early pregnancy

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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:
- 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 C RL 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.

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).