Infertility:
Failure to conceive after 1 year of regular unprotected intercourse is called infertility.

Primary infertility: Infertility without any previous pregnancy.

Secondary infertility: When there has been a previous pregnancy.

Sterility: Is when there is no chance for a pregnancy. This is different from infertility which generally represents a reduced potential for pregnancy. 

Incidence:
It is 12-15% of reproductive age groups.

Natural fertilization:
In females with regular & normal menstrual cycle (28-30 days), ovulation occurs in between 12-16th day of the cycle & ovum is picked up by fimbrial end of the fallopian tube. This ovum remains live for 24 hours & if there is intercourse during this period then released spermatozoon fertilizes the ovum thus, fertilization takes place in the fallopian tube and then zygote is transferred in the uterine cavity.