Anatomical Screening of the Second Trimester (19–21 weeks): Fetal Organ Checklist
Regulatory Framework of the Study
The conduct of routine mid-trimester screening is regulated by ISUOG recommendations (Salomon LJ, Alfirevic Z, Berghella V, et al. ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2022;59:840-856). Additionally, AIUM practice parameters for detailed second- and third-trimester diagnostic examinations (AIUM practice parameter for the performance of detailed second- and third-trimester diagnostic obstetric ultrasound examinations. J Ultrasound Med. 2019;38:3093-3100) and SOGC recommendations on the content of routine second-trimester examination (Cargill Y, Morin L. No. 223. J Obstet Gynaecol Can. 2017;39:e144-e149) are applied.
Components of Systematic Examination
According to the sources, a complete mid-trimester examination includes the following mandatory sections (Clinical Ultrasound in Gynecology and Obstetrics, Chapter 6):
SectionContentBiometryStandard fetal biometry and interval growth assessmentAnatomySystematic review of fetal anatomyPlacenta and Umbilical CordPlacenta localization, umbilical cord attachment site, amniotic fluid volume assessmentCervixCervical length and preterm birth risk screeningDocumentationStandards for documenting the mid-trimester examination
Organ System Checklist
A detailed list of organs and structures to be visualized during routine second-trimester screening is provided in the ISUOG Practice Guidelines (2022) [note — the specific list of structures in the provided excerpts is not detailed].
Assessment of CNS structures includes the examination of the thalami and derivatives of the median brain structures; specifically, in differential diagnosis, cysts of the cavum velum interpositum are considered, the size and prevalence of which in the second trimester are described in separate studies (Moradi B, et al. Taiwan J Obstet Gynecol. 2019;58:814-819).
Remarks on Timing
Some structural anomalies can be detected as early as the first trimester with a standardized anatomical protocol (Liao Y, et al. Am J Obstet Gynecol. 2021), however, a complete systematic anatomical examination pertains to the mid-trimester (ISUOG, 2022). The exact optimal timing of 19–21 weeks is not specified in the provided excerpts [clarify].
Frequently asked questions
Which document regulates the anatomical screening of the second trimester?
ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan (Salomon LJ, et al. Ultrasound Obstet Gynecol. 2022;59:840-856), as well as AIUM parameters (2019) and SOGC recommendation No. 223 (2017).
What mandatory sections are included in the examination?
Biometry and growth assessment, systematic review of anatomy, placenta localization and umbilical cord attachment site, amniotic fluid volume, cervical length, and standardized documentation (Clinical Ultrasound, Chapter 6).
Is it necessary to assess cervical length?
Yes, according to the sources, the assessment of cervical length and preterm birth risk screening are included in the content of the mid-trimester examination.
Can anomalies be detected earlier than the second trimester?
Some structural anomalies are detected during first-trimester screening with a standardized protocol (Liao Y, et al., 2021), but a complete systematic anatomical examination pertains to the mid-trimester (ISUOG, 2022).
Salomon LJ, Alfirevic Z, Berghella V, et al. ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2022;59:840-856; AIUM practice parameter for the performance of detailed second- and third-trimester diagnostic obstetric ultrasound examinations. J Ultrasound Med. 2019;38:3093-3100; Cargill Y, Morin L. No. 223. J Obstet Gynaecol Can. 2017;39:e144-e149; Moradi B, et al. Taiwan J Obstet Gynecol. 2019;58:814-819; Clinical Ultrasound in Gynecology and Obstetrics (Rhett Holland, 2026), Chapter 6.