The links between infertility and endometriosis are not clear. Many gynaecologists and researchers agree that severe endometriosis causes infertility. However, there is considerable dispute amongst the experts as to whether or not minimal and mild endometriosis cause infertility.
There is probably more than one cause of infertility in women with endometriosis. Many different theories have been proposed to try to explain the causes and it is likely that some of the theories will eventually be proven but much research still needs to be done. Some of the more widely known theories are outlined below.
Reduced intercourse
Painful intercourse — often a symptom of endometriosis — can contribute to infertility simply by reducing the frequency of sexual activity, thereby reducing the potential number of times that fertilisation can occur.
Failure to ovulate
A failure to ovulate, or anovulation, is not uncommon in women with endometriosis but whether it is due to their endometriosis is not known as many infertile women without endometriosis do not ovulate regularly either. A failure to ovulate will obviously cause infertility as there is no ovum to be fertilised.
Luteal phase defects
There are a range of abnormalities of the luteal phase of the menstrual cycle which may contribute to infertility. The most common abnormality is that the corpus luteum degenerates too quickly and the luteal phase becomes too short. Therefore, progesterone is not produced for a long enough time, or insufficient progesterone is produced, to enable the uterus to properly prepare its lining for implantation of the fertilised ovum.
The research into luteal phase defects in women with endometriosis has been contradictory. Some studies have shown that luteal phase defects are more common in women with endometriosis while others have not. In addition, at this stage it is not known whether or not luteal phase defects cause infertility in endometriosis.
Prolactin problems
Prolactin is a hormone that is secreted by the pituitary gland which helps to control ovulation. Research has shown that the levels of prolactin in women with endometriosis are higher than that found in women in general. The high levels of prolactin may contribute to infertility in women with endometriosis but the role that they play is unknown.
Macrophages
Macrophages, sometimes referred to as scavenger cells, are a special type of white blood cell which are found throughout the body. The function of these cells is to consume or ‘eat up’ and eliminate any unwanted debris or foreign material in the body, including sperm cells.
Women with endometriosis have an increased number of macrophages in their bodies, particularly in their pelvic cavity and fallopian tubes. Consequently, the macrophages are able to consume and destroy larger numbers of sperm than normally occurs, reducing the number of sperm available for fertilisation of the ovum.
Auto-immune response
Some women with endometriosis develop an abnormal immune response against their endometrial implants. It is thought that the immune system of these women for some, as yet unknown, reason thinks that the implants are foreign material and therefore begins to produce antibodies against them in an attempt to destroy them.
The immune system also perceives the normal endometrium in the uterus as being foreign material and therefore produces antibodies against it as well in an attempt to destroy it. If the immune reaction is strong enough the antibodies produced in the uterine endometrium may prevent implantation of the fertilised ovum.
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