Prenatal development
There are three phases of prenatal development: germinal, embryonic, and fetal.
Germinal stage (weeks 1–2)
- Conception happens when a sperm fertilizes an egg, forming a zygote, which is a single-cell entity containing DNA from both parents. Over the first week, the zygote undergoes mitosis (cell division), expanding from one cell to hundreds of cells (a blastocyst) by around day 5. In this germinal stage, the cell cluster has not yet embedded itself in the uterine lining; once it does, the next stage begins.
Embryonic stage (weeks 3–8)
- About 7–10 days after fertilization, once the cell mass has around 150 cells, it moves down the fallopian tubes and attaches to the uterine wall and is known as an embryo.
- During this phase, blood vessels develop into the placenta, which delivers nourishment and oxygen to the embryo through the umbilical cord. Basic structural regions form, which will eventually become the head, chest, and abdomen. Cardiac cells begin beating, organs begin to develop and function, and a neural tube appears, destined to become the spinal cord and brain.
Fetal stage (weeks 9–40)
- At approximately nine weeks, the** embryo** is termed a fetus, about the size of a kidney bean. From weeks 9–12, sex organs differentiate. By week 16, the fetus is about 4.5 inches long, with fully formed fingers and toes, and visible fingerprints. Around week 24, the fetus may weigh up to 1.4 pounds, with developed hearing and sufficiently formed internal organs (e.g., lungs, heart, stomach, intestines) that a premature birth has some chance of survival.
- The brain continues rapid growth, nearly doubling in size between weeks 16 and 28. By week 36, the fetus typically weighs about 6 pounds and measures around 18.5 inches. By week 37, the organ systems are mature enough for survival outside the uterus with fewer complications. Growth in both weight and length continues until near week 40, when there is little space left to move, and birth becomes imminent.
Motor development
- Motor skills refer to how we move our bodies and manipulate objects.
- Fine motor skills engage smaller muscles (in fingers, toes, and eyes) and control actions like gripping a toy or holding a spoon.
- Gross motor skills rely on larger muscle groups (in arms and legs) to perform movements like running, jumping, and balancing.
Infant reflexes
- Sucking reflex: Triggered when something touches the roof of the baby’s mouth.
- Palmar grasp reflex: Elicited by stroking the baby’s palm, causing a tight grasp.
- Tonic neck reflex: When the baby’s head turns to one side, that arm extends while the other bends.
- Blinking reflex: Activated by a bright light or a gust of air.
- Rooting reflex: A gentle stroke on the cheek prompts the baby to turn and open the mouth, ready to nurse.
- Stepping reflex (walking reflex): Touching the soles of the feet to a flat surface triggers step-like motions.
- Babinski reflex: Stroking the sole of the foot causes the big toe to lift while the other toes fan out.
- Swimming reflex: Placed face down in water, the baby moves in a coordinated swim-like manner.
- Startle (Moro) reflex: A sudden noise or motion makes the baby cry and extend the neck and limbs, then retract them.
Progression of infancy motor development
| ~2 months
|
Prone but lifts head
|
| ~3 months
|
Roll over, lift chest with arms
|
| ~6 months
|
Sit up unassisted
|
| ~7 months
|
Stand with support
|
| ~9 months
|
Crawl
|
| ~10 months
|
Walk with support
|
| ~12 months
|
Stand alone
|
| ~13 months
|
Walk alone
|
Lifespan motor development
| Age (years)
|
Physical
|
Personal/social
|
Language
|
Cognitive
|
| 2
|
Kicks a ball; walks up and down stairs
|
Plays alongside other children; copies adults
|
Points to objects when named; puts 2–4 words together in a sentence
|
Sorts shapes and colors; follows 2-step instructions
|
| 3
|
Climbs and runs; pedals tricycle
|
Takes turns; expresses many emotions; dresses self
|
Names familiar things; uses pronouns
|
Plays make believe; works toys with parts (levers, handles)
|
| 4
|
Catches balls; uses scissors
|
Prefers social play to solo play; knows likes and interests
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Knows songs and rhymes by memory
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Names colors and numbers; begins writing letters
|
| 5
|
Hops and swings; uses fork and spoon
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Distinguishes real from pretend; likes to please friends
|
Speaks clearly; uses full sentences
|
Counts to 10 or higher; prints some letters and copies basic shapes
|
Table adapted from OpenStax
Developmental changes in adolescence
Although the order of physical changes in puberty is predictable, its start and progression vary significantly. Adrenarche and gonadarche mark the maturation of the adrenal and sex glands, respectively, leading to growth in primary and secondary sexual characteristics.
- Primary sexual characteristics: Organs necessary for reproduction (ovaries, uterus, testes).
- Secondary sexual characteristics: Visible indicators of sexual development not directly involved in reproduction.
- Female developments: Breast and hip formation, pubic and underarm hair, menarche (first menstrual cycle) around ages 12–13.
- Male developments: Facial hair, deeper voice, pubic and underarm hair, spermarche (first ejaculation) around ages 13–14.
