Using high-resolution real-time ultrasound in the first trimester of pregnancy, we have been able to demonstrate, in addition to the fetus, the yolk sac and the amniotic membrane. Pitfalls to avoid in first-trimester pregnancies include (a) mistaking the yolk sac for a second separate fetus, (b) including the yolk sac in the crown-rump length, and (c) misinterpreting the amniotic membrane for a septum between two separate gestational sacs.
No takes yet. Share an insight, caveat, or question.
Sauerbrei et al. (1980) studied this question.
Synapse has enriched one closely related paper. Consider it for comparative context: