Obstetrical Ultrasound

/

Sonographic nomenclature breaks down into three main types of ultrasound:

  1. Limited US: for a specific, singular purpose (i.e., viability, placental location, presentation, cervical length).

  2. Standard US: A more thorough examination, which requires a number of elements including:

    • Fetal presentation and number 

    • Amniotic fluid volume

      1. AFI (normal 5-25) or DVP (normal 2-8)

      2. SMFM recommends using DVP over AFI to diagnose oligo in the third trimester as using AFI leads to more interventions without improving perinatal outcomes 

    • Fetal Heart Rate / Rhythm

    • Placental location 

      1. Previa covers cervical os, either partially or wholly.

      2. Low lying placental edge is within 2cm of os.

        1. Partial or marginal previa is unfavored terminology.

      3. Suspicion for abnormal placentation for previa in setting of history of cesarean. With previa, history of 

        1. 1 prior cesarean = 3% risk of PAS

        2. 2 prior cesarean = 11% risk of PAS

        3. 3 prior cesarean = 40% risk of PAS

        4. 4 prior cesarean = 61% risk of PAS

    • Fetal biometry/anatomic survey.

    • Cervix and adnexa when clinically appropriate and when technically feasible. 

  3. Specialized US: A highly technical ultrasound that focuses on particular organ systems or uses dynamic measures.

    1. Examples include level 2 ultrasounds which provide further anatomic detail than a standard US, or fetal doppler ultrasonography, biophysical profiles, or fetal echocardiograms.

Important Facts for Ultrasound, by Trimester:

First Trimester: Know the absolute and relative criteria for early pregnancy failure, from SRU’s publication in NEJM:

(c) NEJM, 2013

2nd / 3rd Trimester: Know components of a basic growth scan by the Hadlock formula. Some representative photos are here! Images courtesy of www.fetalultrasound.com. Be sure to check out The OBG Project 2nd Trimester Ultrasound Atlas if you’re a chief resident with access to OBG First!

Appropriate plane for BPD and HC measurements

Appropriate plane for fetal abdominal circumference.

Appropriate plane for fetal abdominal circumference.

Appropriate plane and landmarks for fetal femoral length measurement.