Throughout the menstrual cycle, fertility changes as a result of different levels of hormones. An imbalance or abnormal level of hormones can affect fertility and the chances of conceiving.

Overview of the menstrual cycle

Most individuals with a uterus experience the menstrual cycle. The menstrual cycle is a coordinated cycle of events lasting approximately 28 days, but this number varies from person to person. During the menstrual cycle, an egg is released from the ovaries and the mucous lining of the uterus (endometrium) thickens to accommodate a potential pregnancy. It is controlled by signalling molecules called hormones.¹

The follicular phase

The follicular phase of the menstrual cycle lasts from approximately day 1 to day 14. The ovaries contain hundreds of thousands of follicles, and during the follicular phase, one of these follicles matures into an egg to prepare it to be released during ovulation. The endometrium begins to thicken.

Ovulation

Ovulation occurs 14 days before menstruation. During this phase, the egg is released from the ovary and travels through the fallopian tube to the uterus. The endometrium continues to thicken, preparing for the egg to become implanted if it is fertilised. Fertility is usually at its highest during ovulation.

The luteal phase

The luteal phase lasts from approximately days 14-28. During this stage, the endometrium receives an increased blood supply, while the thickness decreases slightly as it no longer needs to support a fertilised egg.

Menstruation

The beginning of menstruation, or a period, marks the start of the next menstrual cycle. The average length of menstruation is 3-5 days, but it can sometimes be as long as 8. The endometrium is shed, containing blood, tissue debris and prostaglandins which contract the uterus to assist in shedding the lining. The amount of blood lost during menstruation is usually around 30ml but can range anywhere from a small amount of spotting to 80ml.¹

Key hormones involved in female fertility

Follicle-stimulating hormone (FSH)

FSH belongs to a family of hormones called gonadotropins. It is secreted by the pituitary gland during the follicular phase of the menstrual cycle, causing a follicle to mature into an egg. The mature follicle releases oestrogen and a type of hormone called inhibin, which suppresses the secretion of FSH. The release of FSH peaks sharply just before ovulation.²

Oestrogen

Oestrogen is a steroid hormone secreted by the ovaries, which plays a key role in fertility. Oestrogen levels rise steadily during the second half of the follicular phase of the menstrual cycle and reach a sharp peak during ovulation. One of the roles of oestrogen is to make the endometrium thicken. During the follicular phase, oestrogen is involved in a negative feedback loop (a regulatory response which slows down or stops a reaction) which causes the pituitary gland to secrete less FSH. Just before ovulation occurs, oestrogen is involved in a positive feedback loop (a regulatory response which increases the rate of a reaction), which makes the pituitary gland more sensitive to a hormone called gonadotropin-releasing hormone (GnRH). GnRH is responsible for increasing the secretion of luteinising hormone (LH) and FSH by the pituitary gland, leading to ovulation.³

Luteinising hormone (LH)

LH, similar to FSH, belongs to the gonadotropin family. It is secreted by the pituitary gland in response to the release of GnRH. LH is only secreted once during the menstrual cycle; in a surge prior to ovulation. The release of LH initiates ovulation.⁴ LH also stimulates the enzymes which weaken the ovarian wall, which allows the mature follicle to pass through.³

Gonadotropin-releasing hormone (GnRH)

GnRH is a peptide hormone which is secreted by a part of the brain called the hypothalamus. It stimulates the pituitary gland at different pulse frequencies to secrete FSH and LH. FSH is secreted at low-frequency GnRH pulses and LH is secreted at high-frequency GnRH pulses.³

Progesterone

Progesterone is a steroid hormone which is produced by the adrenal glands and the ovaries. It’s important in the maintenance of pregnancy, but also plays a role in the menstrual cycle. It’s secreted during the luteal phase and induces the growth of blood vessels in the endometrium, preparing it to support an implanted egg. If there is no pregnancy, progesterone levels decrease and the endometrium is shed in a period.⁵

Hormonal imbalances and their impact on fertility

Imbalances in hormone levels can occur as a result of conditions such as polycystic ovary syndrome (PCOS) or thyroid disorders. They can also occur due to issues with glands that secrete hormones such as the pituitary gland, hypothalamus or ovaries.

Deficiency of progesterone

As previously mentioned, progesterone plays a crucial role in the maintenance of the endometrium. A lack of progesterone can mean that the endometrium is unable to thicken and increase its blood supply, making it unsuitable for implantation of a fertilised egg.⁵

Polycystic ovary syndrome (PCOS)

PCOS is an endocrine (hormonal) disorder which affects between 7-15% and is the leading cause of infertility in people who menstruate. The ovaries of people with PCOS contain small sacs of fluid called cysts which contain follicles. These follicles are unable to mature into eggs.⁶

The most common hormonal imbalance observed in PCOS is hyperandrogenism. This is where patients have excess testosterone and progesterone, which can impact the development and maturation of follicles, meaning normal ovulation is unable to take place.

Many PCOS patients also experience a higher frequency of GnRH secretion, therefore resulting in elevated levels of LH. High LH levels can cause a developing follicle to produce progesterone earlier than it should, which can disrupt the process of the egg maturing. This means that normal ovulation and therefore, fertilisation, are unable to occur.⁷

Thyroid disorders

Most cases of infertility in people who menstruate are a result of thyroid disorders. Both hypothyroidism (underactivity of the thyroid) and hyperthyroidism (overactivity of the thyroid) can result in menstrual irregularities.

Hypothyroidism can result in a decrease of GnRH released, resulting in less LH and FSH being secreted. This means that ovulation is not induced, so an egg is not released from the ovary and fertilisation is unable to take place.⁸

Hyperthyroidism, similarly to PCOS, can increase the secretion of androgens and LH. This results in absent or premature ovulation of an immature follicle, meaning that fertilisation can not occur.⁹

The role of hormones in fertility treatment

Induction of ovulation

According to the NHS, there are drugs which can be prescribed to stimulate the secretion of hormones such as GnRH, LH and FSH, and therefore encourage ovulation. These medicines include:

• Clomifene
• Tamoxifen
• Metformin
• Gonadotropins

In-vitro fertilisation (IVF)

IVF involves an egg being removed from the ovaries and fertilised by sperm in a laboratory. The fertilised egg is then inserted into the uterus, where it implants itself in the endometrium to grow.

During IVF treatment, the person who will be carrying the baby is given medications which will suppress the natural menstrual cycle, with the intention of making the treatment more effective. They are then given FSH to stimulate maturation of multiple follicles which can be collected and fertilised.

Summary

The menstrual cycle and fertility are controlled by many hormones: follicle-stimulating hormone (FSH), luteinising hormone (LH), gonadotropin-releasing hormone (GnRH), oestrogen and progesterone. These hormones are released in different amounts at different points of the menstrual cycle to regulate fertility. Imbalances in these hormone levels can result in infertility, and these imbalances may be the result of conditions such as polycystic ovary syndrome (PCOS) or thyroid disorders. There are hormonal treatments available for infertility, such as drugs that stimulate ovulation. Hormones are also involved in in-vitro fertilisation (IVF).