Everything you need to know about progesterone

Progesterone belongs to a group of steroid hormones called ‘progestogens’.

So here’s what happens in a normal cycle: Every single month, a female produces a group of follicles that are released from the storage. One of these follicles will be recruited as the dominant follicle and will ovulate. After ovulation, it switches to the corpus luteum, and it’s the corpus luteum that produces progesterone.

However, when the egg is fertilised, progesterone stimulates the growth of blood vessels that supply the endometrium and it converts that lining into its secretory phase when it's going to be most receptive to the embryo implanting in the lining.

Progesterone makes the cervical mucus much thicker: it prevents extra sperm from coming up in the reproductive tract. It’s basically a contraceptive within the second half of the cycle.

Progesterone is responsible for the thermogenic changes in a menstrual cycle. For example, when you check your temperature to monitor your ovulation, it’s basically the presence of progesterone that you detect when you see a rise in basal body temperature.

In the second half of the cycle, after ovulation, the endometrium matures. There are four days, called the window of implantation, which is when the endometrium is most prime to accept an embryo and progesterone is the hormone that makes that endometrium most receptive.

Progesterone also helps modulate the immune responses of the mother. So that makes the immune system more accepting of the implantation of the embryo.

It helps to reduce miscarriage and also helps to prevent preterm labour. So we use progestogens to help reduce that later on in pregnancy. In fact, when you do drop your progesterone levels naturally in the later stages of pregnancy it facilitates the onset of labour. So progesterone plays a very important role throughout pregnancy.

Even postpartum, progesterone plays an important role as it will inhibit lactation. So after the delivery, we don’t want progesterone to be around because we want women to produce breast milk.

As a fertility doctor here in Perth, I explain to patients who come and see me, that there are two parts to an IVF cycle.

The first one is what we call ‘controlled ovarian hyperstimulation’. This happens over the first couple of weeks in the IVF cycle.

Then we give you a trigger injection, and about 12 to 24 hrs later, you get that maturation of the follicles. They switch from producing predominantly oestrogen to predominantly progesterone, and then you are getting progesterone production from those follicles.

So you may wonder, if the progesterone in the body is maturing the lining, why do we still give you a supplement?

During egg collection, we suck the fluid out of the follicle. The fluid contains granulosa cells and they are the ones that produce the hormones. So by stripping the follicles out, we reduce their ability to produce enough progesterone in the luteal phase to support the pregnancy. And that’s why we prescribe a supplement in the form of a gel, pessaries, or tablets.

Unfortunately, when we see low levels of progesterone, or when progesterone levels drop in pregnancy, this may lead to miscarriage. That’s why so many women worry about their progesterone levels.