Progesterone is an endogenous steroid hormone that is commonly produced by the adrenal cortex as well as the gonads, which consist of the ovaries and the testes. Progesterone is also secreted by the ovarian corpus luteum during the first ten weeks of pregnancy, followed by the placenta in the later phase of pregnancy.

What is the main purpose of progesterone?

The main function of progesterone is to prepare the endometrium (lining of your uterus) for a fertilized egg to implant and grow. If a pregnancy doesn’t occur, the endometrium sheds during your menstrual period. If conception occurs, progesterone increases to support the pregnancy.

Progesterone and menstruation

Ovulation (when your ovary releases an egg) occurs around the middle of a person’s menstrual cycle. The corpus luteum forms from the empty egg follicle and begins producing progesterone. Your corpus luteum is a temporary gland that helps support the beginning of a pregnancy if conception occurs during that cycle. Progesterone works by thickening your uterine lining and creating a good environment for a fertilized egg to implant.

If an egg isn’t fertilized during that cycle (meaning you don’t get pregnant), the corpus luteum breaks down, which decreases progesterone levels. Decreasing progesterone levels means your uterine lining thins and breaks down, causing the beginning of your menstrual period.

Progesterone during pregnancy

If an egg is fertilized by sperm and conception occurs, the corpus luteum doesn’t break down and continues to make more progesterone. Your uterine lining is thick and rich in blood vessels, which provides nutrients for the fertilized egg (now an embryo). Once the placenta forms, it’ll take over progesterone production.

During pregnancy, progesterone levels increase each trimester, reaching their highest level in your third trimester (weeks 28 to 40 of pregnancy). Progesterone levels decline in the years leading up to menopause, when ovulation stops.

What does progesterone do during pregnancy?

Progesterone is critical in supporting a pregnancy because it thickens your uterine lining. A thick uterine lining helps a fertilized egg grow into an embryo, and then to a fetus.

Progesterone levels continue to rise during pregnancy. High progesterone levels prevent your body from ovulating while you’re pregnant. It also suppresses uterine contractions, which helps you avoid preterm labor. Finally, progesterone helps your breasts prepare for breastfeeding (chestfeeding).

Because progesterone is so important in maintaining the early stages of pregnancy, low progesterone levels may make it hard for you to conceive and may put you at higher risk for miscarriage.

What does progesterone do to my body?

Progesterone does several things, including:

• Thickening the lining of your uterus for implantation.
• Regulating bleeding during menstruation.
• Supporting a pregnancy once conception occurs.
• Helping to improve your mood.
• Supporting thyroid function.
• Supporting lactation.