What is progesterone?

Progesterone is a hormone produced by the ovaries and placenta that helps to maintain pregnancy. It's main effects on tissues inside the body include induction of an elaborate network of glands (endometrial glands) in the uterus to help provide nutrition to the early conceptus(baby) and become the maternal side of the placenta (connection between baby and mom). During pregnancy, it helps keep the uterine muscle layers relatively quiet so as not to disrupt a pregnancy. It also helps suppress the mother's immune response to its "foreign" baby as the baby grows and develop its own immune system. Progesterone also provides the stimulus to development of the glandular portions of the mammary glands (breast tissue)- it along with estrogen and other hormones, produce the changes to breast tissue after puberty, throughout pregnancy and during nursing to allow these tissues to produce milk for babies. Progesterone enhances the effects of estrogen on the female's brain to provide outward signs of estrus ("heat"). This is one of the reasons why a bitch's first heat can be silent or go unobserved when there was not sufficient progesterone to prime the brain prior to the secretion of estrogen. Progesterone, also like other steroids, can reduce the body's sensitivity to hormones like insulin that are helpful to glucose control- this is helpful to a pregnant mother with young, growing babies that require large amounts of energy as they develop into late pregnancy, but can lead to poor glucose control (gestational diabetes) in some bitches.

When is progesterone secreted?

Progesterone is not just elevated after ovulation and throughout pregnancy. In fact, in dogs, foxes and wolves, levels of progesterone start to climb around the time of the LH peak (the brain's signal to initiate ovulation). The sites where eggs will be released (antral follicles) from the ovaries start to change prior to ovulation and develop small amounts of luteal (Latin word for yellow) tissue. This luteinization of the follicles assist in follicular rupture and increases after ovulation as the cells in the ovary around the follicle, divide and continue to secrete large amounts of progesterone that will be needed to provide a uterine environment ready for embryos.

This means that progesterone will be produced in substantial amounts around the time of ovulation in the bitch and continue to rise into the first half of pregnancy.

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