12.2: The Human Life Cycle

Learning Objectives

  • Identify and define the different stages of the human life cycle.
  • Explain how the human body develops from infancy through the toddler years.

According to the American Journal of Clinical Nutrition, the human life span, or the maximum length of time possible for human life, is 130 years. Ordovas, J. M. “Living Well to 100: Nutrition, Genetics, Inflammation.” Am J Clin Nutr 83 (2006): 401S490S. Human bodies change significantly over time, and food is the fuel for those changes. People of all ages need the same basic nutrients—essential amino acids, carbohydrates, essential fatty acids, and twenty-eight vitamins and minerals—to sustain life and health. However, the amounts of nutrients needed differ. Throughout the human life cycle, the body constantly changes and goes through different periods known as stages. The major stages of the human life cycle are defined as follows:

  • Pregnancy. The development of a zygote into an embryo and then into a fetus in preparation for childbirth.
  • Infancy. The earliest part of childhood. It is the period from birth through age one.
  • Toddler years. Occur during ages two and three and are the end of early childhood.
  • Childhood. Takes place from ages four to eight.
  • Puberty. The period from ages nine to thirteen, which is the beginning of adolescence.
  • Older adolescence. The stage that takes place between ages fourteen and eighteen.
  • Adulthood. The period from adolescence to the end of life and begins at age nineteen.
  • Middle age. The period of adulthood that stretches from age thirty-one to fifty.
  • Senior years, or old age. Extend from age fifty-one until the end of life.

Changes during Pregnancy

In this chapter, we will focus on the human life cycle from the prenatal period into early childhood. We begin with pregnancy, a developmental marathon that lasts about forty weeks. It begins with the first trimester (weeks one to week twelve), extends into the second trimester (weeks thirteen to week twenty-seven), and ends with the third trimester (week twenty-eight to birth). At conception, a sperm cell fertilizes an egg cell, creating a zygote. The zygote rapidly divides into multiple cells to become an embryo and implants itself in the uterine wall, where it develops into a fetus. Some of the major changes that occur include the branching of nerve cells to form primitive neural pathways at eight weeks. At the twenty-week mark, physicians typically perform an ultrasound to acquire information about the fetus and check for abnormalities. By this time, it is possible to know the sex of the baby. At twenty-eight weeks, the unborn baby begins to add body fat in preparation for life outside of the womb.Elaine U. Polan, RNC, MS and Daphne R. Taylor, RN, MS, Journey Across the Life Span: Human Development and Health Promotion (Philadelphia: F.A. Davis Company, 2003), 81–82. Throughout this entire process, a pregnant woman’s nutritional choices affect not only fetal development, but also her own health and the future health of her newborn.

Figure 12.2.1: This ultrasound image shows a four-month-old fetus. Ultrasound image of a fetus in the womb, viewed at 12 weeks of pregnancy (bidimensional-scan). Wolfgang Moroder.

Changes during Infancy

A number of major physiological changes occur during infancy. The trunk of the body grows faster than the arms and legs, while the head becomes less prominent in comparison to the limbs. Organs and organ systems grow at a rapid rate. Also during this period, countless new synapses form to link brain neurons. Two soft spots on the baby’s skull, known as fontanels, allow the skull to accommodate rapid brain growth. The posterior fontanel closes first, by the age of eight weeks. The anterior fontanel closes about a year later, at eighteen months on average. Developmental milestones include sitting up without support, learning to walk, teething, and vocalizing among many, many others. All of these changes require adequate nutrition to ensure development at the appropriate rate.Beverly McMillan, Illustrated Atlas of the Human Body (Sydney, Australia: Weldon Owen, 2008), 248.

Changes during the Toddler Years

Major physiological changes continue into the toddler years. Unlike in infancy, the limbs grow much faster than the trunk, which gives the body a more proportionate appearance. By the end of the third year, a toddler is taller and more slender than an infant, with a more erect posture. As the child grows, bone density increases and bone tissue gradually replaces cartilage. This process known as ossification is not completed until puberty.Elaine U. Polan, RNC, MS and Daphne R. Taylor, RN, MS, Journey Across the Life Span: Human Development and Health Promotion (Philadelphia: F.A. Davis Company, 2003), 108. Developmental milestones include running, drawing, toilet training, and self-feeding. How a toddler acts, speaks, learns, and eats offers important clues about their development.

Key Takeaways

The human body constantly develops and changes throughout the human life cycle, and food provides the fuel for those changes. The major stages of the human life cycle include pregnancy, infancy, the toddler years, childhood, puberty, older adolescence, adulthood, middle age, and the senior years. Proper nutrition and exercise ensure health and wellness at each stage of the human life cycle.

Discussion Starter

  1. In preparation for this chapter and the next, predict how you think nutrient needs might differ at the beginning of life compared to the end of life. Then, after reading this chapter and the one that follows, discuss if your predictions were correct or incorrect.