Cardiotocography

Basic fetometry and assessment of amniotic fluid

Ultrasound fetometry is a key tool for determining whether fetal dimensions correspond to the gestational age. During a standard examination, the doctor measures several strictly defined parameters. These include biparietal diameter (BPD) and head circumference, abdominal circumference, and fetal femur length. These metrics provide an objective assessment of growth rate and developmental proportionality.

An equally important stage is the assessment of amniotic fluid volume, as this indicator is critical for understanding the overall condition of the fetus. The volume of fluid can be assessed in two ways:

• Subjective assessment (via transabdominal approach): the doctor visually determines the volume of fluid between the anterior uterine wall and the fetus. In polyhydramnios, this volume is significantly increased, whereas in oligohydramnios, a sharp reduction in free spaces is observed.
• Objective assessment: semi-quantitative echographic criteria are used. The depth of the maximum free pocket (vertical pocket) is measured, which normally varies from 2 to 8 cm. A more accurate method is calculating the amniotic fluid index (AFI). To do this, the maximum dimensions of the pockets in the four conventional quadrants of the uterine cavity are summed. In a normal pregnancy, the AFI is 8–18 cm.

Placentography and criteria of placental maturity

Placental examination is a mandatory component of obstetric screening. The doctor evaluates its localization, thickness, and internal structure. Most often, the placenta is attached to the anterior or posterior wall extending to the lateral surfaces, less frequently to the uterine fundus. It is important to remember that localization is unstable: as pregnancy progresses, placental migration occurs from the lower segments to the fundus. The final conclusion regarding its position is made only before childbirth. An attachment where the distance from the lower edge of the placenta to the internal os of the cervix exceeds 5 cm is considered normal.

Placental thickness steadily increases with gestational age and normally reaches 3.3–3.6 cm by the end of gestation. Ultrasound may reveal various pathological findings: well-defined echonegative cysts, multiple dense inclusions (calcification), as well as infarctions and tumors.

Structural changes of the placenta are described by its grades of maturity (according to the P. Grannum classification):

1. Grade 0 (up to 30 weeks): chorionic plate is straight and smooth, parenchyma is homogenous, basal layer is not visualized.
2. Grade I (27–33 weeks): the plate becomes slightly wavy, single echogenic zones appear in the parenchyma.
3. Grade II (34–39 weeks): indentations appear on the plate, linear condensations appear in the parenchyma, and the basal layer looks like a dashed line of echogenic zones.
4. Grade III (after 37 weeks): indentations reach the basal layer, rounded condensations with rarefactions are visible in the parenchyma, and the basal layer produces an acoustic shadow due to merged echogenic zones.

Assessment of the uterine scar and modern imaging modes

For patients with a uterine scar, ultrasound is the primary method for monitoring its competence. Norm criteria include tissue thickness greater than 2.5 mm, smooth contours, uniformity, and homogeneous structure of the lower segment. Special attention is paid to this area in cases of low-lying placenta to rule out formidable complications such as uterine aneurysm or placenta accreta (placenta accreta).

Modern diagnostics rely on various scanning modes:

• 2D mode: the standard method for analyzing sequential tomographic slices in real time.
• 3D and 4D ultrasound: used as supplementary methods. 3D provides a static, volumetric realistic image, while 4D allows viewing the object in dynamics, assessing facial expressions and behavioral reactions of the fetus.
• HDlive technologies (Silhouette, Flow): increase the penetration depth of the ultrasound beam. They provide panoramic tomography of internal organs (e.g., visualization of brain hemispheres at 8 weeks) and detailed fetal angiography (visualization of the Circle of Willis at 18 weeks).

Dopplerometric blood flow study

Dopplerometry is used to assess hemodynamics in the mother-placenta-fetus system. In obstetric practice, priority is given to qualitative analysis of Dopplerograms. The clinical value lies not in the absolute value of blood flow velocity, but in the ratio between blood flow velocities in the systole and diastole phases.

Three main hemodynamic parameters (indices) are used to interpret the results:

• Systolic-diastolic ratio.
• Pulsatility index (requires the mean blood flow velocity parameter for its calculation).
• Resistivity index.

Class and exam questions

What else to study on this topic

• Pathological changes of the placenta: cysts, infarctions, tumors, and calcification.
• Diagnosis of placenta accreta using HDlive technologies.
• Method of calculating the amniotic fluid index (AFI) across four quadrants.
• Features of visualizing the fetal Circle of Willis during fetal angiography.
• Application of real-time ultrasound during intrauterine surgical interventions.

Other topics in the "Obstetrics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.