Circulating levels of oestradiol-17beta during early pregnancy in the Alaskan fur seal showing an oestrogen surge preceding implantation

Circulating levels of estradiol-17beta during early pregnancy in the Alaskan fur seal (Callorhinus ursinus) were studied to determine whether an estrogen surge coincided with the period of blastocyst growth immediately before implantation. Blood samples were obtained from 38 adult female seals during the period of blastocyst dormancy and early postimplantation. An estrogen surge (35 pg/ml) occurred immediately before the time of blastocyst reactivation and accelerated growth leading to implantation. A prolonged period (almost 2 months to return to lowest levels of 11 pg/m1) of high estrogen concentration followed ovulation. A return to high levels was characteristic of postimplantation. It is concluded that the Alaskan fur seal, having obligatory delayed implantation, has an estrogen surge near the end of that delay period.