Written by:

Dr Aynsley Cresswell

BA (Oxon), BMBCh, DRCOG, DFSRH, MRCGP

Dr Cresswell is a general practitioner in the North East having recently moved to Yorkshire from Oxford. Aynsley has a special interest in women's and sexual health, and she has gained diplomas in both of these specialist areas. She has a keen interest in teaching and also enjoys caring for patients with long-term health conditions in her day-today practice.

Hormones and the Monthly Cycle

In this article:

  • The role of hormones in the menstrual cycle
  • Infertility caused by hormonal imbalances
  • How to spot if hormones may be out of balance
  • Main Points
  • References

Hormones and the Monthly Cycle

The role of hormones in the menstrual cycle

To understand how hormone imbalances can affect the menstrual cycle and fertility, it is important to understand how a normal menstrual cycle works. It is the result of hormone messages between the pituitary gland in the brain and the ovaries. The length of the cycle is determined by the number of days from the first day of one period to the first day of the next period. On average, a cycle can last from 24-35 days, but it is not abnormal for a woman’s cycle to be shorter or longer than this.

In general, there are five main hormones that control the menstrual cycle and fertility. Two of these hormones are made in the ovaries, while the other three are made in the brain.

Gonadotrophin-releasing hormone (GnRH) is produced in the hypothalamus, an area of the brain. It acts on another part of the brain, the anterior pituitary, which releases follicle-stimulating hormone (FSH) and luteinising hormone (LH). FSH travels to the ovaries where it stimulates the ova (eggs) to mature. LH triggers ovulation and stimulates the formation of important cells called the corpus luteum. The maturing eggs and the corpus luteum produce oestrogen. This helps control the levels of GnRH, FSH and LH so that not too many ova mature at the same time. Oestrogen also helps to maintain the female reproductive organs. Progesterone is released by the corpus luteum and, along with oestrogen, it prepares the lining of the womb for the implantation of a fertilised egg.

At the beginning of the cycle, oestrogen and progesterone levels are low. These low levels send a signal to the pituitary gland which produces FSH to begin the process of developing a mature follicle (which is a fluid-filled sac in the ovary containing an egg). When ovulation happens at about day 12-14, the increased oestrogen levels trigger a sharp rise in LH from the pituitary gland, which results in the release of the egg from the follicle.

Once the egg is released, the follicle ruptures (now known as the corpus luteum) and this produces progesterone and oestrogen, which both continue to prepare the womb for pregnancy. If the egg is fertilised by sperm, the corpus luteum will keep producing hormones. Human chorionic gonadotrophin (hCG), detected on pregnancy tests, is produced by the cells around the embryo and prevents the corpus luteum from breaking down.

If the egg is not fertilised, the corpus luteum cannot live for longer than two weeks. When it is degrading, there are fewer hormones released; that is, the lower levels of oestrogen and progesterone cannot control the amounts of GnRH, FSH and LH. This means that these hormones can increase and encourage new ova to develop, which marks the beginning of another cycle. The lining of the womb degrades and is shed in menstruation in response to the reduction in progesterone.

Overall the cycle happens in three segments: follicular (the first half of the cycle), ovulatory (the mid-point, usually between days 12-16) and luteal (the second half of the cycle).

Graph showing hormones in the monthly cycle
Graph showing hormones in the monthly cycle

Infertility caused by hormonal imbalances

There are many different causes of infertility, and these can affect either the male or female partner. However, in a quarter of cases it is not possible to identify the cause. In women, common causes include lack of regular ovulation and hormonal imbalance, blocked or damaged fallopian tubes and endometriosis. The most common cause of male infertility is poor quality semen.

Importantly, if there are problems with the production of hormones, or there are hormone imbalances, women may find it more difficult to fall pregnant. Some hormone changes prevent an egg being released at all, while others stop an egg being released during some cycles but not others.

Progesterone imbalance is one of the most common hormonal imbalances in women who have fertility concerns. If progesterone levels are too low, the woman will have a very short luteal phase in her menstrual cycle. As above, this is the length of time between ovulation and the beginning of menstruation. If this phase is short, it means that the fertilised egg does not effectively implant in the uterus and there is a higher chance of miscarriage. Progesterone imbalance can be treated and levels can be increased reasonably quickly.

Oestrogen imbalance can lead to sub-optimal fertility. A low level can mean women do not ovulate, and a lack of oestrogen can prevent the lining of the womb thickening to support a pregnancy. Conversely, having too much oestrogen may lead to irregular periods, which can make getting pregnant more difficult.

The hormone prolactin, which controls breast milk production and is produced in the pituitary gland of the brain, can also affect fertility. Increased levels inhibit the release of FSH and thus may suppress ovulation. This is the reason why breastfeeding women (who have naturally high prolactin levels) usually do not fall pregnant. Often, high levels of prolactin may be caused by a prolactinoma, a small, usually benign tumour in the pituitary gland. Other rare causes of high prolactin levels include liver or kidney disease, polycystic ovary syndrome (see below), shingles and hypothyroidism. Other more common reasons for high prolactin include strenuous exercise, poor sleep, and some medications (such as anti-depressants and painkillers). Furthermore, both an over- or under-active thyroid gland can prevent ovulation.

How to spot if hormones may be out of balance

For most women, having regular periods every month can mean that regular ovulation is occurring. Irregular or absent periods may signify a problem with pregnancy hormones and thus ovulation. That is, after the menopause, women stop having periods and stop ovulation. In some, this happens earlier than normal, and if it occurs before the age of 40, it is called premature ovarian insufficiency. Symptoms that women may experience include those associated with the menopause including night sweats, hot flushes, reduced energy, changes in mood and lack of sex drive. Hair may become thinner and joints may ache, but 1 in 4 women do not have any symptoms at all.

Polycystic ovary syndrome (PCOS) also causes problems with ovulation, and symptoms can include excessive hair growth, acne and can be associated with being overweight.

Trying to conceive can be a stressful time. It is important to remember that 84% of couples fall pregnant within one year. There may be underlying reasons why you cannot get pregnant, and if you have been having regular sexual intercourse without getting pregnant for one year, you should see your GP. There may be underlying hormonal imbalances which may be resolved with medication, but if needed, the GP can refer you to a fertility specialist to discuss further investigations and treatment options.

For more information on the topic of infertility related to hormonal imbalances and hormone testing, read Dr Cresswell’s article on Infertility tests and treatments

Main Points

  • Five main hormones control the menstrual cycle and fertility: three that are made in the brain – Gonadotrophin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), and luteinising hormone (LH) – and two produced in the ovaries – oestrogen and progesterone.
  • At the beginning of the cycle, oestrogen and progesterone levels are low. These low levels send a signal to the pituitary gland which produces FSH to begin the process of developing a mature follicle. During ovulation, a sharp rise in LH causes an egg to be released from the follicle during ovulation.
  • If the egg is fertilised by sperm, the corpus luteum will keep producing hormones. Human chorionic gonadotrophin (hCG), detected on pregnancy tests, is produced by the cells around the embryo and prevents the corpus luteum from breaking down. If the egg is not fertilised, the corpus luteum cannot live for longer than two weeks. When it is degrading, there are fewer hormones released:
  • The cycle happens in three segments: follicular (the first half of the cycle), ovulatory (the mid-point, usually between days 12-16) and luteal (the second half of the cycle).
  • Hormonal imbalance is one of the most common causes of infertility in women along with having blocked or damaged fallopian tubes and endometriosis.
  • Progesterone imbalance is one of the most common hormonal imbalances in women who have fertility concerns. Low levels can lead to the fertilised egg not effectively implanting in the uterus, increasing the risk of miscarriage.
  • Oestrogen imbalance can lead to sub-optimal fertility. A low level can mean women do not ovulate, and a lack of oestrogen can prevent the lining of the womb thickening to support a pregnancy.
  • The hormone prolactin, which controls breast milk production and is produced in the pituitary gland of the brain, can also affect fertility. Increased levels inhibit the release of FSH and thus may suppress ovulation.
  • Symptoms of hormonal imbalance include those associated with the menopause including night sweats, hot flushes, reduced energy, changes in mood and lack of sex drive. Hair may become thinner and joints may ache, but 1 in 4 women do not have any symptoms at all.

References

Fertility Problems; NICE Quality Standards, October 2014

Manders M, McLindon L, Schulze B, et al; Timed intercourse for couples trying to conceive. Cochrane Database Syst Rev. 2015 Mar 173:CD011345. doi: 10.1002/14651858.CD011345.pub2.