Anatomical detail visible in ultrasound images. Ultrasound images (A, C) and H&E histological sections (B, D) of implantation sites at E6.5 (A, B) and E7.5 (C, D). Divisions in the scale in A and C are 100 μm apart. The conceptus in histological sections is smaller than in vivo due to shrinkage during tissue preparation (fixation and dehydration). AC, amniotic cavity; Al, allantois; Emb, embryo; EPC, ectoplacental cone region; Exo, exocoelomic cavity.