Progesterone – a steroid hormone – plays a vital role in your fertility. Progesterone helps your body build strong bones and healthy hair. It’s also said to help you sleep well and prevents endometrial cancer. But progesterone plays its most impactful role when it comes to your ability to conceive and carry your pregnancy to term.

After ovulation, progesterone causes the uterine lining (endometrium) to thicken, creating the ideal environment for a fertilised egg/embryo. If you have low levels of progesterone, you might experience abnormal menstrual cycles. You might also struggle to fall pregnant.

“Progesterone prevents muscle contractions from happening in the uterus that would cause your body to reject an egg,” says Dr Natalia Novikova, a gynaecologist and endoscopic surgeon at Mediclinic Cape Town. “If your body is not producing enough of this hormone, you aren’t able to create the proper environment for a fertilised egg to grow.”

In addition, women with low progesterone levels and who fall pregnant are at higher risk for miscarriage or preterm delivery, because the hormone helps maintain the pregnancy.

During the early stages of pregnancy, progesterone is produced by a specific part of your ovary called the corpus luteum. Progesterone is essential for establishing the placenta. At about eight weeks, once the placenta is established, it then takes over producing progesterone.

During your pregnancy, progesterone plays an important role in the development of the foetus. It is thought that it has anti-inflammatory activity and influence over your immune system. This helps protect you from miscarriage and preterm labour.

Progesterone also increases blood supply to the breasts and pelvic area, stimulates the growth of maternal breast tissue and strengthens the pelvic wall muscles in preparation for labour. Your levels rise steadily throughout pregnancy until your baby is born. During the early days of your pregnancy, your levels will increase, making you feel tired. At the same time, this increase in also slows the movement of food through the digestive system, which can cause gas or constipation.

Not having enough of the pregnancy hormone can also cause you to have too much oestrogen. A high level of oestrogen can decrease your sexual desire, cause weight gain, and affect your gallbladder. ‘If you aren’t pregnant, signs that you might have low progesterone levels include headaches or migraines, mood changes (such as anxiety or depression) and an irregular menstrual cycle,’ says Dr Novikova. ‘If you are pregnant, signs that you might have a low progesterone level include: uterine bleeding and spotting.’ If you’re having trouble getting pregnant, or if you’re undergoing fertility treatments, your doctor may recommend that you take progesterone hormone therapy.