you should see:
  • The midline falx
  • Thalami
  • Septum cavi pellucidi (labelled in the image below)
• You should not see the cerebral hemispheres.
• The structures on each side of the midline falx should be symmetrical.
• Activate the BPD measurement package on the OB/GYN preset.
• The caliper line should be perpendicular to the midline falx.
• Place the near caliper (top of the screen) on the outside of the parietal bone wall.
• Place the far caliper (bottom of the screen) on the inside of the parietal bone wall.

Head Circumference

• Use the same view that you acquired for the biparietal diameter measurement.
• Activate the head circumference (HC) measurement package on the OB/GYN preset.
• Trace the head circumference.
• If an ellipse tool is not available on your machine, measure the long axis diameter (OFD) of the head. The machine should calculate the head circumference from the BPD and OFD.

Abdominal Circumference

• Along the same transverse plane, scan down towards the fetus’ upper abdomen.
• You should see the following structures in your view:
  • Portal sinus
  • Umbilical vein
  • Stomach
• You should not see the kidneys.
• Activate the abdominal circumference (AC) measurement package on the OB/GYN preset.
• Trace the abdominal circumference.

Femur Length

• Acquire a long axis view of one of the femurs.
• Activate the femur length (FL) measurement package on the OB/GYN preset.
• Place the calipers on each end of the bone.

Amniotic fluid accompanies the fetus in the amniotic sac. It functions as a cushion for the fetus, helps protect it from infection, and promotes muscle, lung, and digestive system development. Polyhydramnios refers to having too much amniotic fluid while oligohydramnios refers to having too little. Polyhydramnios is associated with fetal malformations, developmental delay, and neurologic disorders. Oligohydramnios is associated with increased perinatal morbidity and mortality.

Amniotic fluid volume can be assessed with ultrasound by measuring the maximum vertical pocket of amniotic fluid.

• Oligohydramnios: <2 cm
• Normal: 2-8 cm
• Polyhydramnios: >8 cm

Here’s how to measure the maximum vertical pocket:

• Scan the amniotic sac from left to right in a sagittal view.
• Estimate a location that has the deepest vertical pocket and measure it with the calipers.
• The caliper line must be in a vertical orientation.
• The caliper line must be free of any fetal parts or the umbilical cord.

Fetal lie refers to the relative orientation of the fetal and maternal spines and can either be longitudinal, oblique, or transverse.

To determine the fetal lie, acquire a mid-sagittal view of the fetal spine and compare it to the maternal spine.

Longitudinal Lie – The maternal and fetal spines are parallel (can be breech or cephalic presentations)

Oblique Lie – The fetal spine is at an oblique angle to the maternal spine

Transverse Lie – The maternal and fetal spines are perpendicular

The presentation of the fetus refers to which part of the fetus is closest to the birth canal.

• Obtain a midline sagittal view of the lower uterus.
• Determine which part of the fetus is closed to the maternal pelvic inlet.

Cephalic Presentation — Head first

Breech Presentation — Buttocks/Feet first

Shoulder Presentation — Shoulder/Arm first

• This presentation will show a transverse (short axis) or oblique cross-section of the fetus in a mid-sagittal view of the uterus.

Cardiac Activity (Fetal Heart Rate Measurement)

Like in the 1st trimester, identifying fetal cardiac activity is essential to determining the viability of a pregnancy. Now that the fetus has grown to the point where we can identify its anatomy, we can directly visualize the 4 chambers of the heart. We can still use M-mode to measure fetal heart rate. Use the steps described above to calculate the fetal heart rate.

• Acquire a 4-chamber view of the fetal heart. This view can be found by scanning through the fetus in a transverse plane.
• Optimize the magnification and gain so that the fetal heart takes up the majority of the screen.
• The left atrium will be closest to the spine.
• The axis of the heart will be pointing to the left of the fetus.
• Use the calipers to measure the length of 1 (or 2 depending on the machine) cardiac cycle(s). The fetal heart rate should be calculated by the machine.