Estrogen and progesterone: Why are they so important?

Estrogen and progesterone are the primary female sex hormones. They are essential for female reproductive health and pregnancy, but they also have a number of other roles in the body too. In this article, we will go over the major roles of these two hormones in females.

The primary function of estradiol (the strongest estrogen hormone) in females is to mature and maintain the reproductive system, including the mammary glands, uterus, and vagina. During each menstrual cycle, estradiol levels increase to trigger the maturation and release of the egg. Increasing levels of progesterone are required to cause the thickening of the uterus lining to allow a fertilized egg to implant (if conception occurs) (1).

In females of reproductive age, estradiol levels rise and fall twice during each menstrual cycle. At the beginning of the follicular phase of the cycle (when menstruation occurs), estradiol levels are low, with levels gradually increasing, until a rapid increase just before ovulation occurs (when an egg is released), followed by a rapid decrease. During the luteal phase, estradiol levels gradually increase again, along with an increase in progesterone to prepare the uterus and to stimulate the secretion of nutrients to provide the perfect environment for the implantation of a fertilized egg. Progesterone also inhibits muscular contractions of the uterus that would prevent a fertilized egg from implanting (2).

During pregnancy, estradiol levels keep increasing, until they reach levels as high as 40,000 pg/mL during the third trimester (3). After a fertilized egg becomes implanted, the placenta forms and takes over the production of progesterone at around 6-12 weeks. The placenta continues to produce progesterone for the remainder of the pregnancy, with progesterone levels steadily rising throughout pregnancy (4).

Throughout the pregnancy, progesterone is important for the development of the fetus, as well as stimulating the growth of maternal breast tissue in preparation for breastfeeding and strengthening the pelvic wall muscles in preparation for labor (4).

Studies have shown that both estrogen and progesterone levels influence sexual desire. Higher levels of estrogen are linked to increased libido (but with a two-day lag). In contrast, higher levels of progesterone have a negative effect on libido. This influence on sexual desire, particularly for higher estrogen levels, is thought to be linked to the time of the menstrual cycle when a woman is the most fertile (5).

Increasing estrogen and progesterone levels in young girls stimulate the development of female secondary sexual characteristics, (such as breasts, female fat distribution, and pubic hair), as well as triggering the menstrual cycles to begin (6). 

Bone health
Estrogen and progesterone work in sync for optimum bone health. Estrogen controls the rate of bone resorption, while progesterone is important for bone formation (7). In healthy bone, resorption (breakdown) and formation are kept in balance. However, if estrogen and progesterone levels are low, bone resorption exceeds bone formation, resulting in a weakening of the bone and an increased likelihood of osteoporosis.

After menopause, both estrogen and progesterone are at much lower levels than in pre-menopausal women. This contributes to increased bone loss during postmenopause, with an average bone mineral density loss of 1–2% each year after menopause (8), while some women can lose up to 20% of their bone density in just the 5–7 years following menopause (9).

Skin and hair health
Estrogen plays a prominent role in overall skin health. It is associated with increased collagen production, so is beneficial for maintaining skin thickness, as well as reducing the appearance of wrinkles. Estrogen also helps maintain skin moisture and topical estrogen has even been found to improve wound healing (10). Progesterone is also important for skin health, with evidence showing how topical progesterone can increase skin elasticity and decrease wrinkling (11).

Estrogen and progesterone are also required for optimum hair growth and health, which is evident with slower hair growth and thinner hair in postmenopausal women with lower levels of these two important hormones. The sharp decline in estrogen and progesterone that occurs at menopause is also linked to increased or imbalanced androgen levels (e.g., testosterone). Androgens tend to shrink hair follicles on the head, resulting in hair loss and thinner hair, but can also cause more hair to grow on the face. This is why some women at perimenopause and postmenopause have increased hair on the chin and upper lip (12).

Cardiovascular system
Estrogen has many protective effects on the cardiovascular system, including increasing nitric oxide activity (in coordination with progesterone) to improve blood flow (13) and lower blood pressure (14), soaking up free radicals that can damage arteries and other tissues, and helping regulate blood clotting (15). Estrogen is also important to keep cholesterol levels under control (16), which helps explain why females generally have lower cholesterol levels than males until postmenopause when estrogen levels are very low in females.

How can I measure my estrogen and progesterone levels?
Estradiol is the strongest of the three estrogen hormones and we offer several different tests to measure your estradiol levels. We also have tests available to measure your progesterone levels. These tests just require a small blood sample collected from a simple finger-prick in the privacy of your own home. Test results are available through our online portal, thereby avoiding the need to make any doctor or lab appointments. Of course, you may also wish to discuss your results with your healthcare professional, and this is highly recommended if you receive any results that fall outside the normal range.

The Estradiol (E2) and Progesterone (P4) tests measure just your estradiol levels and progesterone levels, respectively. For females of reproductive age, it is helpful to know what stage of your menstrual cycle your sample was collected at, as normal levels vary throughout each cycle.

There are various combination tests available for females that include estradiol and/or progesterone along with other important hormones:

1. Oestradiol. You and your Hormones, an education resource from the Society for Endocrinology. Reviewed Mar 2018.
2. Progesterone. Encyclopedia Britannica. (Edited August 2020)
3. Speroff L, Glass RH, and Kase NG. (1994). The Endocrinology of Pregnancy. In: Mitchell C, editor. Clinical Gynecologic Endocrinology and Infertility, 5th ed. Baltimore: Williams and Wilkins. 251-289.
4. You and Your Hormones. An education resource from the Society of Endocrinology. (Reviewed July 2021)
5. University of California – Santa Barbara. “Hormone levels and sexual motivation among young women.” ScienceDaily. 25 April 2013.
6. Estrogen’s Effects on the Female Body. John Hopkins Medicine.
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9. What Women Need to Know. Bone Health & Osteoporosis Foundation.
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14. Thomas P & Pang Y. (2013). Protective actions of progesterone in the cardiovascular system: potential role of membrane progesterone receptors (mPRs) in mediating rapid effects. Steroids. 78(6): 583-588.
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16. What is Estrogen? Hormone Health Network. Updated August 2018.