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== Introduction ==

== Introduction ==

* The ruminant conceptus undergoes rapid elongation between day 14 and 16.

* The ruminant conceptus undergoes rapid elongation between day 14 and 16.

* During this period, IFN-τ is secreted at high levels before decreaseing.

* During this period, IFN-τ is secreted at high levels before decreasing.

* Small embryos produce much less IFN-τ than larger embryos.

* Small embryos produce much less IFN-τ than larger embryos.

* There is a hige proportion of embryo loss in the first 3 weeks of gestation, as small embryos fail to secrete enough IFN-τ in this critical period.  Thus they do not block luteolysis.

* There is a hige proportion of embryo loss in the first 3 weeks of gestation, as small embryos fail to secrete enough IFN-τ in this critical period.  Thus they do not block luteolysis.

* The lifespan of the corpora lutea in a pregnant queen is extended to ~60 days by the action of pituitary prolactin.

* The lifespan of the corpora lutea in a pregnant queen is extended to ~60 days by the action of pituitary prolactin.

* There is no need for a conventional 'maternal recognition of pregnancy'.

* There is no need for a conventional 'maternal recognition of pregnancy'.