The corpus luteum is a cyst that forms from the egg follicle after ovulation. This body secretes progesterone into your system in the luteal phase of your cycle, as well as if you fall pregnant in that particular cycle.

To understand what the corpus luteum is, it’s necessary to start at the beginning, with your cycle

The stages of your menstrual cycle

Each cycle has three stages: the follicular phase, fertile phase and luteal phase.

The start of your follicular phase is menstruation. Next, your body’s levels of estrogen and FSH increase and the follicles in your ovaries are stimulated to grow and mature. It’s quite common for between 10 and 20 eggs to be stimulated in one cycle. Eventually, one of them reaches maturity and bursts free of the follicle. This is the moment of ovulation.

The fertile phase of your cycle overlaps with the other two phases. You start to become fertile just before ovulation and stay that way for about two or three days after the event. After ovulation, your LH levels rise and you may see a very wet vaginal discharge similar to that of the whites of a raw egg.

After ovulation, the leftover follicle on your ovary turns into the corpus luteum. This is the body that is responsible for secreting progesterone. Your hormone levels now shift again as progesterone rises and estrogen decreases. This change causes your endometrium to thicken and prepare for a fertilized egg. If conception doesn’t happen, your progesterone levels drop and you begin your cycle again with menstruation.

The corpus luteum can confuse even doctors

There are various kinds of cysts that can occur on the ovaries. In an ultrasound, your doctor can sometimes confuse a corpus luteum for something a bit more problematic. If they are looking for a reason that you may be struggling to fall pregnant or if your cycle isn’t regular, they may see cysts on your ovaries as a potential cause.

However, it is important to remember that not all cysts on your ovaries are cause for concern. Most of the time, they will reduce in size or disappear entirely all on their own. If your doctor notes that you have a functional or secretory cyst, this is actually a good sign if you want to fall pregnant.

In the case of the corpus luteum, it’s important for the cyst to remain there for the duration of your cycle, and for the duration of your pregnancy if you conceive. This is because it is responsible for the production of progesterone, which will keep the lining of your uterus thick and welcoming to your growing embryo.

The symptoms of early pregnancy

Once the ovum is fertilized, it begins to multiply rapidly. It also takes a journey from the fallopian tubes through to the uterus, where it implants into the endometrium. At this point, your progesterone levels will increase even more. The corpus luteum that was created in that cycle will stay in place and cause these high levels of progesterone. This will, in turn, cause the symptoms of early pregnancy to appear.

You’ll start to notice a few changes that are quite subtle at first and can be confused for your usual PMS. These symptoms include sensitive breasts, cramps and headaches.

If you start to experience these symptoms, it’s important to wait until your menstruation is late before getting your hopes up. If the symptoms continue and you are now around five days late, then take a pregnancy test to confirm your theory. If you take the test too early, you may get a false negative.

Readers’ questions:

How does the corpus luteum form?

The corpus luteum forms on the ovary each cycle. It is the remains of the follicle from which the mature egg burst during ovulation. This cyst is responsible for secreting progesterone to get the lining of your uterus to thicken in preparation for a fertilized egg. If no egg is implanted, the corpus luteum will stop producing progesterone and disappear.

How do you identify the corpus luteum?

Your doctor will be able to see the cyst formation through an ultrasound scan. They can use this to verify that you have in fact ovulated that cycle.

What happens to the corpus luteum during pregnancy?

When a fertilized embryo implants into the lining of the uterus, the corpus luteum stays in place and can actually grow in size. This is so that it can produce more progesterone to maintain your pregnancy.