These may have a deformed structure or chromosomal abnormalities. In some cases it would appear that eggs are produced, and mature correctly within the follicle which then goes on to become a corpus luteum without however first bursting to release the egg. The luteal phase is the part of the cycle that follows after the egg has been released from the ovary. It may be inadequate in one way - and this is called a luteal phase defect. The corpus luteum produces the hormone called progesterone.
Progesterone is essential for preparing the endometrium to receive the fertilized egg.
Another possibility is a defective endometrium that does not respond properly to the progesterone. The immune system can react against the man's sperm, and kill them, immobilize them or make them stick together. Women can also develop an immune reaction to the coating of their own eggs, which can prevent sperm from attaching to them. Infections Certain infections have been shown to be responsible for some cases of unexplained infertility. For example, mycoplasma or chlamydia may be present in numbers that are not enough to show up in a clinical examination, but which nevertheless cause infertility.