What Is Progesterone?

So-called “female hormones” play an important role in keeping your reproductive cycle on track. The body ramps up production of the female hormone progesterone during pregnancy, but what exactly does it do? Let’s run through the functions and effects of progesterone throughout your cycle, plus what it does for pregnant moms.

Progesterone: What is it?


Progesterone is one of the hormones secreted by the ovaries just after ovulation. When you ovulate, the egg cell that up until then had been wrapped up in a bundle of tissue called the ovarian follicle breaks free. The egg is now ready to be fertilized, and the leftover follicular matter in the ovary turns into a temporary gland called the corpus luteum. The corpus luteum secretes the hormone progesterone.

The corpus luteum hangs around secreting progesterone for about two weeks after ovulation – in other words, up until your next period starts. The increase in progesterone causes a slight increase in body temperature. During these two weeks, it encourages the endometrium lining of the womb to be thick and soft in case it needs to support a fertilized egg. This is why progesterone is often called the pregnancy hormone.

What does progesterone do?

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After ovulation, progesterone kicks in to get the uterus ready to support a pregnancy. That includes triggering the following physical changes:

Developing the mammary glands

In order to prepare for breastfeeding the baby that would ultimately grow from a fertilized egg, progesterone stimulates the mammary glands that will be responsible for milk production.

Maintaining the endometrium

The endometrium lining of the womb begins to be several millimeters thick in the weeks leading up to ovulation, but in order for a fertilized egg to attach there and start developing, the endometrium lining needs to be about 1 centimeter thick. After ovulation, progesterone increases bloodflow to the endometrium and surrounding tissues, also raising the body temperature. This makes the endometrium soft and gives it enough nutrients in case a fertilized egg attempts to settle in.

Raising Basal Body Temperature (BBT)

It’s thought that the rise in body temperature helps support a nurturing environment for a fertilized egg to implant in the uterus. Higher progesterone means your body basal temperature stays slightly elevated.

Elevated progesterone levels during the luteal phase leading up to a menstrual period are associated with the symptoms of PMS (pre-menstrual syndrome). When progesterone is secreted too much compared with estrogen, it can really throw you for a loop! In that case, you can blame progesterone for pre-menstrual moodiness, rough skin and constipation.

However, healthy levels are a requirement for getting pregnant, but if you’re not producing enough, it could be a sign of a condition called luteal phase defect (also called luteal insufficiency). This makes it more to difficult to fall pregnant or carry a pregnancy to term, and is also associated with symptoms like depression, weight gain and gall bladder problems. As with many things in life, it’s all about balance when it comes to progesterone and estrogen.

Progesterone levels over the menstrual cycle

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Assuming you’re not pregnant, your progesterone levels will rise and fall in rhythm with your menstrual cycle. Progesterone levels rise after ovulation, but if a fertilized egg doesn’t set up shop in the uterus in that cycle, then progesterone levels decline, body temperature drops back down, and the endometrium breaks down and bleeds out in the next menstrual period.

On the other hand, if you do fall pregnant, progesterone continues to be secreted and body temperature remains elevated. This is why an elevated Basal Body Temperature after the day when your period is due is considered a tell-tale sign of pregnancy.

What happens to progesterone when you’re pregnant?

During pregnancy, the corpus luteum in the ovaries steps up production of progesterone to support the womb development and help establish the placenta. The placenta will be finished in Week 16, at which point it will take over production of progesterone for the remainder of the pregnancy.

The increased progesterone in the bloodstream increases the carbon dioxide (CO2) sensitivity in the respiratory center of the brain. Although the number a pregnant mom needs to breathe stays the same, she’s going to need to breath more deeply with each time to get rid of the CO2.

Muscle-relaxing effects of progesterone don’t just affect the uterus: it also affects the movement of the stomach and guts, which can result in constipation. This may worsen as pregnancy progresses and the growing womb increases pressure on the surrounding organs.

Progesterone levels tend to peak during Month 7 to 8, then gradually decline, and finally plummet once the baby is born. Progesterone release is then inhibited in relation to prolactin (the hormone that facilitates breastfeeding), but as the ovaries and uterus come back online and the ovulation cycle resumes, then progesterone returns to normal.

Progesterone is important for maintaining pregnancy

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On one hand, progesterone is the hormone responsible for PMS moodiness, constipation and rough skin. On the other, it makes pregnancy possible, so it does kinda allow for the continuation of the human species. Whether or not you’re planning to get pregnant, it’s an important hormone to know about! Speak to your doctor if you think your progesterone levels aren’t quite where they should be.