Structures generally develop in the following predictable sequence.
- Gestational sac
- Double decidual reaction
- Yolk sac (yolk sac with intrauterine gestational sac confirms early intrauterine pregnancy)
- Embryonic heartbeat.
The timing of structure development is also fairly predictable.
- Gestational sac:
- Visible at approximately 5 weeks gestation, ± 4 days.
- Yolk sac:
- Visible at approximately 5½ weeks gestation, ± 4 days.
- Embryo and heartbeat:
- Visible at approximately 6 weeks gestation, ± 4 days.
Correlate with ultrasound appearances (see below) and refer to local clinical guidelines.
Figure 1: βhCG chart
The presence of a yolk sac within the intrauterine gestational sac confirms an intrauterine pregnancy and essentially excludes ectopic pregnancy. Heterotopic pregnancy is very rare but should be considered if there are suggestive ultrasound features, particularly in the setting of assisted reproductive technology.
Growth is approximately 1.2 mm per day, but may be less in a normally developing pregnancy. Interval growth of CRL alone should not be used as a determinant of pregnancy loss.
Figure 2: Crown-rump length
Embryonic cardiac activity should always be visualised with a CRL ≥7 mm.
Slow embryonic heart rate of <80 bpm may suggest a guarded prognosis for the pregnancy. Suggest a follow-up scan if clinically appropriate.