When considering the causes of infertility it is useful to consider the image of the female reproductive organs shown above. In heterosexual intercourse male semen containing about 1-200 million motile sperm is deposited in the upper vagina close to the cervix. These sperm have to pass through the mucus of the cervix and ascend up the uterine cavity to enter the fallopian tube. This process is rapid occurring within 1 hour of ejaculation but only a few dozen sperm successfully complete this journey. The sperm move along the narrow tortuous tube to meet the egg (ovum) at the end of the tube.
The meeting of the sperm and egg can only occur in a narrow time frame within 24 hours after ovulation although sperm can survive for a few days waiting for ovulation to occur. Ovulation, which usually occurs about half way through the woman’s menstrual cycle, represents the release of a mature egg from the dominant ovarian follicle of the ovary. At the beginning of the menstrual cycle several follicles are recruited in a race to ovulation and as they grow they secrete the hormone oestrogen which thickens and prepares the lining of the uterine cavity (endometrium) for possible implantation. In each cycle usually only one egg is released and the following month the recruitment process begins again with a new set of follicles. In the cartoon the end of the tube is in close contact with the ovary but this only occurs at the time of ovulation so that its finger like tissue processes (fimbriae) can capture the egg and sweep it into the tube. Fertilisation of the egg now occurs to form an embryo and it begins to divide (cleave). With the help of tiny hair-like structures lining the fallopian tube (cilia) gently, the embryo moves towards the uterus. During this process which takes 4 days cleavage is rapid so that after about 4 days when the embryo reaches the uterine cavity is contains about 100 cells. The process of implantation of the embryo (now called a blastocyst) in the endometrium occurs 2 days after the embryo reaches the uterine cavity.
Causes of infertility:
We will go through the above account again to highlight causes of infertility.
- Erectile dysfunction or a narrow vagina may mean that semen is not deposited in the vaginal vault.
- There may be problems with the quality of the male sperm, such as a low count, low motility or a low number of normal forms. This will affect the ability of the sperm to penetrate the cervical mucus and reach the fallopian tube and will also affect normal fertilisation.
- The fallopian tube may be damaged by previous infection such as chlamydia or gonorrhoea. In some cases this will cause an obstruction preventing the sperm reaching the egg or the fertilised egg reaching the uterus. In these cases the tube may become swollen with fluid (hydrosalpinx). Less severe damage can affect the fimbriae affecting ovum pickup or the cilia lining the tube leading to ectopic pregnancy. Another condition called endometriosis where endometrial deposits around the ovary cause adhesions and distortion of the anatomy may prevent the fimbriae reaching the ovary.
- The egg may not be released at the expected time. This will cause irregular menstrual cycles making timing of intercourse for conception difficult. Women with irregular cycles may have polycystic ovarian syndrome (PCOS) where the ovary contains a large number of small follicles that may not mature to release an egg or may do so irregularly and infrequently.
- The number and quality of the eggs may be low. The total number of eggs falls and the number of poor quality eggs rises after the age of 35 but this may occur earlier. This will affect fertilisation resulting in infertility or early miscarriage. Factors other than age which can affect egg quality are smoking, a high BMI, underactive thyroid gland, raised prolactin levels, endometriosis of the ovary and treatment with steroids.
- Implantation of the embryo in the endometrium may be adversely affected by local abnormalities such as adhesions across the cavity of the uterus. Uterine fibroids are benign tumours of the wall of the uterus which can prevent implantation if they are close to the endometrium. Sometimes the endometrium does not respond adequately to oestrogen produced by the follicles and remains thin; the reasons for this are not always understood. The main factor affecting implantation is the quality of the embryo.
Professor Stuart Campbell.