The main function of progesterone is to prepare the lining of your uterus (endometrium) for the implantation and growth of a fertilized egg. If pregnancy does not occur, the endometrium is shed during menstruation. If conception occurs, progesterone increases to support pregnancy.
Progesterone and Menstruation
Ovulation (the release of an egg from the ovary) occurs mid-cycle. The corpus luteum forms from the empty follicle and begins to produce progesterone. The corpus luteum is a temporary gland that helps support the start of pregnancy if conception occurs during the menstrual cycle. The role of progesterone is to thicken the endometrium to create a good environment for the fertilized egg to implant.
If the egg is not fertilized during that cycle (i.e. you do not become pregnant), the corpus luteum breaks down, which reduces progesterone levels. Reduced progesterone levels mean that your endometrium thins and breaks down, causing menstruation to start.
Progesterone during pregnancy
If the egg is fertilized and conception occurs, the corpus luteum does not break down and continues to produce more progesterone. The endometrium is thick and rich in blood vessels to provide nutrients to the fertilized egg (now an embryo). Once the placenta forms, it takes over progesterone production.
During pregnancy, progesterone levels increase every three months, reaching their highest levels in the third trimester (weeks 28 to 40 of pregnancy). Progesterone levels drop in the years before menopause, when ovulation stops.
What role does progesterone play during pregnancy?
Progesterone is essential for pregnancy because it thickens the lining of the uterus. A thick lining helps a fertilized egg develop into an embryo and, in turn, a fetus.
Progesterone levels continue to rise during pregnancy. High progesterone levels prevent your body from ovulating during pregnancy. It also suppresses uterine contractions, which helps you avoid premature birth. Finally, progesterone helps prepare your breasts for breastfeeding (chest feeding).
Because progesterone is so important for maintaining an early pregnancy, low progesterone levels may make it difficult for you to get pregnant and may increase your risk of miscarriage.
What does progesterone do in my body?
Progesterone has a variety of roles, including:
Thickens the lining of the uterus to facilitate implantation.
Regulates bleeding during menstruation.
Supports pregnancy once conception occurs.
Helps improve your mood.
Supports thyroid function.
Supports lactation.