Uneven Startle Reflex in Newborns (Asymmetric Moro)
Medically reviewed by Dr. Michael Okonkwo, MD, FAAP · Board-Certified Neonatologist
Content reviewed against published AAP, NIH guidelines
Last reviewed:
If your baby has been diagnosed with or you suspect uneven startle reflex in newborns (asymmetric moro), here is what the evidence says.
The short answer
An asymmetric Moro reflex, where one arm responds differently than the other during startle, is an important finding that should be evaluated by your pediatrician. It may indicate a birth injury such as a broken clavicle (collarbone) or brachial plexus injury (Erb's palsy). Early evaluation and treatment lead to the best outcomes.
Key takeaways
- An asymmetric Moro reflex, where one arm responds differently than the other during startle, is an important finding that should be evaluated by your pediatrician. It may indicate a birth injury such as a broken clavicle (collarbone) or brachial plexus injury (Erb's palsy). Early evaluation and treatment lead to the best outcomes.
- Usually normal when: Both arms respond symmetrically during the Moro reflex, fanning out and then coming back in together
- Call your doctor if: One arm is completely limp or your baby shows no movement in one arm, which could indicate a significant nerve injury or fracture
- Varies by age — see the age-by-age breakdown below
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What Parents Should Know
According to AAP, NIH guidelines, an asymmetric Moro reflex, where one arm responds differently than the other during startle, is an important finding that should be evaluated by your pediatrician. It may indicate a birth injury such as a broken clavicle (collarbone) or brachial plexus injury (Erb's palsy). Early evaluation and treatment lead to the best outcomes. At 0-1 month, during a normal Moro reflex, both arms should fling out symmetrically. If one arm does not respond, moves less, or is held differently, this could indicate a birth injury. The most common causes are a fractured clavicle (collarbone) or a brachial plexus injury (damage to the nerves controlling the arm), also known as Erb's palsy. These injuries are more common after difficult deliveries, large babies, or shoulder dystocia. Your pediatrician should evaluate any asymmetry promptly. Most clavicle fractures heal on their own in 2-3 weeks, and many brachial plexus injuries improve within the first few months. It is generally considered normal when both arms respond symmetrically during the Moro reflex, fanning out and then coming back in together. However, you should contact your pediatrician promptly if one arm is completely limp or your baby shows no movement in one arm, which could indicate a significant nerve injury or fracture.
Normal vs. Concerning
By Age
What to expect by age
0-1 month
During a normal Moro reflex, both arms should fling out symmetrically. If one arm does not respond, moves less, or is held differently, this could indicate a birth injury. The most common causes are a fractured clavicle (collarbone) or a brachial plexus injury (damage to the nerves controlling the arm), also known as Erb's palsy. These injuries are more common after difficult deliveries, large babies, or shoulder dystocia. Your pediatrician should evaluate any asymmetry promptly. Most clavicle fractures heal on their own in 2-3 weeks, and many brachial plexus injuries improve within the first few months.
1-3 months
If a birth injury was identified, your pediatrician will be monitoring recovery. Most clavicle fractures are fully healed by this time. Brachial plexus injuries typically show significant improvement by 3 months. If there is no improvement in arm movement by 3 months, referral to a pediatric neurologist or orthopedist may be recommended for further evaluation and possible therapy.
3-6 months
The Moro reflex naturally fades by this age, making asymmetry harder to assess. However, overall arm use should be assessed. If your baby is consistently not using one arm as well as the other, physical therapy and continued specialist follow-up may be recommended. About 80-90% of brachial plexus injuries recover fully within the first year.
6-12 months
If a brachial plexus injury has not recovered by this age, surgical options may be considered. Your baby should be using both arms for reaching, grasping, and playing. Continued physical therapy can optimize function even if some nerve damage is permanent.
What to Tell Your Pediatrician
- Describe when you first noticed uneven startle reflex in newborns (asymmetric moro) and how it has changed over time.
- Note your baby's current age and which age-specific patterns you are seeing.
- Mention if you notice one arm moves less than the other during startle responses.
- Mention if one arm seems to be held in a different position (such as against the body or internally rotated).
- Let your doctor know if you have noticed any related concerns, such as changes in feeding, sleep, or movement patterns.
- Bring a list of any questions or observations you want to discuss at the appointment.
What Should You Do?
When to take action
- Both arms respond symmetrically during the Moro reflex, fanning out and then coming back in together
- You notice one arm moves less than the other during startle responses
- One arm seems to be held in a different position (such as against the body or internally rotated)
- Your baby does not seem to move one arm as freely as the other
- One arm is completely limp or your baby shows no movement in one arm, which could indicate a significant nerve injury or fracture
- Swelling, bruising, or tenderness over the collarbone area, especially after a difficult delivery
What You Can Do at Home
- Keep track of when you notice uneven startle reflex in newborns (asymmetric moro) — noting the time of day, duration, and any triggers can help your pediatrician.
- Remember that both arms respond symmetrically during the Moro reflex, fanning out and then coming back in together — this is generally within the range of normal.
- At 0-1 month, focus on observation rather than intervention unless your pediatrician advises otherwise.
- Follow any care instructions from your pediatrician. Keep a written log of symptoms to bring to appointments.
- While monitoring at home, seek immediate care if one arm is completely limp or your baby shows no movement in one arm, which could indicate a significant nerve injury or fracture.
Related Conditions
Erb's Palsy (Brachial Plexus Birth Injury)
A brachial plexus injury (Erb's palsy) occurs when the nerves controlling the arm are stretched during delivery, most often during shoulder dystocia. The affected arm may appear limp or have limited movement. The good news is that 80-90% of cases resolve within the first 3-6 months with conservative management and physical therapy.
Broken Collarbone (Clavicle Fracture) at Birth
A broken clavicle (collarbone) is one of the most common birth injuries, occurring during difficult deliveries, especially in large babies or when shoulder dystocia occurs. While concerning to parents, clavicle fractures in newborns heal very quickly (usually within 2-3 weeks) without any specific treatment. The prognosis is excellent.
Excessive Moro (Startle) Reflex in Newborns
The Moro (startle) reflex is a normal primitive reflex present in all newborns. Some babies seem to have a more active or sensitive startle reflex, which can cause them to wake frequently during sleep. This is a normal variation and typically diminishes by 3-4 months of age. Swaddling can help reduce the impact of the Moro reflex on sleep.
Related Resources
Frequently asked questions
Is uneven startle reflex in newborns (asymmetric moro) normal?
When should I call the doctor about uneven startle reflex in newborns (asymmetric moro)?
When is uneven startle reflex in newborns (asymmetric moro) normal?
What causes uneven startle reflex in newborns (asymmetric moro)?
What should I mention to my pediatrician about uneven startle reflex in newborns (asymmetric moro)?
Is uneven startle reflex in newborns (asymmetric moro) normal at 0-1 month?
Is uneven startle reflex in newborns (asymmetric moro) normal at 1-3 months?
Should I go to the ER for uneven startle reflex in newborns (asymmetric moro)?
Does uneven startle reflex in newborns (asymmetric moro) go away on its own?
References
Doctor Visit Checklist
Bring this checklist to your next pediatrician visit to discuss Uneven Startle Reflex in Newborns (Asymmetric Moro).
Things to mention
- Describe when you first noticed uneven startle reflex in newborns (asymmetric moro) and how it has changed over time.
- Note your baby's current age and which age-specific patterns you are seeing.
- Mention if you notice one arm moves less than the other during startle responses.
- Mention if one arm seems to be held in a different position (such as against the body or internally rotated).
- Let your doctor know if you have noticed any related concerns, such as changes in feeding, sleep, or movement patterns.
- Bring a list of any questions or observations you want to discuss at the appointment.
Observations to share
- You notice one arm moves less than the other during startle responses
- One arm seems to be held in a different position (such as against the body or internally rotated)
- Your baby does not seem to move one arm as freely as the other
Urgent signs to report immediately
- One arm is completely limp or your baby shows no movement in one arm, which could indicate a significant nerve injury or fracture
- Swelling, bruising, or tenderness over the collarbone area, especially after a difficult delivery
My notes
From ismybabyalright.com — free, evidence-based baby health guides
All content follows our editorial policy and is reviewed against published clinical guidelines.
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Related Resources
Bottom line
Most cases of uneven startle reflex in newborns (asymmetric moro) are normal. Talk to your pediatrician if one arm is completely limp or your baby shows no movement in one arm, which could indicate a significant nerve injury or fracture.
Trust your instincts. If something feels wrong, reach out to your pediatrician. Worrying about your baby means you care — that is a good thing.
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Related Medical Concerns
Erb's Palsy (Brachial Plexus Birth Injury)
A brachial plexus injury (Erb's palsy) occurs when the nerves controlling the arm are stretched during delivery, most often during shoulder dystocia. The affected arm may appear limp or have limited movement. The good news is that 80-90% of cases resolve within the first 3-6 months with conservative management and physical therapy.
Broken Collarbone (Clavicle Fracture) at Birth
A broken clavicle (collarbone) is one of the most common birth injuries, occurring during difficult deliveries, especially in large babies or when shoulder dystocia occurs. While concerning to parents, clavicle fractures in newborns heal very quickly (usually within 2-3 weeks) without any specific treatment. The prognosis is excellent.
Excessive Moro (Startle) Reflex in Newborns
The Moro (startle) reflex is a normal primitive reflex present in all newborns. Some babies seem to have a more active or sensitive startle reflex, which can cause them to wake frequently during sleep. This is a normal variation and typically diminishes by 3-4 months of age. Swaddling can help reduce the impact of the Moro reflex on sleep.
My Baby's Head Shape Looks Abnormal
Many babies develop temporary head shape irregularities that are completely normal. A cone-shaped head from vaginal delivery reshapes within days. Mild positional flattening (plagiocephaly) from sleeping on the back is very common and usually improves with repositioning and tummy time. However, head shape changes involving ridges, a persistently bulging fontanelle, or rapid head growth changes should be evaluated to rule out craniosynostosis.
Achondroplasia (Dwarfism) in Babies
Achondroplasia is the most common form of short-limbed dwarfism, affecting about 1 in 15,000 to 40,000 births. It is caused by a mutation in the FGFR3 gene and is usually apparent at birth with characteristic features including short limbs, a larger head, and a prominent forehead. Intelligence is normal. With monitoring for specific complications and supportive care, children with achondroplasia lead full, active, and independent lives.
Adenoid Hypertrophy and Breathing
Adenoids are lymphoid tissue located behind the nose that help fight infection in young children. When adenoids become enlarged (adenoid hypertrophy), they can block the nasal airway, causing chronic mouth breathing, snoring, nasal speech, and sleep-disordered breathing. Enlarged adenoids are most common between ages 2-7 and are a leading cause of obstructive sleep apnea in young children. Treatment ranges from watchful waiting and nasal steroids to surgical removal (adenoidectomy) if breathing or sleep is significantly affected.