Fertility is the natural capability of giving life. As a measure, "fertility rate" is the number of children born per couple, person or population. Fertility differs from fecundity, which is defined as the potential for reproduction (influenced by gamete production, fertilisation and carrying a pregnancy to term). Infertility is a deficient fertility.

Human fertility depends on factors of nutrition, sexual behavior, culture, instinct, endocrinology, timing, economics, way of life, and emotions.

Both women and men have hormonal cycles which determine both when a woman can achieve pregnancy and when a man is most virile. The female cycle is approximately twenty-eight days long, but the male cycle is variable. Men can ejaculate and produce sperm at any time of the month, but their sperm quality dips occasionally, which scientists guess is in relation to their internal cycle.

Furthermore, age also plays a role, especially for women.

Knowing your own body and being able to tell when you are fertile / ovulating is an important skill and one that every woman should have.

The fact is, there are only certain times in a woman's cycle when she is fertile or ovulating and can get pregnant. So it pays off to know when those times are.

Your body will give you clear clues and signals as to when fertility is approaching and occurring.

A brief note, this system of observing fertility changes has nothing to do with the rhythm method. The rhythm method does not use bodily signals and is often very unreliable due to factors like the ignorance of the natal lunar phase, sickness, stress etc. If you are simply counting days and guessing when you should be fertile, this could be slowing down your bid to conceive.