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Living organisms such as humans are genetically diverse because of sexual reproduction. In this process, two kinds of sex cells or gametes are involved: the sperm (male gamete) and the egg or ovum (female gamete).
When ejaculation (the release of semen) occurs during sexual intercourse, approximately 200 million sperm or spermatozoa are deposited into the vagina. These sperms swim through the cervix, propelled by the whip-like motions of their tails called flagella, after which the muscular contractions of the uterus direct them to the fallopian tube. The sperm then travels through the fallopian tube and penetrates the zona pellucida (membrane surrounding the egg or ovum), fusing with it to form a zygote (fertilized egg). Once the zygote is formed, it travels to and becomes implanted in the uterus. Further growth takes place when cells start dividing, tissues begin forming and these tissues become organs, resulting in the formation of an organ system (where the zygote is now called “embryo”), and ultimately developing into a fetus (BYJU’S, 2020).
Natural fertility is the fertility that exists without birth control. It is a concept developed by the French historical demographer Louis Henry (Rosental, 2003) which refers to the level of fertility that would prevail in a population that makes no conscious effort to limit, regulate, or control fertility so that it depends only on the physiological factors affecting fecundity (probability or potential for reproduction).
On the other hand, controlled fertility refers to the use and application of either natural or artificial contraceptive methods, with the primary objective of preventing unwanted pregnancies and childbirths.
References
BYJU’S. (2020, September 1). Sexual reproduction: An overview. https://byjus.com/biology/sexual-reproduction-an-overview/
Rosental, P. (2003). La nouveauté d'un genre ancien: Louis Henry et la fondation de ladémographie historique. Population, 58,103-136. https://doi.org/10.3917/popu.301.0103