Prenatal development

There are three phases of prenatal development: germinal, embryonic, and fetal.

Germinal stage (weeks 1-2)

• Conception occurs when a sperm fertilizes an egg, forming a zygote. A zygote is a single cell that contains DNA from both parents. During the first week, the zygote undergoes mitosis (cell division), growing from one cell to hundreds of cells. By about day 5, it becomes a blastocyst. In the germinal stage, this cluster of cells has not yet embedded in the uterine lining. Once implantation occurs, the next stage begins.

Embryonic stage (weeks 3-8)

• About 7-10 days after fertilization, when the cell mass has around 150 cells, it travels down the fallopian tubes, attaches to the uterine wall, and is then called 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 that will later become the head, chest, and abdomen. Cardiac cells begin beating, organs begin to develop and function, and a neural tube appears, which will become the spinal cord and brain.

Fetal stage (weeks 9-40)

• At about nine weeks, the embryo is called a fetus and is roughly 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 are the abilities that let us move our bodies and manipulate objects.
• Fine motor skills use smaller muscles (in the fingers, toes, and eyes) to control actions such as gripping a toy or holding a spoon.
• Gross motor skills use larger muscle groups (in the arms and legs) for movements such as 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: When 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

Age	Milestone
~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

Table adapted from OpenStax

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	Knows songs and rhymes by memory	Names colors and numbers; begins writing letters
5	Hops and swings; uses fork and spoon	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 sequence of physical changes in puberty is fairly predictable, the timing of when puberty begins and how quickly it progresses varies widely. Two key processes are adrenarche and gonadarche, which mark the maturation of the adrenal glands and the sex glands, respectively. Together, these changes contribute to the development of primary and secondary sexual characteristics.

• Primary sexual characteristics: Organs necessary for reproduction (ovaries, uterus, testes).
• Secondary sexual characteristics: Visible signs of sexual development that are 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.