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External Characteristics Useful for Estimation of Gestational Age


28 weeks

32 weeks

36 weeks

40 weeks

Ear cartilage

Pinna soft, remains folded

Pinna slightly harder, but remains folded

Pinna harder, springs back

Pinna firm, stands erect from head

Breast tissue



1-2mm nodule

6-7mm nodule

External genitalia – male

Testes undescended, smooth scrotum

Testes in inguinal canal, few rugae

Testes high in scrodum, more rugae

Testes descended, pendulous scrotum covered with rugae

External genitalia – female

Prominent clitoris, small, widely separated labia

Prominent clitoris, larger separated labia

Clitoris less prominent, labia majora cover labia minora

Clitoris covered by labia majora

Plantar surface


1-2 anterior creases

2-3 anterior creases

Creases over sole

Level of Alertness

28 weeks

32 weeks

36 weeks

40 weeks

<28 wks – persistent stimulation leads to eye opening and alerting for seconds

>28 weeks – gentle stimulus can arouse baby and for several minutes

Spontaneous arousal

Eyes remain open with roving movements

Sleep-wake alternation

Increased alertness

Vigorous crying when awake

Distinct periods of attention to visual and auditory stimuli



28 weeks

32 weeks

36 weeks

40 weeks

Passive Tone

Minimal resistance to manipulation in all limbs

Flexor tone in lower extremities

Prominent flexor tone in LE palpable in UE

Strong flexor tone in all extremities

Pull to sit


Complete head lag


Head held in same plane as body for several seconds

Primary Neonatal Reflexes


28 weeks

32 weeks

36 weeks

40 weeks



Hand opening

Extension and abduction,

Audible cry

Mature response by 37 weeks: extension and anterior flexion of upper extremities followed by audible cry

6 months

Palmar grasp



Mature response by 37 weeks: infant can be lifted from bed

Voluntary grasp 2 mo

Tonic neck response



Mature response by 35 weeks: “fencing posture,” strongest at 1 mo postnatal

6-7 months

Cranial Nerve Function


28 weeks

32 weeks

36 weeks

40 weeks

I olfaction


Mature response: arousal, sucking

II vision

Blink to light (26 wks)

Eye closure to light

Visual fixation

Tracks (34 wks)

Turns to soft light (37 wks)

Consistent fix and follow

VA 20/150

II, III Pupils


Consistent reaction to light (32-35 wks)

Increases in amplitude with gestational age

III, IV, VI oculomotor, trochlear, abducens

Oculocephalic present (25 wks)

Spontaneous roving

Calorics (>30wk)

Fix and follow

Initially jerky

Smooth and accurate by 3mo

VIII audition

Startle/blink to loud noise



Baby alerts, eyes widen, turns to sound

V, VII, IX, X, XII suck and swallow

Suck and swallow

Not synchronized with breathing

Better synchrony of breathing with suck and swallow

Mature response at 37 weeks:

fully synchronized and coordinated suck and swallow, best after 48 hours

Gag reflex


Source: Courtesy of Dr. Hannah C. Glass