SIZE: 0.1 - 0.2 mm

TIME PERIOD: 7 - 12 days post-ovulation

Trophoblast cells continue to engulf and destroy cells of the uterine lining creating blood pools and stimulating new capillaries to grow - beginning the growth of the placenta.

The blastocyst inner cell mass differentiates into two layers:

EPIBLAST The top layer of cells (dark blue) which will become the embryo and amniotic cavity.

HYPOBLAST Lower layer of cells (yellow) which become the yolk sac.

Ectopic pregnancies - implantation sites not in the uterus - can occur at this time and may continue up to 16 weeks unnoticed. Quick diagnosis can pharmacologically treat without surgery preserving the site of the pregnancy.