Technology for imaging in obstetrics has advanced a great deal over the past decade, but the purpose remains the same: to evaluate the anatomy and well-being of the fetus and the mother. The primary imaging modality in obstetrics is the 2-dimensional (2D) ultrasound because it is safe and widely available. Three- and 4-dimensional (3D/4D) ultrasound and magnetic resonance imaging (MRI) are also being used in certain situations to enhance imaging techniques, but the cost of these modalities limits their widespread use. Finally, computed tomography (CT) scan has limited utility because of safety issues but may be necessary in evaluating certain maternal conditions.
Ultrasound evaluation uses sound waves at a frequency greater than that which the human ear can hear (> 20,000 cycles per second or Hertz [Hz]) to obtain images. An ultrasound examination is performed in real time with images or video clips stored for review. Ultrasound probes contain a transducer that creates the ultrasound waves at different frequencies. Higher frequency transducers provide better resolution but have less tissue penetration, whereas low-frequency transducers have lower resolution but better tissue penetration.
In obstetrics and gynecology, ultrasound imaging is generally performed 1 of 2 ways: either with a transvaginal probe or transabdominal probe. The choice of which probe to use generally depends on the structure of interest and its distance from the probe. For instance, imaging of the cervix or an early gestation is generally best achieved with a transvaginal probe, whereas evaluation of the fetus in the third trimester of pregnancy is best accomplished with transabdominal imaging.
Ultrasound during pregnancy when performed for medical indications is considered to be safe. There are no documented harmful effects to the fetus from diagnostic ultrasound. Ultrasound waves, however, are a form of energy and have been shown to raise tissue temperature with high energy output or prolonged exposure. To minimize this risk, it is recommended that energy output, as measured by the Mechanical Index, be kept < 1.0 and that ultrasound be used for diagnostic purposes only (ie, not for entertainment purposes). Imaging of the fetus can be divided into ultrasound evaluations in the first trimester and evaluations in the second and third trimesters. In each trimester, the goals and the ability to evaluate the fetal anatomy differ.
First-Trimester Ultrasound Examination
There are a number of indications to perform first-trimester ultrasound. They include confirmation of an intrauterine pregnancy, assessment of pelvic pain and vaginal bleeding, estimation of gestational age, confirmation of viability, evaluation of number of gestations, genetic screening, evaluation of basic anatomy, and assessment of uterine and adnexal anomalies and pathology.
To begin with, a first-trimester ultrasound can be used to confirm an early pregnancy by documenting the location of a gestational sac and the presence or absence of a yolk sac and fetal pole. However, one should use caution in determining the location of a pregnancy based solely on the presence of a gestational sac because sometimes an intrauterine fluid collection could be a pseudogestational sac from an ectopic pregnancy and not a normal gestational sac. Please see Chapter 13 for a more thorough discussion of using ultrasound to evaluate for ectopic ...