“Allons, mes enfants.”

– Come on, my children.

The Human Life Cycle

Human bodies change significantly over time, and food fuels 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. This chapter will focus on pregnancy, infancy, and the toddler years. Chapter 14 will focus on childhood through adolescence, and Chapter 15 will focus on the stages of adulthood. The significant 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 the first birthday.
• Toddler years. Occur during ages one to three and are the end of early childhood.
• Childhood. The period from birth to eighteen.
• Onset of Puberty. It can occur from ages nine to fourteen, which is the beginning of adolescence.
• Older adolescence. The stage that takes place between the ages of fourteen and eighteen.
• Adulthood. The period from adolescence to the end of life begins at age eighteen in the US and nineteen in some other countries.
• Middle age. The period of adulthood that stretches from age forty-five to sixty-four.
• Senior years, or old age. Extend from age sixty-five until the end of life.

Pregnancy

We begin with pregnancy, a developmental marathon that lasts about forty weeks. It starts 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. It implants itself in the uterine wall, where, by the end of the 8th week after conception, it develops into a fetus. Some of the significant changes that occur include branching nerve cells to form primitive neural pathways at eight weeks. At the first trimester and twenty-week mark, physicians typically perform an ultrasound to acquire information about the fetus and check for abnormalities. By 16 weeks, it is possible to know the sex of the baby. At twenty-eight weeks, the unborn baby adds body fat to prepare for life outside the womb.^[1] The ability to coordinate sucking and swallowing, necessary for feeding at birth, is not present until 32-34 weeks, and matures around 36-38 weeks gestation.^[2]

The placenta is the organ that intermediates the transfer of nutrients between the mother and fetus during pregnancy. The perfusion of the intervillous spaces of the placenta with maternal blood allows the transfer of nutrients and oxygen from the mother to the fetus and the transfer of waste products and carbon dioxide back from the fetus to the maternal blood. Placental nutrient metabolism was found to play a key role in limiting the transfer of some nutrients. Adverse pregnancy situations, such as those involving maternal diabetes or obesity, can increase or decrease levels of nutrient transporters in the placenta, potentially resulting in overgrowth or restricted growth of the fetus.^[3] A normal placenta weighs around  510 g at birth.^[4] Throughout this process, a pregnant woman’s nutritional choices affect not only fetal development, but also her health and the future health of her newborn.