Second Trimester Ultrasound: Exploring Fetal Development
The second trimester ultrasound is an essential part of monitoring the health of both the mother and the fetus, typically performed between the 18th and 22nd week of pregnancy. This ultrasound provides valuable information about the baby’s development, helps detect any abnormalities early, and monitors the mother’s health.
Objectives of the Second Trimester Ultrasound:
- Fetal Age Determination: If the mother missed the first trimester ultrasound, the second trimester scan helps determine the exact fetal age, allowing for appropriate monitoring of the baby’s development.
- Fetal Size Measurements: The second trimester ultrasound provides data on the fetus’s length, head circumference, abdominal circumference, and femur length. These measurements help track the baby’s growth and detect early signs of fetal growth restriction.
- Fetal Anatomy Survey: This is the most crucial objective of the second trimester ultrasound. The doctor will carefully examine the fetus’s internal organs, skeleton, face, and brain to detect early signs of fetal anomalies.
- Placenta Evaluation: The ultrasound helps determine the placenta’s location, structure, and thickness. This information helps monitor the baby’s health and detect early signs of placental aging, low placental attachment, placental bleeding, etc.
- Estimated Fetal Weight: Based on the measurements obtained, the doctor can estimate the baby’s weight.
- Uterus and Ovaries Assessment: The second trimester ultrasound can also be used to check the mother’s uterus and ovaries for any health issues.
Measurements in Second Trimester Ultrasound:
- BPD (Biparietal Diameter): The diameter between the two sides of the fetal head.
- Measurement Method: Ultrasound beam perpendicular to the midline.
- Location: Transverse section through the thalamus.
- Note: Two cerebral hemispheres are symmetrical across the midline. The midline is interrupted by the thalamus and the transparent septum. The cerebellum is not visible.
- Measurement: From the outer edge of the skull of one hemisphere to the inner edge of the skull of the opposite hemisphere.
- HC (Head Circumference): Head circumference.
- Measurement Method: Same section as BPD measurement.
- Measurement: Circumference of the line touching the outer edge of the skull.
- AC (Abdominal Circumference): Abdominal circumference.
- Measurement Method: Transverse section through the abdomen.
- Note: Umbilical cord and the umbilical cord insertion point. Gastric fluid pouch. The two kidneys and the umbilical cord insertion point on the abdomen are NOT visible.
- Measurement: Circumference of the line touching the skin surface.
- FL (Femur Length): Femur length.
- Measurement Method: Ultrasound beam perpendicular to the axis of the femur (deviation of 0-15 degrees).
- Measurement: The probe is placed at the distal end of the femur ossification point (not including the upper femoral epiphysis).
Anatomical Scan Planes:
- Transverse Plane Through the Thalamus, Lateral Ventricles, and Posterior Fossa:
- Assessment: Lateral ventricles (including choroid plexus), transparent septum, midline, thalamus, cerebellum, and cisterna magna.
- Lateral Ventricle Horn Measurement: The probe is placed at the level of the occipital sulcus, measuring from inside to inside and perpendicular to the ventricle wall.
- Transverse Plane for Face Examination:
- Includes: Transverse plane for eye examination and transverse plane for upper lip and jawbone examination.
- Transverse Plane for Eye Examination:
- Two eyeballs are present.
- Two normal lenses.
- Measurement: Size of eyeballs, distance between sockets.
- Abnormalities: Small eyes, no eyes, eyes close to/far from each other.
- Transverse Plane for Upper Lip and Jawbone Examination:
- The integrity of the upper lip should be visible in the frontal plane.
- The integrity of the upper jawbone should be visible.
- Examination of the lower jawbone.
- Mid-Sagittal Plane:
- Helps Diagnose: Prominent forehead, small chin, flat face, nasal bone abnormalities, and assesses the size of the corpus callosum.
- Coronal Plane:
- Assessment: Corpus callosum, frontal horns of lateral ventricles, and transparent septum.
- Chest Plane:
- Thoracic Cavity: Symmetrical shape, continuous with the abdomen. The ribs have a normal curvature and are not deformed. The two lungs have a similar echogenicity and there is no displacement of the mediastinum.
- Diaphragm: A hypoechoic line, continuous. Separates the chest and abdomen.
- Heart:
- 4B Plane: The heart is located on the left side of the chest (same side as the stomach). The heart is
- Outflow Tract Plane of the RVOT and LVOT: The size is approximately equal. They cross each other at the exit from the two ventricles.
- 3 Vessel and Trachea Plane: Evaluates the anatomical relationship and size of the RVOT, descending aorta, and right superior vena cava. Size decreases: RVOT, descending aorta, right superior vena cava. RVOT is anterior and the superior vena cava is posterior.
- Locate the organs in the abdomen: The stomach is on the left. The intestines are in the abdomen. The gallbladder is located in the upper right quadrant, next to the liver. The umbilical cord is inserted into the abdominal wall intact (excluding midline abdominal wall defects such as omphalocele, gastroschisis). Number of blood vessels in the umbilical cord. Bladder and two kidneys (if enlarged, measurement is required).
- Need to examine 3 planes: Longitudinal, transverse, coronal.
- The spinal cord usually ends at L2.
- Most common severe spinal column abnormalities: Open spina bifida often accompanied by head manifestations (banana-shaped cerebellum, lemon-shaped skull).
- The presence of both upper limbs and lower limbs.
Minimum Requirements for Second Trimester Morphology Assessment:
- Head: Cranial vault intact. Transparent septum present. Midline. Thalamus. Two cerebral hemispheres symmetrical. Occipital horn of lateral ventricles. Cerebellum. Cisterna magna.
- Face: Two eyeballs, lenses. Profile, nasal bone. Mouth, upper lip, continuous upper jawbone.
- Neck, Chest, Heart: No masses (e.g., cervical lymph node cysts). Normal shape and size of chest + lungs. Heart is functioning. Normal heart position. 4B, RVOT outflow tract, LVOT outflow tract. No diaphragmatic hernia.
- Abdomen: Stomach in normal position. Intestines not dilated. Two kidneys. Umbilical cord insertion into the abdominal wall normal, with three blood vessels.
- Bones: No tumors, no spina bifida (longitudinal and transverse planes). Normal limbs.
- Genitourinary: Sex, anus.
Placenta:
- Location: Relationship between the lower border of the placenta and the internal cervical os (if it is close to/extends beyond the internal cervical os, ultrasound should be repeated in the third trimester).
- Placental Abnormalities: Bleeding, honeycomb appearance (in triploidy), placental tumors (e.g., chorioangioma).
- History of Cesarean Section, Low-Lying Placenta Anterior, Placenta Previa => Look for Placenta Accreta Signs: Presence of irregular blood vessel spaces – lacunae, abnormalities in the border area between the uterus and bladder.
Amniotic Fluid:
- Assessment: Subjective/ measurement.
- Two assessment methods: Largest amniotic fluid pocket/ 4-quadrant amniotic fluid index.
Umbilical Cord:
- Insertion site.
- Number of blood vessels.
- Umbilical cord knot.
Measurement of Cervical Canal Length:
- Transvaginal ultrasound + full bladder.
- Place the probe in the anterior fornix.
- Identify: Cervical mucosa, internal os, external os.
- Do not apply pressure to the cervix.
- Cervical canal magnification >75%.
- Measure 3 times, take the smallest value.
Note:
- The second trimester ultrasound is an important part of maternal and fetal health care.
- Following the doctor’s scheduled appointments for the second trimester ultrasound is essential.
- Ask your doctor about any concerns you may have about the ultrasound results.
- If any abnormalities are detected, consult your doctor for timely advice and follow-up.
Conclusion:
- The second trimester ultrasound is a useful tool for monitoring fetal health, detecting early abnormalities, and supporting prenatal care.
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