What is the difference between infertility, sterility and subfertility?

Infertility and sterility are two terms that make reference to the inability to have children. However, and although they are often used synonymously, even by a number of professionals, their meaning is not exactly the same. Subfertility, on the other hand, is defined as the condition of being capable of conceiving, yet less fertile than normal.

In short, the main difference between being infertile and being sterile lies in the precise moment when the problem that prevents pregnancy appears. A subfertile couple are not infertile, but will find conception more difficult. The purpose of this article is to define each one of these concepts in detail.

The various sections of this article are assembled in the following table of contents.

Meaning of ‘sterility’ and ‘infertility’

The term sterility is defined as the inability to achieve a pregnancy because the fusion between the egg and the sperm, that is, fertilization never occurs.

On the other hand, by infertility we mean the inability to carry a pregnancy to term due to a problem that develops when fertilization has already taken place. In this case, even though the egg-sperm binding does occur, the resulting embryo is unable to implant or cannot develop till the end of the pregnancy, that is, delivery.

In short, the purpose of having a baby cannot be fulfilled in any case. Nevertheless, in cases of infertility, the chances of creating a family are a little bit higher than in cases of sterility, although it might not be possible either.

In everyday language, some examples of words or terms used as synonyms of infertile and sterile include the adjectives barren and unproductive.

Primary and secondary sterility

It should be noted that the concept of sterility can be split into two different types:

Primary sterility
A couple whose gametes (egg and sperm) have never been able to cause a pregnancy.
Secondary sterility
A couple who have been able to have children previously, but conception does not occur when trying to conceive another child.

Primary and secondary infertility

Also, in the case of infertility, there exist two different types as well, which are defined hereunder:

Primary infertility
Those couples who have never achieved a full-term pregnancy.
Secondary infertility
The couple have already had a child, but is unable to achieve a full-term pregnancy again.

Causes of sterility in humans

Even though both sterility and infertility are two-way things, the cause behind it might be related to the man, the woman, or both members of the couple. The following is a detailed explanation of the most common causes of sterility and infertility in males and females:

Female sterility

The most frequent causes of sterility in females are:

Endocrine causes
It comprises the set of problems related to the hormone control of the menstrual cycle and ovulation. It can lead to alterations such as poor egg quality, anovulation, amenorrhea, premature ovarian failure (POF), etc.
Tubal causes
Alterations in the Fallopian tubes, which is the place where the egg and the sperm fuse together to create an embryo. A tubal blockage, endometriosis, or salpingitis are some of the most common abnormalities that occur in the tubes.
Cervical causes
The presence of functional and/or anatomical alterations in the cervix can make it difficult for the sperm to ascend in the female genital tract, thereby preventing fertilization.
Uterine causes
Malformations, myomas, uterine synechiae, endometritis, polyps, etc. can prevent the sperm from reaching the oocyte.

These factors can also lead to female infertility, as fertilization still occurs in some cases in spite of these abnormalities, though the resulting embryo is unable to develop properly.

If you want to learn more, we recommend you to read the following post: Female factor infertility.

Male sterility

As for the causes leading to sterility in males, the following are the main factors:

Pre-testicular causes
As in the case of females, abnormalities in the regulation of the hormones that regulate spermatogenesis (i.e. the process by which spermatozoa are produced) can have a negative impact on the development of sperms, which can affect their fertilization potential.
Testicular causes
Pathologies such as orchitis, hydrocele, cryptorchidism, testicular trauma, or varicocele are some of the most usual examples.
Post-testicular causes
Even though sperm production occurs normally, the man experiences sperm ejection problems. Some examples of post-testicular male infertility include: blockage of the vasa deferentia, seminal tract infections, impotence, retrograde ejaculation, etc.
Sperm issues
It is also possible that sperm quality is insufficient to cause a natural pregnancy. Problems related to sperm motility, sperm morphology, or sperm count can prevent a couple from conceiving naturally.

The terms impotence and infertility in males are often confused, but there is an important distinction between them:

  • Impotence: inability to achieve and maintain an erection; in fact, it is a post-testicular cause of infertility.
  • Infertility: inability to achieve a successful pregnancy.

As in the previous case, even though this set of causes is basically related to sterility, sometimes fertilization happens, but the problem is that the pregnancy cannot be carried to term. In such scenario, it would be a case of male factor infertility.

Are you interested in learning more about fertility issues in men? Then do not miss the following article: Male factor infertility.

‘Infertility’ versus ‘subfertility’

As explained earlier, infertility allows the couple to go one step further in the journey toward pregnancy. In this case, the fusion between the egg and the sperm does occur and results in an embryo. However, the problem with infertility is that the embryo is unable to attach to the maternal uterus or to develop properly until childbirth.

For this reason, one can define the term infertility as the inability to achieve a full-term pregnancy, and bring a healthy baby home. On the other hand, by subfertility we refer to a couple being able to achieve a successful pregnancy, but with more difficulty.

For example, a woman in her 40s is considered to be subfertile, as the chances for pregnancy exist, but are lower in comparison to younger females.

In addition to the reasons listed above, which sometimes can lead to sterility and, in others, to infertility, there exist other causes related to the embryo and its development/growth. The most common ones include:

Implantation failure
This abnormality prevents the woman from having a full-term pregnancy, as the embryo is unable to implant into the endometrial lining.
Miscarriage or pregnancy loss
Pregnancy occurs, but stops at some point of fetal development, which causes the woman to miscarry.

In short, the cause that prevents full-term pregnancies can appear at early pregnancy, such as in cases of implantation failure, in which embryo implantation is not possible. Pregnancy might also stop progressing at more advances stages of fetal development, which leads to miscarriage as well.

Conversely, common causes of subfertility include endometriosis, tubal disease, ovulatory disorders, sperm abnormalities, advancing female age, uterine abnormalities, and peritoneal adhesions.

Male factor infertility, female factor infertility, or even both can be what causes a couple to experience trouble conceiving. Sometimes, however, the cause remains unknown, which is known as unexplained infertility.

FAQs from users

Is infertility hereditary?

No, it is not. There are, however, medical conditions that are hereditary, and might interfere with a person’s fertility. Polycystic ovary syndrome (PCOS), endometriosis, thyroid disorders, etc. are some examples.

Do cats or dogs cause infertility in humans?

To date, there is no evidence of a link between common pets and infertility. Having dogs, cats, hamsters or others will not affect your fertility. However, there exist certain parasitic infections that can be passed from them to humans and might have a negative impact on human fertility. As for cat allergy, there exist no studies that show a link between it and human infertility either.

On the other hand, people who work with livestock could be infected with certain diseases such as brucellosis, which symptoms include fever, chills, aches, sweating… In short, it might cause the body not to work optimally, and therefore affect a couple’s reproductive efforts.

Are impotence and infertility linked?

To some extent, yes. As a matter of fact, as mentioned earlier, impotence is a post-testicular cause of male infertility. However, a man with the impotence problem can produce normal sperm and his sperm count be okay, and the same applies the other way round: it is not necessary for an infertile person to have erection problems.

Impotence can be cured with medications such as Viagra, but infertility might require the use of fertility treatments such as IVF or ICSI to produce offspring.

What is Y chromosome infertility?

It is a condition that affects sperm production (i.e. spermatogenesis), making it very difficult for these males to father children naturally. An affected man may produce:

Suggested for you

If you want to learn more about what causes male and female infertility, the most common symptoms, treatments, and more, our advice is that you visit any of the following links:

To learn the details of all fertility treatment options available nowadays for you, do not miss this: Infertility treatments: definition and options available.

When infertility cannot be solved with infertility treatments, couples still have the option to fulfill their dream of having a family via donor conception and/or surrogacy. Do you want to find out what are your options? Check this out: Guide to third-party reproduction.