The ability of both animals and people producing numerous healthy offsprings is called fertility. This term was first applied only to the female; however, with better understanding of the reproductive mechanisms, the importance of the male's fertility too is gaining importance. A person or animal that cannot produce offsprings is considered to be infertile.

With humans, fertility in a person depends on various issues like sexual behavior, nutrition, culture, endocrinology and instinct. Along with this timing, emotions, economics and the way of life all play an important part in the fertility of a person. Animal fertility also follows such factors and displays great mechanisms. Vegetables in farmlands and plants too experience fertility, which is the capacity of producing large yields of fruits, seeds or vegetables.

The term fertility rate is the measure of number of children per woman. This was first considered a good indicator of population growth. However, it is not so much so in Asia now. This is due to selective abortion and various other factors that the number of women here is reducing drastically. With this, the fertility rate is no longer considered to be an authoritative in measuring population growth in the Asian countries of China, Myanmar and India.

The hormonal cycles in both men and women help to tell when the woman can conceive, and when the man is very fertile. Though the hormonal cycle of the female is constant, about twenty eight days long, the male cycle is variable. The fourteenth day of the female cycle is the most fertile period for females. This is because they ovulate at this time. There is no such period for men to ejaculate and produce sperms; it is produced any time of the month. However, it is their libido that may dip and this is what scientists infer to be their hormonal cycle.

It is during the fourteenth week of fetal growth that a woman's eggs forms in the ovaries. They remain there till puberty where the eggs mature one by one. On ovulation, the egg bursts from the ovary. This may sometimes because a small and sharp pain called mittelschmerz. The egg breaks down within twenty fours into its protein components if the egg is not fertilized by a male's sperm. These components are then reabsorbed by the body. As the quality of men's sperms deteriorates after 35, it is recommended by fertility clinics that they freeze their sperms before this if they plan to have kids later. However, there are men who produce healthy sperms long after 40!

Women are most fertile in their twenties, and starts diminishing after 30. It is better to get pregnant at a young age as the possibility of miscarriages increases with age. Even the possibility of birth defects in the baby increases with the age of the mother. So by multiplying the possibility rate of a conception with the miscarriage rate and the birth defect rates give the best possibility of a healthy birth. This figure; when multiplied by it gives the rough possibility of additional children.