Diagram of the molecular structure of progesterone.
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Hormones 101: What is Progesterone?

Jen on the roof terrace at Natural Cycles headquarters.

Written by Jennifer Gray

Jennifer Gray

Jennifer Gray is an award-winning writer with more than five years’ experience covering reproductive topics ranging from birth control to planning pregnancy. She is passionate about providing women with accurate information grounded in science they can use to take charge of their own health - while also dispelling myths that exist within the field of women’s health. She holds a Master of Science from the University of Edinburgh and currently lives in Ireland.

Medically reviewed by Jack Pearson, Medical Affairs Manager at Natural Cycles

Jack Pearson

Dr. Jack Pearson is a previously HCPC registered Embryologist with a PhD in reproductive medicine. Prior to joining Natural Cycles leading Medical Affairs, he worked for more than 10 years in a clinical setting working at some of the busiest fertility clinics in the UK. Today he spends most of his time working with experts at the world’s leading institutions to carry out important research with the vision to further the field of female health. He earned his PhD from the University of Sheffield specializing in Sperm Metabolism and currently lives in London.

Understand the role sex hormones play in the menstrual cycle with our quick guide to hormones. Here at Natural Cycles, we aim to break down the science of reproductive health and answer questions like ‘how do hormones work?’, 'what is progesterone?’ and much more.

How do hormones work?

To understand what progesterone is and how it works, it’s important we first understand the role of hormones more generally. Hormones are produced in glands and, in simple terms, work as a sort of messenger for the body.

Hormones are made out of a chemical substance that travels through the tissue in order to reach different parts of the body. When a hormone gets to its destination, the message is ‘delivered’ and this starts a process. For example, the sex hormone estrogen is produced in the ovaries and is important in triggering different phases in the menstrual cycle.

What is progesterone?

Progesterone is a sex hormone found in both men and women. As with estrogen, progesterone plays a key role in female reproductive health and, in particular, is important in the later stages of the menstrual cycle and during pregnancy.

Progesterone and the menstrual cycle

Progesterone is a big player in the latter phase of the menstrual cycle (the luteal phase). This is because progesterone is the hormone that keeps the uterus lining thick. If an egg cell is fertilized, then progesterone levels stay steady and this prevents uterine contractions and shedding.

If a woman gets pregnant, progesterone levels stay stable in order to stop the cycle from ending and her period starting. Progesterone is also essential to the body’s function during pregnancy, as it also helps get the breasts ready for milk production. Alternatively, if an egg is not fertilized, then progesterone levels decline, the uterus lining sheds (a woman gets her period), and a new cycle begins.

Progesterone in birth control

Hormonal birth control works by using synthetic estrogen and/or progesterone to keep hormones steady. Without a surge or decline in hormones, different phases of the menstrual cycle cannot be triggered. For example, a steady level of progesterone prevents ovulation from taking place. So if the levels are kept stable, no egg will be released and so cannot be fertilized.

There are several types of hormonal birth control which only use progesterone (and not estrogen), these include the mini-pill (or the progesterone-only pill) and the contraceptive injection. These types of birth control can work well for women who cannot use birth control with synthetic estrogen but are still looking for hormonal birth control options.

Could non-hormonal birth control work for me?

If you are searching for a non-hormonal birth control method that adapts to your unique cycle, then the Natural Cycles birth control app might be what you’re looking for. The intelligent app works by analyzing daily basal body temperature readings and identifying your fertile window. This means you will know where you are in your cycle and can use condoms or abstain from sex on fertile days to prevent pregnancy.

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