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

Radiographic features

Ultrasound 

• 0-4.3 weeks: no ultrasound findings

• 4.3-5.0 weeks: 

  • possible small gestational sac

  • possible double decidual sac sign (DDSS)

  • possible intradecidual sac sign (IDSS)

• 5.1-5.5 weeks:

  • ​gestational sac should be visible by this time

• 5.5-6.0 weeks 

  • yolk sac should be visible by this time

  • gestational sac should be ~6 mm in diameter

  • double bleb sign

• >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

CT/MRI

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 ³.

• 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

• ectopic pregnancy

• missed abortion

• gestational trophoblastic disease

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 ⁵.

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

See also

• failed early pregnancy

• pregnancy of uncertain viability