Stages of Development
Learning Objectives
By the end of this section, you will be able to:
- Describe the stages of prenatal development
- Identify factors that influence prenatal development
- Explain theories of cognitive and psychosocial development
- Describe the major changes that occur in adolescence
Development from conception into a fully grown human involves major alterations to anatomy, physiology, cognition, and behaviour, and many of these changes follow a predictable, age-based progression. This section will discuss prenatal, infant, child, adolescent, and adult development across three domains: physical, cognitive, and psychosocial.
From beginning as a one-cell structure to your birth, your prenatal development occurred in an orderly and delicate sequence. There are three main stages of prenatal development: germinal, embryonic, and fetal. Let’s explore what happens to the developing baby in each of these stages.
Germinal Stage (Conception–Week 2)
Each biological parent contributes DNA at the moment of conception, when sperm and egg unite to form a single-celled zygote (Figure 4.7). Growth and development of the zygote are guided by these inherited genetic instructions, coded in DNA within 46 chromosomes, 23 from each parent. In mammals, sex is determined by contribution of specific chromosomes, females are typically XX and males are typically XY, so it’s the male parent’s contribution that determines genetic sex, a female parent only has Xs to contribute.
Fetal Stage (Week 8-9 – Onward)
At about nine weeks, the embryo becomes a fetus. At this stage, the fetus is about the size of a kidney bean and begins to take on the recognizable form of a human being. It’s during the fetal stage that the sex organs begin to differentiate. At about 16 weeks, the fetus is approximately 4.5 inches long. Fingers and toes are fully developed, and fingerprints are visible. By the time the fetus reaches the sixth month of development (24 weeks), it weighs up to 1.4 pounds. Hearing has developed, so the fetus can respond to sounds. The internal organs, such as the lungs, heart, stomach, and intestines, have formed enough that a fetus born prematurely at this point has a chance to survive outside of the womb. Throughout the fetal stage the brain continues to grow and develop, nearly doubling in size from weeks 16 to 28. Around 36 weeks, the fetus is almost ready for birth. It weighs about 6 pounds and is about 18.5 inches long, and by week 37 all of the fetus’s organ systems are developed enough that it could survive outside the uterus without many of the risks associated with premature birth. The fetus continues to gain weight and grow in length until approximately 40 weeks. By then, the fetus has very little room to move around and birth becomes imminent. The progression through the stages is shown in Figure 4.8.
Reproductive Development and Sex Assignment
The first step in differentiation of the reproductive organs happens with a sexless collection o