A variety of conditions are known to pose a threat to eggs and sperm.
One of the most significant threats to fertility is cancer treatment. The drugs used during cancer treatment (known as chemotherapy) work by blocking the survival of cancer cells. Unfortunately, eggs and sperm are also very sensitive to many of these agents and may also be damaged during chemotherapy. Certain cancer agents such as the alkylating agents (e.g. cyclophosphamide) are especially toxic to sperm and eggs. The threat to ovarian function is greatest when function is already low, for instance, for women in their forties.
Natural ageing is a major threat to a woman’s fertility. This is because women are born with a fixed quota of eggs, which age as a woman gets older. Egg numbers also dwindle with age until they eventually run out at the time of the menopause. Eggs begin showing signs of deterioration in quality when women are in their thirties, and therefore at a relatively young age. Quality is the key property of the egg that is important for pregnancy success, so this deterioration has a significant impact on pregnancy chances, becoming especially marked by the late thirties. Ageing is less detrimental to men because they retain sperm-making cells in the testes (called germline stem cells) that can generate new sperm throughout life. Indeed, men make millions of sperm per day in contrast to women who are born with a relatively small number of around 1-2 million eggs.
In women with endometriosis who have endometriotic cysts (also known as endometriomas or “chocolate cysts”), these cysts can become quite large. As cysts enlarge, they erode normal ovarian tissue including their contained eggs. Surgery on endometriotic cysts can also be very dangerous to the egg reserve since removing endometriomas will also remove some of the normal surrounding ovarian tissue. Due to damaging effects that surgery can have on the ovarian reserve, it is intensely debated whether endometriomas should be removed or not prior to undergoing IVF, with many experts tending towards a more conservative approach of not undertaking surgery.
In rare instances, females may be born with genetic changes that predispose to the early loss of eggs. In extreme cases, like most instances of Turner syndrome (these girls are born with only one X chromosome instead of two), the ovaries may not have any eggs at birth. In other instances, there may be eggs at birth but they then undergo an early decline leading to premature menopause. There are a wide variety of conditions and genetic mutations associated with an early decline in egg numbers; some examples are the Fragile X premutation, Galactosemia and BPES (Blepharophimosis-Ptosis-Epicanthus inversus Syndrome).