Introduction
The ability to reproduce, that is, the formation of new generations of individuals of the same species, is one of the main features of living organisms. Species transfer the genetic material from one generation to the next in the process of reproduction. This process ensures the survival of this species over long periods, despite the death of its representatives. The method of reproduction of humans as a species is sexual reproduction, which scientists divide into specific stages, presented in the table.
Sexual intercourse and the passage of sperm to the egg
Internal fertilization is an essential part of the reproductive cycle in terrestrial organisms. Humans do this thanks to a male organ – the penis inserted into the vagina. Sexual intercourse occurs with an increase in blood pressure on the penis and activation of neurons. As a result, all future components of the seminal fluid are pushed into the proximal part of the urethra, where they mix to sperm. Pressure on the urethra leads to muscle activity at the base of the penis, and contractions of these muscles push sperm through the urethra, and it gets ejected (ejaculation). After ejaculation, sperm enter the upper part of the uterus, passes through the cervix and fallopian tubes, and approach the ovum.
Fertilization
Fertilization is the fusion of the sperm nucleus with the egg nucleus, resulting in the formation of a diploid cell (zygote). Schatten (2017) claims that “fertilization is a complex but essential phenomenon for the formation of an embryo, which further develops into a new organism” (p. 313). The fertilized zygote undergoes several crushing divisions, as a result of which a blastocyst forms and plunges into the uterine wall.
Implantation and germinal stage
The implantation period lasts from the moment of fertilization to two weeks. This period is characterized by the rapid and systematic crushing of the fertilized egg, its advancement through the fallopian tube to the uterine cavity, and implantation on the 6–7th day after fertilization and further formation of the membranes. The uterus creates the necessary conditions for the development of the embryo, provides nutrition, creates a fluid habitat, and mechanical protection (fetal bladder fluid).
Embryonic stage
The embryonic period lasts from the third to the twelfth week of pregnancy. During this period, the rudiments of all the most important organs and systems of the unborn baby form, and the formation of the trunk, head, and limbs occurs. Leung & Qiao (2018) claim that “early embryonic development occurs in a sex-independent manner” (p. 5) At the end of this period, the embryo becomes a fetus with a baby-like configuration.
Fetal stage
The fetal period begins with the third month of pregnancy and ends with the birth of a baby. Placenta carries out the nutrition and metabolism of the fetus. As Weitkamp et al. (2018) mark, “innate immune system shields the fetus from infection” (p. 457). Rapid fetal growth occurs and the formation of new functional systems that ensure the life of the fetus in the womb and child after birth.
Labor
Labor takes place in three stages, and the first period can be excruciating and last more than a day, this is the period of the expansion of the cervix, which is characterized by contractions. The second period is the appearance of the child and the beginning of breathing, and an important part is cutting the umbilical cord and preventing infections (Agarwal et al., 2017). The third stage is the contraction of the uterus and the removal of the placenta through the vagina.
Neonatal period and infancy
A new-born is a child under 28 days of age. Providing adequate feeding and care during this period is crucial to improve the child’s chances of survival and lay the foundations for a healthy life. Infancy’s features are intensive growth and development. At the age of one to three months, the growth of the child increases monthly (Symonds et al., 2017). During the first year of life, a child’s height and body weight increase by an average of several times.
References
Agarwal, A., Sharma, R., Gupta, S., Harlev, A., Ahmad, G., Plessis, S. S., Esteves, S. C., Wang, S. M., & Durairajanayagam, D. (2017). Oxidative stress in human reproduction: shedding light on a complicated phenomenon. Springer.
Leung, P. C., & Qiao, J. (2018). Human reproductive and prenatal genetics. Academic Press.
Schatten, H. (2017). Human reproduction: updates and new horizons. John Wiley & Sons.
Symonds, M. E., Bloor, I., Ojha, S., & Budge, H. (2017). The placenta, maternal diet, and adipose tissue development in the newborn. Annals of nutrition and metabolism, 70(3), 232–235.
Weitkamp, J. H., Lewis, D. B., & Levy, O. (2018). Immunology of the fetus and newborn. Avery’s diseases of the newborn, 7. 453–481.