Blastocyst oestrogen synthesis and the maternal recognition of pregnancy

The maternal recognition of pregnancy takes a number of forms in different species; among the eutherian mammals the maintenance of luteal function and cessation of oestrous or menstrual cycles is an important event in early pregnancy. In the pig the embryos signal their presence in the uterus between Days 10 and 12 post coitum; this time corresponds to the onset of blastocyst synthesis of oestrogens, which are luteotrophic in this species, and it has been suggested that oestrogens may constitute an embryonic signal responsible for maintained luteal function in pregnancy. Although oestrone sulphate, which is formed from oestrogens of embryonic origin by uterine sulphotransferase, has been found in the maternal circulation in high concentrations after Day 15 p.c., its appearance is late relative to the time of maternal recognition of pregnancy. Therefore an alternative mechanism has been sought. The recent finding that oestradiol is capable of reducing uterine prostaglandin F2 alpha secretion (i.e. acting as an antiluteolysin), and that it is present in uterine venous blood as early as Day 12 p.c. in pregnant pigs, suggests a mechanism whereby blastocyst oestrogens may be capable of influencing luteal function.