Chorionic gonadotrophin in the mouse from implantation to term.

Chorionic gonadotrophin (CG) in implantation sites from Days 5 to 12 and placentae from Days 13 to 19 of pregnancy in the mouse was determined by utilizing a cross-reaction with human CG in a radioimmunoassay. Significant amounts of CG could be detected throughout the period of gestation investigated. The amount of CG per implantation site increased steadily from Day 5 to a peak on Day 11. In the second part of pregnancy a second peak was observed with the highest amount of CG per placenta found on Day 16. No gonadotrophic activity could be detected in blastocysts flushed from the uteri on Day 5 of pregnancy or in uteri of non-pregnant mice.