Causes of Female Infertility – Structural Issues With the Reproductive Tract

The major structural issue faced by women who are struggling to conceive, is the presence of abnormal tissue somewhere in the reproductive system. Depending on where the blockage is, movement of the egg from the ovary, along the fallopian tube, and to the uterus may be impeded and/or the male’s sperm may not be able to reach the egg. An abnormally shaped uterus may hinder implantation of a fertilised egg, or prevent a pregnancy from proceeding to term.

Endometriosis

Endometriosis occurs when endometrial-like tissue forms outside of the uterus, leading to a chronic inflammatory reaction and distortion of the pelvic anatomy. Women with endometriosis often experience heavy, painful periods and discomfort in the areas where the endometrial deposits have formed. It is a very common condition, affecting 6-10% of the female population, although 25% of these will be asymptomatic. Amongst infertile women, the prevalence of endometriosis is particularly high, at 25-50%; and of those who have clinically diagnosed endometriosis, 30-40% will struggle to conceive.  

In cases of severe endometriosis the resulting pelvic anatomy distortion can be so extensive that it provides a physical barrier against fertilisation. However, there are other mechanisms that contribute to endometriosis-induced infertility. Often women with the condition have concurrent endocrine/ovulatory disorders, which cause impaired follicular growth and disrupted hormone secretions. The immune system is also involved because it has been found that some women with endometriosis have antibodies in their endometrium that prevent embryo implantation. This is a topic that is not well understood and requires further research into its prevalence, causes and implications.

Laparoscopic removal of the endometrial deposits is the first line treatment approach for many women with the condition, particularly those who are struggling to conceive. This has been shown to improve fertility in women with mild endometriosis. It is generally considered that in more severe cases, or when there are co-existing (male or female) fertility issues, a multi-disciplined approach to treatment will be necessary.

Uterine Fibroids

Uterine fibroids are the most common form of non-cancerous growths in females of reproductive age. They are found in 5-10% of infertile women and, as the name suggests, they are located in and around the uterine cavity. It is unclear exactly what causes fibroids to grow, although there is thought to be a genetic component and a link with oestrogen and progesterone levels. Some studies have suggested that there is an increase in oestrogen and progesterone receptors in fibroid tissue compared to the surrounding tissue, indicating a role for these hormones in the growth of the fibroids. As the levels of oestrogen and progesterone decline with age, so does the growth of fibroids. They usually first form during the reproductive years, when a female’s hormone levels are high, then reduce in size as she approaches the menopause. 

Only those fibroids that are present in the muscular walls of the uterus (intramural) or project into the cavity of the uterus (submucosal) have been shown to impact fertility, suggesting that the predominant issue is formation of a structural barrier to conception. Depending on their size and exact position, fibroids can alter the position of the cervix, the shape of the uterus, block the fallopian tubes and/or alter the blood flow to the uterus. This can make it more difficult for the sperm to successfully reach the egg, as well as hindering implantation of a fertilised egg.

The first line approach for treating fibroids that are affecting fertility is to perform a myomectomy. This surgery involves using laparoscopy to visualise and excise the fibrotic tissue. This technique significantly improves fertility rates, although in up to 30% of cases the fibroids will regrow over time. There are newer, non-surgical, ablative techniques, however, many of these are not suitable for those who wish to have children, as they alter the integrity of the endometrium and increase the risk of miscarriage during subsequent pregnancies.

Endometrial Polyps

Polyps are non-cancerous growths found in various body tissues, including the nasal lining, the vocal cords and within the colon. Those that affect fertility are found in the endometrium and consist of endometrial glands, connective tissue and blood vessels. They are the most common uterine structural abnormality, affecting approximately 10% of the female population. Despite this, many women only learn that they have endometrial polyps when they undergo clinical investigations for abnormal bleeding or unexplained infertility. Most polyps are diagnosed following transvaginal ultrasound investigation or hysteroscopy.

Exactly how endometrial polyps contribute to poor fertility and increased risk of miscarriage is not well understood. They seem to hinder embryo implantation; a theory supported by the finding that they are the most commonly observed abnormality in those who experience recurrent implantation failure following IVF. It has also been suggested that they impede sperm transport and make the endometrium less receptive to receiving a fertilised egg.

Polyps are most commonly found in women between the ages of 35 and 55. However, it has been suggested that they are more common in younger women than was previously thought; existing in a latent state and only being identified following investigative procedures for other issues, including unexplained infertility. One suggestion for why younger women are less likely to develop polyps is that the cells of their endometrium undergo continuous cycling and, as such, any polyps that do form, spontaneously regress without causing any issues. 

As with uterine fibroids, there seems to be a hormonal association, with increased oestrogen and progesterone receptors in polyp tissue. Furthermore, women on certain hormonal medications are at greater risk, for example, Tamoxifen. This is a partial oestrogen receptor agonist used in the treatment of breast cancer and women who take it are at greater risk of developing polyps. Hormone Replacement Therapy (HRT) may also increase the risk, but this seems to be dependent on the dose of oestrogen and progesterone a woman is prescribed. There are also studies that suggest that women who have non-communicable diseases (NCDs) such as diabetes, hypertension and obesity have an increased likelihood of developing endometrial polyps, however, more work is needed to substantiate these findings.

In some cases, diagnosed polyps do resolve spontaneously; and this seems to be associated with increased age according to limited studies. 

If a doctor suspects that polyps are contributing to a couple’s infertility, he or she will probably recommend a procedure known as a polypectomy, to excise the growths. Usually this procedure will be carried out in advance of any additional fertility treatment (Assisted Reproductive Techniques), as it can improve the overall outcome.

Other Structural Abnormalities

Scarring to the uterus that has occurred as a result of previous infections, injuries, or surgical procedures can impede implantation and, therefore, affect fertility.

Having an unusually shaped uterus can make falling pregnant difficult. It can also reduce the chances of carrying a baby to term.