In reality, there are probably hundreds of "causes" of infertility. What this means is that there are a lot of things that have to happen perfectly in order to conceive and have a baby.
As an overly simplified example of the science involved:
- The hormones that stimulate egg development must be made in the brain and pituitary and be released properly
- The egg must be of sufficient quality and be chromosomally normal
- The egg must develop to maturity
- The brain must release a sufficient surge of the LH hormone to stimulate final maturation of the egg
- The follicle (eggs develop in structures called follicles in the ovaries) must rupture and release the follicular fluid and the egg
- The tube must "pick up" the egg
- The sperm must survive their brief visit in the vagina, enter the cervical mucous, swim to the fallopian tube and "find" the egg
- The sperm must be able to get through the cumulus cells around the egg and bind the shell (zona pellucida) of the egg
- The sperm must undergo a biochemical reaction and release their DNA package (23 chromosomes) into the egg
- The fertilized egg must be able to divide
- The early embryo must continue to divide and develop normally
- After 3 days, the tube should have transported the embryo down into the uterus
- The embryo must continue to develop and expand into a blastocyst
- The blastocyst must hatch out of its shell
- The endometrial lining of the uterus must be properly developed and receptive
- The hatched blastocyst must attach to the endometrial lining and "implant"
- Many more miracles in early embryonic and fetal development must then follow...
A weak link anywhere in this chain will cause failure to conceive