early pregnancy

Early pregnancy roughly spans the first ten weeks of the first trimester.

Radiographic features

Antenatal ultrasound 
  • 0-4.3 weeks: no ultrasound findings
  • 4.3-5.0 weeks: 
  • 5.1-5.5 weeks:
  • 5.5-6.0 weeks 
  • >6.0 weeks
    • fetal pole may be identifiable on endovaginal ultrasound (1-2 mm)
    • fetal heart rate (FHR) should be ~100-115 bpm
    • gestational sac should be ~10 mm in diameter
  • 6.5 weeks
    • crown rump length (CRL) should be ~5 mm
  • 7-8 weeks
    • CRL is between 11-16 mm
    • cephalad and caudal poles can be identified
  • 8-9 weeks
    • CRL is between 17-23 mm
    • limb buds appear
    • head can be seen as separate from the body
  • 9-10 weeks
    • CRL is between 23-32 mm
    • fetal heart rate 170-180 bpm
    • fetal movement can be seen
    • a round hypoechoic structure in the fetal brain represents a developing embryonic/fetal rhombencephalon
    • nuchal translucency may begin to be seen
Transvaginal/endovaginal (TV/EV) scanning
  • intradecidual sac sign (IDSS): early sign on a TV scan
  • when the MSD measures 25 mm, an embryo must be visible
  • when the CRL measures >7 mm, an embryo must show cardiac activity
  • an embryo should be seen <=14 days after a scan with a gestational sac without a yolk sac
  • an embryo should be seen <=11 days after a scan with a gestational sac and a yolk sac
Transabdominal (TA) scanning
  • when the MSD measures 20 mm a yolk sac should be visible
  • when the MSD measures 25 mm, an embryo must be visible

Occasionally, early pregnancy is unintentionally imaged by CT or sometimes MRI is done for some concurrent pathology, and its important to know the imaging findings.

  • fluid-filled cystic structure in endometrial cavity (well identified on MRI, and may be visible on CT especially on delayed post-contrast images)
  • developing placenta seen as curvilinear enhancing structure
  • fetal pole may be seen in delayed first trimester imaging
  • corpus luteal cyst may be visible in one of the ovaries
    • unilocular <3 cm cyst with irregular crenated and enhancing walls

Differential diagnosis to be considered with a positive urinary pregnancy test includes

If urinary pregnancy test is negative similar findings may suggest submucosal fibroid or retained products of conception.

Practical points

The earlier in pregnancy a scan is performed, the more accurate the age assignment from crown rump length. The initial age assignment should not be revised on subsequent scans .

Overall, the accuracy of sonographic dating in the first trimester is ~5 days (95% confidence range).

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