Role of Sonography in Obstetrics — Comprehensive Study Notes (Obstetric Sonography)
Introduction
Sonography is the primary tool for evaluating the developing fetus during pregnancy.
Obstetric sonography allows clinicians to assess fetal development, growth, and well-being.
When an abnormal condition is recognized prenatally, obstetric management may be altered to provide optimal care for the fetus and child.
Prenatal diagnosis has led to prenatal treatments performed under ultrasound visualization.
Sharing sonographic images and diagnostic results facilitates prenatal parental education and counseling.
Sonographers performing fetal studies must understand both sonographic and obstetric principles.
Sonographers must accurately and thoroughly compile pertinent information to provide optimal sonographic assessment of the fetus.
Practice guidelines produced by the American College of Radiology (ACR), American Institute of Ultrasound in Medicine (AIUM), and American College of Obstetricians and Gynecologists (ACOG) recommend specific components of a standard obstetric sonography examination.
Ethical/Practical note: Improved diagnostic capability supports counseling and informed decision-making, but must be balanced with patient safety considerations and informed consent.
Relevance to practice: Sets expectations for comprehensive data collection and standardized examination components.
Indications for Obstetric Sonography
Estimation of gestational (menstrual) age for patients with uncertain clinical dates or verification of dates for patients undergoing scheduled elective repeat cesarean delivery, indicated induction of labor, or elective termination of pregnancy. Sonographic confirmation of dating permits proper timing of cesarean delivery or labor induction to avoid premature delivery.
Evaluation of fetal growth, for example when the patient has identified causes of uteroplacental insufficiency (e.g., severe preeclampsia, chronic hypertension, chronic renal disease, or severe diabetes mellitus) or other medical pregnancy complications in which fetal malnutrition (e.g., intrauterine growth restriction [IUGR] or macrosomia) is suspected. Measuring fetal growth by sonography at 2- to 4-week intervals permits assessment of impact on the fetus and guides pregnancy management.
Vaginal bleeding of undetermined cause in pregnancy. Sonography often allows determination of source of bleeding and status of the fetus.
Serial evaluation of cervical length in pregnant women with increased risk for recurrent or primary preterm birth.
Evaluation of abdominal or pelvic pain in pregnancy that may be associated with ectopic pregnancy, abruptio placentae, or maternal appendicitis, renal calculi, pelvic mass, or other conditions.
Determination of fetal presentation when presenting part cannot be adequately determined in labor or fetal presentation is variable in late pregnancy. Accurate knowledge of presentation guides management of delivery.
Suspected multiple gestation based on detection of more than one fetal heart beat pattern, fundal height larger than expected for dates, or prior use of fertility drugs. Pregnancy management may be altered in multiple gestation.
Adjunct to amniocentesis. Sonography permits guidance of needle to avoid placenta and fetus, to increase chance of obtaining amniotic fluid, and to decrease the chance of pregnancy loss.
Significant discrepancy between uterine size and clinical dates. Sonography permits accurate dating and detection of conditions such as oligohydramnios and polyhydramnios, along with multiple gestation, IUGR, and anomalies.
Evaluation of pelvic mass. Sonography can detect location and nature of mass and aid in diagnosis.
Hydatidiform mole suspected on the basis of clinical signs (hypertension, proteinuria), presence of ovarian cysts on pelvic examination, or failure to detect fetal heart tones with a Doppler device after 12 weeks. Sonography permits accurate diagnosis and differentiation of neoplasm from fetal death.
Adjunct to cervical cerclage placement. Sonography aids in timing and proper placement of the cerclage for patients with incompetent cervix.
Suspected ectopic pregnancy or pregnancy after tuboplasty or prior ectopic gestation. Sonography is a valuable diagnostic aid for this complication.
Evaluation of suspected fetal death. Rapid diagnosis enhances optimal management.
Suspected uterine abnormality (e.g., clinically significant leiomyomata, congenital structural abnormalities such as bicornuate uterus or uterus didelphys). Serial surveillance of fetal growth and state enhances fetal outcome.
Evaluation for fetal well-being. Biophysical evaluation after 28 weeks’ gestation may include assessment of amniotic fluid, fetal tone, body movements, breathing movements, and heart rate patterns.
Evaluation of suspected amniotic fluid abnormalities such as suspected polyhydramnios or oligohydramnios. Confirmation of diagnosis and identification of cause of condition in certain pregnancies may be necessary.
Suspected abruptio placentae. Confirmation of diagnosis and extent of abruption assists in clinical management.
Adjunct to external version from breech to vertex presentation. Visualization provided by sonography facilitates performance of the procedure.
Estimation of fetal weight and presentation in premature rupture of membranes or premature labor. Information provided by sonography guides management decisions on timing and method of delivery.
Evaluation following maternal serum biochemical marker results. Elevated AFP increases risk for open defects (e.g., neural tube defects). Other biochemical markers in first trimester or quad screen in second trimester may indicate increased risk for certain obstetric or fetal conditions.
Follow-up observation of identified fetal anomaly. Sonographic assessment of progression or lack of change may assist in clinical management.
Follow-up evaluation of placenta location for suspected placenta previa.
Evaluation for those with history of previous congenital anomaly. Detection of recurrence may be facilitated, or psychological benefit to patients may result from reassurance of no recurrence.
Evaluation of fetal condition in late registrants for prenatal care. Assessment of gestational age and fetal size assists in pregnancy management decisions.
To assess findings that may increase risk of aneuploidy.
To screen for fetal anomalies.
Types of Obstetric Sonography Examinations
Three practice guidelines define major types of sonographic examinations performed in the second and third trimesters using terms “limited,” “standard,” and “specialized.”
18 weeks standard examination includes evaluation of gestational age by fetal biometry, fetal number, placental position, cardiac activity, amniotic fluid volume, and fetal anatomic survey including all elements specified in guidelines. May include maternal cervix and adnexa when clinically appropriate and technically feasible.
Limited obstetric sonography examination (CPT code 76815) is used to answer a specific clinical question such as presentation of fetus, placental location, cervical length, amniotic fluid volume, or verification of fetal heart motion; done when there is a previous standard obstetric examination recorded.
Repeat obstetric sonography examination (CPT code 76816) is similar to standard obstetric examination and typically includes biometry to evaluate fetal growth and reevaluation of anatomy that may or may not have been well visualized on standard examination; repeats when there is a previous standard examination recorded and a second examination is ordered for the same indication.
Specialty obstetric sonography examination (CPT code 76811); also known as targeted sonogram. Performed when anomaly is suspected based on maternal history, biochemistry, or results of previous obstetric sonogram.
Specialty obstetric sonography examination includes all components of the standard examination plus more in-depth view of fetal anatomy. Includes additional views of fetal heart; may include color Doppler views of heart; may also include additional views of extremities and focus on areas of anomalous or expected findings associated with patient history.
First trimester examination (CPT code 76801) performed before 13 weeks and 6 days of gestation. Includes uterus, cervix, maternal adnexa as well as gestational sac and embryo. Pregnancy dated based on embryonic size and fetal heart motion documented if these findings are present. Uterine anomalies and pelvic masses associated with pregnancy are more easily seen in first trimester examinations. Chorionicity and amnionicity of multiple gestations should be documented at this time.
First trimester risk assessment examination (CPT code 76813); also known as nuchal translucency examination. Performed only when women opt for first-trimester screening tests for aneuploidy. Includes measurement of fetal crown-rump length and measurement of nuchal translucency using standard criteria.
Patient History
Open-ended questions (e.g., “Do you have concerns?”) and closed questions (e.g., “When was your last normal period?”) are used to gather important patient information.
Gravidity and Parity
Gravidity (G) is the number of pregnancies, including the present one.
Parity (P) is reported using a numeric system that describes all possible pregnancy outcomes.
The letter “P” is followed by four numbers in sequence, P0000, commonly used.
Numbers represent, in order, full-term deliveries, premature births, and stillbirths, early pregnancy loss or termination, and living children.
Example: G4P2103 describes a patient undergoing her fourth pregnancy with two full-term deliveries, one premature birth, no early pregnancy losses, and three living children.
Clinical Dates
First date of the last normal menstrual period (LMP) is the standard dating method in the United States.
Human pregnancy lasts 266 days ± 10 days.
If conception occurs on day 14 from LNMP, pregnancy duration from LNMP is 280 days or 40 weeks.
Pregnancy divided into trimesters of approximately 13 weeks each: 1st trimester up to 13 weeks 6 days GA; 2nd trimester 14–26 weeks 6 days GA; 3rd trimester begins at 27 weeks GA and lasts until term.
In reality, assessment of gestational age is often not precise due to irregular cycles, irregular ovulation, etc.
Sonographer should ask for the first day of the LMP. If not remembered, ask for the expected date of delivery (EDD).
Also ask if prior sonographic examinations before 20 weeks exist and whether the EDD was determined by the earliest exam.
Dates established by sonography in the first or second trimester typically take precedence over menstrual dates when there is a discrepancy >7 days in the first trimester or >10 days in the second trimester.
Sonography may be considered to confirm menstrual dates if gestational age agreement is within one week by crown-rump length or within 10 days by second-trimester fetal biometry.
Pregnancy should not be dated by sonographic measurements in the third trimester.
Dates should not be changed after having been calculated from an earlier exam.
Nägele’s Rule
EDD = LMP – 3 months + 7 days
LMP = EDC + 3 months – 7 days
Example: LMP of 10/17 would result in EDD of 7/24 (10/17 – 3 months = 7/17; then +7 days = 7/24).
Maternal Risk Factors
Ask if patient has latex allergies.
Ask if patient has experienced supine hypotension (difficulty lying on back during pregnancy).
Instruct patient to report any warmth, dizziness, or syncope during sonography.
Sonographer should know if patient is taking any medications or has had pregnancy problems (bleeding, decreased fetal movement, pelvic pain).
If there have been problems with previous pregnancies (e.g., incompetent cervix, fibroids, fetal macrosomia or growth restriction, congenital or chromosomal anomalies), document this information.
Sonographer should know if patient has maternal risk factors for anomalies that may impact examination or interpretation.
Risk factors for congenital anomalies include increased maternal age, abnormal maternal serum biochemistry values, first-trimester increased nuchal translucency, maternal disease (e.g., diabetes mellitus, systemic lupus erythematosus), and uterine cavity size relative to dates (too small or too large).
Other risk factors include a previous child with a chromosomal disorder or exposure to known teratogenic drugs or infectious agents.
Some anomalies are caused by chromosomal number abnormalities (aneuploidies), most commonly Down syndrome (trisomy 21).
Safety of Ultrasound
Animal experiments and some human studies raise concerns that high-intensity ultrasound may modify biological structures and functions.
Major biologic effects include thermal effects (temperature rise) and mechanical forces including cavitation (production and collapse of gas-filled bubbles).
Sonographers can minimize thermal effects by not staying in one spot (especially over fetal bone) for long periods and by focusing the beam deeper into the body as needed to obtain adequate images.
Cavitation is dependent on the presence of gas preexisting within tissue.
AIUM-Prudent Use and Safety: No independently confirmed adverse effects caused by exposure from present diagnostic ultrasound instruments have been reported in human patients in the absence of contrast agents.
Biological effects (e.g., localized pulmonary bleeding) have been reported in mammalian systems at diagnostically relevant exposures, but the clinical significance of such effects is unknown.
Ultrasound energy today is higher than earlier equipment; Doppler imaging can increase energy exposure.
The studies of ultrasound bioeffects are not definitive; continued research is essential.
Sonographers have responsibility to be knowledgeable regarding ultrasound bioeffects and to use the least amount of energy necessary to obtain clinical information.
AIUM-adopted ALARA principle: The potential benefits and risks of each examination should be considered; ALARA should be observed when adjusting controls that affect acoustical output and by considering transducer dwell times.
Safety of Doppler for the Obstetric Patient
Doppler ultrasound provides a noninvasive method to assess physiology and pathophysiology of fetal and maternal circulations when required for diagnosis.
In most cases, pulsed-wave Doppler is used in the fetus rather than continuous-wave Doppler.
Use of Doppler in the first trimester is controversial due to higher energy levels, difficult examinations, prolonged dwell times, and total body insonation of a small embryo.
Benefits of Doppler imaging likely outweigh risks when there are specific indications requiring Doppler interrogation.
Fetal sonography with or without Doppler should be performed only when there is a valid medical reason, and the lowest possible ultrasonic exposure settings should be used to obtain necessary diagnostic information.
Guidelines for First Trimester and Standard Second and Third Trimester OB Sonography Examinations
AIUM, ACOG, ACR: Professional societies collaborate on clinical statements and include identical wording related to classification and specifications for examinations; some sections differ in detail between organizations (e.g., physician qualification, documentation, quality control, background, and clinical recommendations).
Guidelines are often cited as legal standards, though that was not the original intent.
Adherence to guidelines and referral of suspicious studies for further evaluation optimize detection possibilities.
It is important that all sonographers strive to consistently meet or exceed minimum standards during every obstetric sonographic examination.
Quality standards include: components of examination protocol, qualifications of personnel, documentation, equipment specifications, fetal safety, quality control, safety, infection control, and patient education concerns.
National board certification (RDMS) ensures the sonographer has necessary knowledge, skills, and experience.
Documentation standards require a permanent record of measurements and anatomic findings; images must be labeled with patient name, date, and orientation where appropriate.
A written report of the examination must be maintained in patient records.
Availability of real-time sonography equipment with transabdominal and transvaginal transducers is essential to confirm fetal life, view anatomy and movements, and obtain biometric parameters used to determine fetal age and growth.
Transducer frequency selection and equipment settings should balance imaging resolution and penetration; lowest possible exposure should be used per ALARA.
Laboratory-level policies and procedures should address patient information, infection control, quality control, and safety; policies may include musculoskeletal injury concerns for sonographers.
National board certification (RDMS) ensures qualifications; documentation standards require maintenance of permanent records with measurements and labeled images; written reports are kept in patient records.
Laboratories should have clearly stated policies and procedures related to patient information, infection control, quality control, and safety.
These guidelines emphasize standardization to optimize detection while safeguarding patient well-being and professional accountability.
Nägele’s Rule (Summary Formulas)
Estimated Due Date (EDD) can be calculated from Last Menstrual Period (LMP) by: EDD = ext{LMP} - 3 ext{ months} + 7 ext{ days}.
Conversely, LMP = ext{EDD} + 3 ext{ months} - 7 ext{ days}.
Example: If LMP = 10/17, then EDD = 7/24 (October 17 minus 3 months plus 7 days).
Additional dating convention: pregnancy duration from LMP is 266 ext{ days} \, ext{±} \, 10 ext{ days}. If conception occurs on day 14 of the cycle, duration is 280 ext{ days} = 40 ext{ weeks}.
Key Equations and Numeric References
Pregnancy duration: 266 ext{ days} \pm 10 ext{ days}.
Full-term pregnancy duration (from conception): 280 ext{ days} = 40 ext{ weeks}.
Nägele’s Rule relation: EDD = LMP - 3 ext{ months} + 7 ext{ days}.
Alternative dating: LMP = EDD + 3 ext{ months} - 7 ext{ days}.
Practical and Ethical Implications
Counseling and parental education are enhanced by sharing ultrasound findings, raising considerations for informed consent and ongoing education.
Adherence to ALARA and prudent use principles minimizes potential, albeit theoretical, bioeffects on the fetus.
Normalization of guidelines across professional societies supports consistent quality and reduces medico-legal risk; however, individual patient circumstances may require deviation for safety or diagnostic clarity.
Systematic documentation and permanent records support continuity of care, medicolegal protection, and quality assurance.
Connections to Practice and Real-World Relevance
Standardization of dating, growth assessment, and anomaly screening underpins decision-making for timing of delivery, management of preterm labor, and evaluation of fetal well-being.
Differentiation among standard, limited, repeat, and specialty obstetric sonography allows tailored imaging to answer specific clinical questions while minimizing exposure.
First-trimester screening (nuchal translucency) and integration with maternal serum markers illustrate how ultrasound complements biochemical screening to assess aneuploidy risk.
Safety guidelines (AIUM/ALARA, Doppler use) reflect a balance between diagnostic benefit and potential fetal exposure, emphasizing the need for judicious, evidence-based use of ultrasound.
Quick reference: CPT codes (for exam types)
76801: First trimester examination (before 13 weeks 6 days)
76811: Specialty obstetric sonography examination (targeted)
76813: First trimester risk assessment examination (nuchal translucency)
76815: Limited obstetric sonography examination
76816: Repeat obstetric sonography examination
Notes: Standard 18-week exam includes comprehensive assessment as described above.
Concluding note
The content emphasizes comprehensive data gathering, standardized examination components, patient safety, and adherence to professional guidelines to optimize fetal outcomes and patient care during pregnancy.