4- 6 months

Learning Objectives for Mid-to-Late Infancy Development

• Identify Developmental Milestones and Skills: Cataloging gross motor, fine motor, oral motor, communication, cognitive, and play skills across the mid-to-late infancy spectrum ($3$ to $12$ months).

• Identify Body Structure Changes: Documenting the physiological and anatomical transformations occurring during this developmental window.

• Describe Postural Control Mechanisms: Explaining the underlying mechanisms that facilitate and enable the acquisition of motor skills.

General Characteristics and Body Structure Changes: 4-6 Months

• General Characteristics:

  • Increased symmetry and midline orientation of the body.

  • Development of beginning gravity-resisted control in both prone and supine positions.

  • Stronger extensor control observed in prone positions.

  • Heavier flexor control observed in supine positions.

  • Improvement in postural control reactions, which directly supports the enhancement of motor control.

  • The beginning stages of sitting emerge.

• Central Nervous System (CNS) Changes:

  • This period marks the first critical period of brain growth.

  • By $6$ months of age, the brain has doubled in size and reached approximately $\frac{1}{2}$ the weight of an adult brain.

  • Clinical Caveat: Malnutrition during this phase can lead to a decrease in the absolute number of glial cells within the CNS.

• Pulmonary Changes:

  • Rib Angle: The ribs transition to a more downward orientation.

  • Thoracic Cavity: The cavity becomes larger.

  • Vital Capacity: There is an increase in total vital capacity.

  • Efficiency: Overall lung efficiency increases.

  • Respiration: The infant begins to engage in active expiration.

• Musculoskeletal (MSK) Changes:

  • Newborn Phase: Characteristics include moderate genu varum (bowed legs).

  • 6-Month Phase: Transition to minimal genu varum.

Skill Development: 4-6 Months

• Gross Motor Skills (Prone):

  • Early (4 Months):

    • Increasing spine and leg extension.

    • Presence of the Landau reflex.

    • Positions himself/herself prone on elbows with the chest off the floor.

    • Engages in weight-bearing and weight-shifting activities.

    • Reaching behaviors begin.

    • Rolls to the side.

  • Later (6 Months):

    • Strong Landau reflex present.

    • Positioned prone on hands (arms extended).

    • Advanced weight-bearing and weight-shifting.

• Gross Motor Skills (Supine):

  • Early (4 Months):

    • Hands-to-knees and eyes-to-knees exploration.

    • Rolls to side; transitions from prone to supine.

    • Bilateral and unilateral reaching for toys.

  • Later (6 Months):

    • Hands-to-feet exploration.

    • Head and neck flexion against gravity resistance.

    • Reaches for persons or objects with full elbow extension.

    • Capable of holding a bottle.

    • Rolls consistently between supine and prone.

• Gross Motor Skills (Sitting):

  • Early (4 Months):

    • Pull to Sit (PTS): Increased flexor activity and decreased head lag.

    • Sits only with external support.

    • Head is held vertical and in the midline.

    • The back remains straight through the thoracic spine.

  • Later (6 Months):

    • PTS: Lifts head independently during the transition.

    • Sits Without Hands: Increased back extension with a "high guard" position of the upper extremities (UEs); lower extremities (LEs) are placed in external rotation and abduction.

    • Sits With Hands: Uses protective extension forward. This requires the "co-contraction" of the trunk muscles.

• Gross Motor Skills (Standing):

  • Maintains a supported stance with a wide base of support (BOS).

  • Infants typically love to stand at this stage.

  • Begins to play with knee flexion, which serves as a precursor to bouncing.

• Play Development:

  • Philosophy: Play is considered the "work" of children. Motor exploration allows children to learn about size, shape, texture, color, space, gravity, cause/effect, and specific actions.

  • Self-Identity: Helps determine the boundary between "me" and "not me."

  • Sensorimotor Play: Dominated by the mouth (e.g., "Everything goes in the mouth!") including hands, toys, and feet. Exploration of hands in the midline and feet-to-feet contact.

• Fine Motor Development:

  • Visually guided reaching and grasping.

  • Ability to transfer objects from hand to hand.

  • Grasp Patterns: Consistent palmar grasp; ulnar grasp transitions to an emerging radial grasp.

  • Ability to turn and manipulate objects.

  • Hand Preference: No true preference or dominance is established by the end of this period, even if one hand is used more frequently.

• Communication and Social Skills:

  • Cooing, babbling, and imitation of sounds.

  • Uses voice to engage others in "conversations."

  • Expresses needs via squeals, laughs, grunts, scowls, coos, and whines.

  • Shows anticipation through games like Peek-a-boo.

• Oral Motor Development:

  • Greater proximal stability leads to greater distal control.

  • Improved trunk control and co-contraction translate to better control of the neck, lips, cheeks, and tongue.

  • Reduced liquid loss when using a bottle or breast (though liquid loss remains high with a cup).

  • Attempts to assist with a spoon; uses the upper lip to clear food.

  • Ability to keep lips closed during play is dependent on the stability of the infant's current position.

• Vision:

  • Full visual control is achieved.

  • Eye movement becomes independent from head movement.

• Developmental Red/Yellow Flags:

  • Asymmetry.

  • Persistent fisted hands.

  • Significant head lag during pull-to-sit maneuvers.

  • Tremors.

  • General stiffness or floppiness.

  • Inability to vision or use hands to explore the environment due to lack of stability when prone.