Obstetric imaging examinations are primarily used to assess the condition of the mother, fetus, and related structures. They include ultrasound, magnetic resonance imaging (MRI), and computed tomography (CT), performed when necessary.
Ultrasound Examination
Ultrasound is a non-invasive, convenient, and repeatable imaging modality, making it an essential tool during pregnancy.
Examination Routes
The main routes for ultrasound examination are transabdominal, transvaginal, and transperineal.
Transabdominal Examination
This is the most common route, performed with the patient in a supine position. It is primarily used to observe and measure fetal structures and related anatomy.
Transvaginal Examination
This is conducted after the patient empties the bladder and assumes the lithotomy position. It is mainly used to measure cervical length and the distance between the lower placental edge and the internal cervical os.
Transperineal Examination
This is performed with the patient in the lithotomy position to observe the lower edge of the placenta, the cervix, the fetal head position, and fetal presentation.
Examination Methods
M-Mode Ultrasound
This is often used for heart rate measurement and cardiac function assessment.
B-Mode Ultrasound
This is the most commonly used mode in obstetrics, providing anatomical information for diagnosis and treatment.
Doppler Ultrasound Examination
Doppler ultrasound employs the Doppler effect, which measures ultrasound waves reflected or scattered by moving objects. It is frequently utilized to evaluate hemodynamics. Color Doppler ultrasound focuses on blood flow origin, pathways, and distribution. Spectral Doppler enables the measurement of hemodynamic parameters, with resistance index (RI), pulsation index (PI), and systolic phase/diastolic phase (S/D) ratio being the three most commonly used parameters in obstetrics.
Three-Dimensional Ultrasound Imaging
Three-dimensional ultrasound utilizes computer-based image processing to reconstruct continuous two-dimensional images into three-dimensional images. Real-time three-dimensional ultrasound, also known as four-dimensional ultrasound, combines a series of three-dimensional volume images over a specific time period. This technique may aid in diagnosing fetal facial abnormalities, neural tube defects, fetal tumors, and skeletal anomalies but is not a substitute for two-dimensional ultrasound.
Applications in Obstetrics
First Trimester of Pregnancy
Before 10 Weeks of Pregnancy
Evaluations focus on determining whether the pregnancy is intrauterine and assessing pathological conditions of the cervix, uterine body, and adnexa.
The viability of the embryo is confirmed by observing the gestational sac (GS), yolk sac, fetal pole, and amniotic sac.
Crown-rump length (CRL) is measured to determine gestational age. Before six weeks of pregnancy, it is typically not possible to distinguish the head and tail of the embryo, so the longest diameter of the embryo is measured. Between six and nine weeks, the mid-sagittal section of the entire fetus is captured. During this period, the fetus is highly flexed, so what is measured is the nuchal-to-rump length, although it is conventionally referred to as the crown-rump length. The ultrasound-estimated gestational age needs to be verified against the last menstrual period. If there is a significant discrepancy, the last menstrual period must be re-evaluated.
The number of embryos is determined, and in the case of multiple pregnancies, chorionicity and amnionicity are assessed.
Between 11 and 13 Weeks + 6 Days of Pregnancy
Gestational age is reassessed. At this stage, the estimation of gestational age is most accurate, with a margin of error of no more than five days in 95% of cases.
The anatomical structures of the fetus are evaluated. By the end of the first trimester, ultrasound can identify gross structural abnormalities in the fetus. Early pregnancy screening has a sensitivity of more than 70% for severe malformations. However, many fetal structural abnormalities develop later in pregnancy, and even with the best equipment and the most experienced ultrasound specialists, abnormalities may not be detectable in early pregnancy.
The nuchal translucency (NT) thickness is measured. Increased NT thickness is associated with an elevated risk of chromosomal abnormalities in the fetus.
Observations of the ductus venosus and tricuspid valve blood flow can be conducted where feasible. Abnormal ductus venosus blood flow and tricuspid regurgitation can assist in screening for chromosomal abnormalities and congenital cardiac abnormalities in the fetus.
Second Trimester of Pregnancy
Biometric Measurements
Commonly used measurements include biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), which are used to assess fetal growth.
Screening for Major Fetal Structural Abnormalities
The optimal gestational age for screening is between 20 to 24 weeks. Screening includes the following:
- Fetal Head: Assessing the skull for completeness, cavum septi pellucidi, falx cerebri, thalamus, bilateral ventricles, cerebellum, and cisterna magna.
- Face: Evaluating the continuity of bilateral orbits and the upper lip.
- Neck: Checking for masses.
- Chest/Heart: Observing the shape and size of the thoracic cavity/lungs, fetal heart activity, four-chamber heart position, great vessels such as the aorta and pulmonary artery, and examining for diaphragmatic hernia.
- Abdomen: Evaluating the position of the stomach bubble, bowel dilation, kidneys, and the insertion site of the umbilical cord.
- Skeleton: Inspecting for defects or masses along the spine and observing the relationship of the arms, hands, legs, and feet.
- Placenta: Examining its location and the presence of abnormalities such as masses or accessory placentas.
- Amniotic Fluid: Measuring the maximum depth of amniotic fluid pools.
- Umbilical Cord: Confirming the presence of two umbilical arteries and one umbilical vein.
The determination of fetal sex is performed only when medically indicated.
Ultrasound Markers for Genetic Screening
Indicators such as choroid plexus cysts, lateral ventricle enlargement, increased echogenicity of bowel, single umbilical artery, renal pelvis dilation, and echogenic cardiac foci may suggest an increased risk of aneuploid chromosomal abnormalities.
Cervical Length Measurement
Measuring cervical length is one of the methods for predicting preterm birth. A cervical length ≤25 mm in the second trimester is the most commonly used cutoff value. Transvaginal ultrasound is the recommended method for measurement.
Late Second and Third Trimesters
Biometric Measurements
Commonly used measurements include BPD, HC, AC, and FL. HC is more reflective of fetal head growth than BPD, while AC is the best indicator for evaluating fetal growth, estimating fetal weight, and identifying growth restrictions in late pregnancy.
Placental Positioning
Determining placental location can provide valuable clinical insights, such as identifying placenta previa. During procedures like amniocentesis, ultrasound monitoring aids in avoiding damage to the placenta and umbilical cord.
Amniotic Fluid Volume
Amniotic fluid appears as anechoic dark areas and has a clear appearance. In late pregnancy, vernix in the amniotic fluid manifests as sparse spot-like echoes. A maximum vertical pocket depth (DVP) ≥8 cm indicates polyhydramnios, while ≤2 cm suggests oligohydramnios. The uterus is divided into four quadrants using the umbilical level as a landmark, and the DVP is measured in each quadrant. The sum of these measurements yields the amniotic fluid index (AFI). An AFI ≥25 cm indicates polyhydramnios, while ≤5 cm suggests oligohydramnios.
Biophysical Profile Scoring
This includes assessing fetal breathing-like movements, fetal movements, fetal tone, and amniotic fluid volume. It is one of the methods for evaluating fetal health in utero.
Obstetric Color Doppler Ultrasound Examination
Color Doppler ultrasound provides maternal and fetal hemodynamic parameters, such as data from the uterine arteries, umbilical artery (UA), umbilical vein (UV), ductus venosus (DV), and middle cerebral artery (MCA).
Uterine Artery Blood Flow
During normal pregnancy, the pulsatility index (PI), resistance index (RI), and systolic-to-diastolic ratio (S/D) of the uterine artery progressively decrease with advancing gestational age. Persistent elevation of PI or RI or the presence of an early diastolic notch may indicate an increased risk of preeclampsia. Measurements of uterine artery blood flow at 20–24 weeks of pregnancy can assist in predicting the occurrence of preeclampsia.
Fetal Blood Flow
Monitoring fetal blood flow generally includes the umbilical artery, umbilical vein, ductus venosus, and middle cerebral artery, with umbilical artery blood flow being the most commonly assessed. During normal pregnancy, umbilical artery PI, RI, and S/D values are closely correlated with gestational age. An increase in umbilical artery blood flow resistance is associated with fetal distress, growth restriction, and preeclampsia. Absence or reversal of end-diastolic umbilical artery blood flow is linked to an increased risk of adverse perinatal outcomes. Elevated peak systolic velocity in the MCA can indicate the severity of fetal anemia or distress.
Application in Diagnosing Fetal Cardiac Structural Abnormalities
Fetal echocardiography is typically performed between 18 and 26 weeks of gestation. It is primarily recommended for high-risk pregnancies, including those with a family history of cardiac disease, a history of delivering fetuses with cardiac abnormalities, exposure to environmental chemicals, conception via assisted reproductive technologies, increased NT thickness, fetal arrhythmias, or suspicion of fetal cardiac structural anomalies during routine ultrasounds.
Application in Twin and Multiple Pregnancies
Ultrasound is used to determine the number of fetuses, evaluate gestational age, and assess chorionicity and amnionicity. Chorionicity is most accurately assessed during early pregnancy. Determining chorionicity is crucial for managing twin and multiple pregnancies, as it is closely related to perinatal outcomes. Chorionicity-guided management includes specific protocols such as biweekly ultrasounds for monochorionic twin pregnancies to monitor for potential complications.
Interventional Ultrasound in Obstetrics
Under ultrasound guidance, procedures such as prenatal diagnostic testing and fetal interventions can be conducted.
Magnetic Resonance Imaging (MRI)
MRI is not routinely used for prenatal screening but serves as a supplemental method to ultrasound when necessary. Fetal MRI is typically performed after 20 weeks of gestation in cases where indications are present. Compared to ultrasound, MRI has advantages in evaluating fetal central nervous system abnormalities or when ultrasound image quality is poor due to maternal obesity or oligohydramnios. MRI is also frequently used in diagnosing placental accreta, with additional value in assessing maternal conditions.
Computed Tomography (CT) Examination
CT remains a valuable diagnostic tool for specific maternal conditions such as pneumonia, pulmonary embolism, and cerebral hemorrhage, when clinically warranted.