Before We Start
Why primitive reflexes are expected to disappear, not persist
Primitive reflexes are present at birth and disappear as the cortex matures — their presence at birth confirms an intact nervous system at that stage, and their disappearance on schedule confirms normal ongoing neurological development. This is the opposite of how most reflexes work in older children and adults, where a reflex's continued presence is usually the expected, reassuring finding.
💡 The Three Key Red Flags
Absent Moro reflex can indicate possible neurological damage. A positive Babinski sign in an ADULT (rather than an infant) is abnormal, suggesting an upper motor neuron lesion. Asymmetric reflexes — present on one side but not the other — should prompt assessment for a birth injury, such as brachial plexus injury.
Mnemonic
MSBGRT — the six primitive reflexes
M — Moro (Startle)
Present birth to 4–6 months
Arms extend then flex in response to a sudden stimulus (like a loud noise or sudden movement). Asymmetric Moro response is a specific red flag for brachial plexus injury — one arm responding while the other doesn't suggests a physical injury to that side rather than a general neurological problem.
S — Sucking
Present until 3–4 months
The infant sucks when anything touches the lips. This reflex is directly essential for feeding, making its presence and strength a meaningful part of early feeding assessment.
B — Babinski
Normal in infants; disappears with cortical maturation
Toes fan outward when the sole of the foot is stroked. This is a completely normal finding in infants — but the same finding in an adult is abnormal, indicating a possible upper motor neuron (UMN) lesion. Context (patient age) entirely changes the significance of an identical physical exam finding.
G — Grasp (Palmar and Plantar)
Palmar disappears 3–4 months; Plantar disappears 9 months
Fingers curl around an object placed in the palm (palmar grasp); toes curl when the sole is pressed (plantar grasp) — two related but separately timed reflexes, with the plantar version persisting notably longer than the palmar one.
R — Rooting
Disappears 3–4 months
The infant turns their head toward a touch on the cheek. Like sucking, this reflex directly facilitates feeding — helping the infant locate and latch onto a feeding source.
T — Tonic Neck (Fencing)
Disappears 4–6 months
When the head is turned to one side, the arm on that same side extends while the opposite arm flexes — producing a posture resembling a fencing stance.
💊 Two reflexes disappear notably later than the rest: plantar grasp (9 months) and — depending on source — tonic neck and Moro both extend to 4-6 months, while sucking, rooting, and palmar grasp cluster earlier at 3-4 months. Grouping them by their approximate timing window, rather than memorizing six isolated numbers, makes recall easier.
🏥 Clinical Scenario — Recognizing an Asymmetric Moro Reflex
During a newborn assessment following a difficult vaginal delivery, the nurse elicits the Moro reflex and observes full arm extension and flexion on the right side, but minimal movement of the left arm.
Recognize the Red Flag Pattern
An asymmetric Moro reflex — present and normal on one side, diminished or absent on the other — is a specific, recognized red flag for brachial plexus injury, particularly relevant given the difficult delivery history. This isn't simply a variation in reflex strength; the asymmetry itself is the significant finding.
Assess Further
The nurse examines the affected arm and shoulder more closely for additional signs of brachial plexus injury (such as decreased spontaneous movement or an abnormal resting position of that arm), and documents the specific asymmetric finding precisely. Given the delivery history, this combination of findings warrants provider notification rather than routine documentation alone.
Communicate and Follow Up
The nurse reports the finding to the provider for further evaluation, understanding that early identification of a possible birth injury supports earlier appropriate intervention and follow-up care. Recognizing this specific reflex asymmetry as clinically meaningful — not just a normal variant — is exactly the skill this lesson is built around.
📌 NCLEX Application
Newborn reflex questions test both the expected timeline and red-flag recognition:
Red flag recognition: "A newborn's Moro reflex is asymmetric, stronger on one side. What should the nurse suspect?" → Possible brachial plexus injury on the weaker side.
Age-dependent interpretation: "A positive Babinski sign is elicited in a 2-month-old infant. Is this a normal or abnormal finding?" → Normal — Babinski is expected in infants; it would only be abnormal in an adult.
Feeding-related reflexes: "Which two primitive reflexes are most directly related to feeding?" → Rooting and sucking.
⚠️ The Trap — Assuming a Positive Babinski Sign Is Always Abnormal
Because a positive Babinski sign is taught as a concerning finding suggesting upper motor neuron pathology, it's easy to apply that same alarm to an infant showing the identical physical response. But a positive Babinski is the expected, normal finding in infants — it only becomes concerning once cortical maturation should have suppressed it, at older ages.
The safeguard: Always interpret Babinski (and other primitive reflexes) in the context of the patient's actual age — the same physical exam finding carries opposite clinical meanings in an infant versus an adult.
✓ Quick Self-Test
Answer before checking:
1. What does MSBGRT stand for?
2. What does an asymmetric Moro reflex suggest?
3. Is a positive Babinski sign normal or abnormal in a newborn? In an adult?
4. Which reflex persists the longest, and until approximately what age?
Answers:
1. Moro, Sucking, Babinski, Grasp, Rooting, Tonic neck.
2. Possible brachial plexus injury.
3. Normal in a newborn; abnormal in an adult (suggesting an upper motor neuron lesion).
4. Plantar grasp — until approximately 9 months.