RB

Role of Sonography in Obstetrics – Key Terms (First to Third Trimester)

First Trimester

  • Roles of ultrasound: evaluate embryo/fetus during the first trimester using either transabdominal or transvaginal transducers. If transabdominal examination is not definitive, use transvaginal or transperineal approaches.
  • Transabdominal exam provides overview of the entire pelvic cavity:
    • Image the uterus from cervix to fundus
    • Evaluate ovaries
    • Assess adnexal areas for abnormal fluid collections or masses
    • Look for free fluid
  • Transvaginal transducer provides a more limited view but allows excellent visualization of:
    • Embryo
    • Yolk sac
    • Amnion
    • Chorion
    • Gestational sac
  • First Trimester Sample Protocol
    1. Uterus and adnexa should be evaluated for presence of gestational sac. If seen, note location (intrauterine vs extrauterine). Record gestational sac when embryo is not identified during zygote/implantation stage. Exercise caution diagnosing gestational sac without yolk sac or embryo since intrauterine fluid collections may resemble.
    2. Presence or absence of yolk sac and embryo should be noted. Crown-rump length (CRL) is the most accurate measurement of gestational age during the first trimester and should be recorded when the embryo is present. CRL is used for gestational dating in this period.
    3. The earliest structure seen within the gestational sac is the yolk sac; its presence indicates an intrauterine pregnancy. The embryo appears at ~4 weeks as an echogenic curved structure adjacent to the yolk sac.
    4. Blood tests (hCG levels) should be positive at 7 to 10 days embryonic age (conception age).
    5. Placenta appears as a thickened density along part of the margin of the gestational sac (trophoblastic reaction).
    6. Bowel herniation into the umbilical cord or abdomen occurs between ~8 to 11 weeks and then returns to the abdominal cavity.
  • Cardiac activity and fetal number in the first trimester
    • 2. Presence or absence of cardiac activity should be reported. Cardiac motion is typically seen when the embryo is ≥5 mm in length.
    • Fetal heart rate (HR) changes with development: early HR ~90 bpm; may rise to ~180 bpm in the middle of the first trimester, then settle to ~120–160 bpm for remainder of pregnancy. ext{HR} ext{ ranges approximately } 90 ext{ to } 180 ext{ bpm}.
    • Fetal number should be documented. Count embryos and yolk sacs to determine multiple pregnancies. In all multiple pregnancies, document membrane structure and number of amniotic and chorionic membranes; chorionicity is most reliably documented in the first trimester.
  • Evaluation of uterus, adnexal structures, and cul-de-sac
    • Document texture of ovaries and presence of corpus luteum or other adnexal masses
    • Look for inhomogeneous uterine texture that may represent leiomyomatous growth stimulated by hormonal changes of pregnancy
  • Summary of first-trimester imaging concepts
    • Early anatomy visualization is limited; prioritize detection of gestational sacs, yolk sacs, embryos, and cardiac activity
    • Use CRL for dating when embryo is visible
    • Recognize limitations and potential pitfalls (e.g., distinguishing intrauterine collections from sacs without yolk/embryo)

Indications for Second and Third Trimester Sonography

  • Evaluation of gestational age and fetal growth
  • Evaluation of vaginal bleeding
  • Evaluation of cervical insufficiency
  • Evaluation of abdominal and pelvic pain
  • Determination of fetal presentation
  • Evaluation of suspected multiple gestation
  • Adjunct to amniocentesis or other procedures
  • Significant discrepancy between uterine size and clinical dates
  • Evaluation of pelvic mass or suspected hydatidiform mole
  • Adjunct to cervical cerclage placement
  • Evaluation of suspected ectopic pregnancy (in appropriate contexts)
  • Evaluation of suspected fetal death or uterine abnormalities
  • Evaluation of fetal well-being and suspected amniotic fluid abnormalities
  • Evaluation of placental abruption
  • Adjunct to external cephalic version (ECV)
  • Evaluation for premature rupture of membranes (PROM) and/or premature labor
  • Evaluation for abnormal biochemical markers
  • Follow-up evaluation of fetal anomaly
  • Follow-up evaluation of placental location for suspected placenta previa
  • Evaluation of those with history of prior congenital anomaly
  • Evaluation of fetal condition in late registrants for prenatal care
  • To assess findings that may increase risk of aneuploidy or screen for fetal anomalies

Protocol for Second and Third Trimester Sonography

  1. Fetal cardiac motion, fetal number, presentation, and activity should be documented. In multiple gestations, perform the following on each fetus: amnionicity, chorionicity, comparison of fetal sizes, estimation of amniotic fluid on each side of the membranes, and fetal gender when visualized. ext{Amnionicity}= ext{number of amniotic sacs},
    ewline ext{Chorionicity}= ext{number of chorionic sacs}.

  2. Qualitative or semiquantitative estimate of amniotic fluid volume should be reported. Abnormal fluid amounts should be described. In early pregnancy, amniotic fluid is produced by the placenta; fetal kidneys begin to produce urine, contributing to the amniotic fluid via swallowing and urination. Fluid increases until ~34 weeks.

    • Semiquantitative methods include four-quadrant amniotic fluid index (AFI), single deepest pocket, and two-diameter pocket. Excessive fluid = ext{hydramnios (polyhydramnios)}; too little fluid = ext{oligohydramnios}.

  3. Placental localization and appearance, and its relationship to the internal cervical os should be recorded. Image the umbilical cord and evaluate the number of vessels when possible. Ensure maternal bladder is adequately filled to visualize the cervical os in the lower uterine segment. Document that the lower end of the placenta is away from the cervical os to rule out placenta previa.

    • Be aware that overdistended bladder or contractions can falsely suggest placenta previa. Placental location in early pregnancy may not correlate with location at delivery.

  4. Gestational (menstrual) age should be assessed by sonographic biometry. The CRL measured in the first trimester remains the most accurate method for dating. In the second and third trimesters, multiple sonographic parameters can be used, and measurement variability increases with gestational progress. If clinical gestational age and sonographic parameters differ significantly, consider fetal growth abnormalities such as macrosomia or IUGR. Common biometric parameters include:

    • ext{BPD} measured in the axial plane that includes the thalamus and cavum septi pellucidi.
    • ext{HC} (head circumference).
    • FL or FDi (femur length/diaphysis length) measured after ~14 weeks; most accurate when the femoral shaft is perpendicular to the acoustic beam.
    • AC (abdominal circumference) measured on a true transverse view at the level of the junction of the umbilical vein and portal sinus; measure at the skin line with visible portal sinus, stomach, and umbilical vein; use AC to estimate fetal weight.
    • Note: Abdominal circumference measurement uses the skin line on a true transverse view; HC and BPD measurements have head shape considerations (brachycephaly vs dolichocephaly).

  5. Evaluation of maternal anatomy including uterine, adnexal, and cervical structures should be performed to document presence, location, and size of uterine or adnexal masses that may complicate obstetric management. Normal maternal ovaries may not be imaged during the 2nd and 3rd trimesters.

  6. Fetal anatomy may be adequately assessed after 18 weeks of gestation. Anatomy imaging may be limited by fetal movement, size, position, maternal scars, or increased wall thickness. If anatomy is not seen due to technical limitations, note the reason and consider a follow-up examination.

  7. Documentation guidelines: During standard obstetric sonography, the following anatomy should be documented; more detailed studies can be performed if anatomy appears questionable or abnormal. Documentation and images should be retained. Anatomy to document:

    • 1) Head and neck: Cerebellum, choroid plexus, cisterna magna, lateral cerebral ventricles, midline falx, cavum septi pellucidi, upper lip
    • 2) Chest: Four-chamber view of the fetal heart; if feasible, extend to include outflow tracts for a basic cardiac exam
    • 3) Abdomen: Stomach (presence, size, situs), kidneys, bladder, umbilical cord insertion into the abdomen, umbilical cord vessels
    • 4) Spine: Cervical, thoracic, lumbar, sacral spine
    • 5) Extremities: Presence or absence of arms and legs
    • 6) Gender: Medically indicated in low-risk pregnancies primarily for those with multiple pregnancies

Diagnostic and Screening Aspects of Obstetric Sonography

  • Diagnostic vs screening roles

    • Diagnostic tests provide definitive information about the clinical question or presence/absence of a finding.
    • Obstetric sonography examinations are typically diagnostic with respect to fetal heart motion, fetal number, fetal biometry, fetal presentation, placental location, presence of maternal pelvic or adnexal masses, and major disruptions of fetal anatomy (e.g., anencephaly).
    • Screening tests do not provide definitive diagnosis but indicate whether the patient or pregnancy is at greater or lesser risk.
    • A normal ultrasound with respect to the findings does not completely rule out anomalies; screening reduces risk but does not provide certainty.

  • Factors influencing sensitivity

    • Sensitivity depends on maternal habitus, expertise of sonographers and responsible physicians, risk level of patients, number and timing of examinations, and type of anomaly.
    • Sensitivity tends to be higher for defects of the central nervous system and urinary system and lower for defects of the heart and great vessels.
    • Higher sensitivity in tertiary care centers and with specialty obstetric examinations.
    • Maternal obesity can impair visualization of anatomy.

  • Counseling and limitations

    • Patients should be counseled about limitations of obstetric sonography.
    • Counseling should include that screening examinations do not detect all anomalies, that false-positive findings are possible, and that the sensitivity of sonography is not certain in any situation.

  • Practical caveats

    • The sensitivity and accuracy of detecting anomalies can vary by defect type and imaging context.
    • Serial imaging can help assess growth and development, particularly for suspected growth abnormalities.

  • Formulas and key variables used in biometric assessment (for quick reference)

    • Crown-Rump Length: CRL (first trimester dating metric)
    • Biparietal Diameter: ext{BPD} (measured in axial plane including thalamus and cavum septi pellucidi)
    • Head Circumference: ext{HC}
    • Femur Length: ext{FL}
    • Abdominal Circumference: ext{AC}
    • Amniotic Fluid Index: AFI (four-quadrant) or single deepest pocket/ two-diameter pocket methods
    • Fetal weight estimation often uses AC, HC, BPD, FL in various formulas; note that best estimates have significant error margins and interval growth is better assessed with serial measurements.

  • Practical implications

    • Regular counseling about limitations and interpretation is essential for expectant parents.
    • Early and targeted imaging improves detection of specific anomalies but cannot guarantee absence of all anomalies.