Happy Mutant 3-Armed Baby Costume




About: I've been posting Instructables since the site's inception, and now build other things at Autodesk. Follow me for food and more!

This year we were a Nuclear Family for Halloween, with our 4-month-old daughter Corvidae dressed up as a 3-armed happy mutant.

While we wanted to be subtle, this was almost too subtle - she wore the costume all day, and hardly anyone noticed!  But when they finally detected a problem, the responses were excellent.

This instructable documents the creation and care of a 3-armed baby.  If you make your own, please post pictures!


Step 1: Find Radiation Sources

As with many superheroes, Corvidae's mutation is the result of irradiation.  in this case, both of her parents take serious rads on a regular basis - Eric as a Beam Engineer at North Central Positronics*, and I as head of the Experimental Radiology department at Mid-World University*.

radiation-induced mutant powers are highly idiosyncratic.  We cannot guarantee your child will be born with super strength or telekinesis, or that any extra limbs will be fully functional and/or not evil.

Since we had to leave our real badges at work (pesky government regulations!) Eric quickly cut duplicates on the Epilog laser cutter.

*Hat tip to Stephen King's Dark Tower.

Step 2: Find Duplicate Outfits

To clothe your adorable 3-armed mutant, you'll need duplicate outfits, as few baby clothes are sold with an extra sleeve.  Choose highly-patterned clothing for best results.

Step 3: Find Suitably-sized Doll

A similarly-sized doll can be useful in sizing the costume for your 3-armed baby.  We'll use one of its arms to mock up the outfit, though of course we'll substitute Corvidae's real third arm when she wears the outfit.

This doll has realistic plastic hands, and a cloth body - perfect for our purposes.  Choose the hand with the most realistic positioning.

Step 4: Prepare Doll Arm

Cut the doll's arm off at the shoulder, leaving as much extra material as possible.  Test against the sleeve of your outfit - the arm should be long enough to fit comfortably, just like your baby's real third arm.

Stuff a bit more cotton in the upper arm/shoulder region, then hand or machine stitch the openings shut. 

Dispose of doll carcass, saving any useful bits for future projects.

Step 5: Prepare Third Sleeve

Cut the sleeve from your sacrificial outfit, again leaving extra material arund the shoulder region to accommodate future sewing.

Test your doll arm inside the sleeve - is it long enough, and properly plump?  Did you use the appropriate matching-side sleeve?

Step 6: Sew in Third Sleeve

Use a seam ripper to open the seam directly below the existing arm hole in your final outfit, extending the hole just far enough to fit your third sleeve inside.  Adjust the new sleeve position so it matches the original sleeves, then turn the outfit inside-out to pin the sleeve in place, right-sides together.

Sew sleeve in place, stitching right inside the edge of the split seam.  (you'll likely have extra bits of cut sleeve protruding beyond, which is fine.)

Turn the outfit right-side out to admire/check your work.

Now you can sew your third arm into the sleeve, anchoring it at the shoulder only for extra-lifelike flop.  Alternatively, go put the suit on your 3-armed baby.

Step 7: Dress Your Happy Mutant

Of course she's happy - she finally has a custom pantsuit, specially fitted to accommodate her third arm!  Be careful, though - with three arms vs. your two, you may need help getting the pantsuit on.

Step 8: Put Her in the Carseat

With five flailing limbs, this may prove difficult! 
Be sure to secure all three arms underneath the shoulder straps.

Step 9: Visit the Farmers' Market

Take you happy mutant out on the town!

Most people were completely oblivious.  When people smiled at Corvidae, I waved her third arm back at them - everyone smiled again and returned the wave, clearly not bothering to count arms.  Only those who had several minutes to examine her up close while I paid for groceries ever noticed something was amiss. You could tell when they saw it - their eyes were suddenly huge, surprised and amused/horrified all at once. Even after we told people she was wearing her Halloween costume, it often took a while for them to catch on.

My favorite anecdote: a lady came up to us at the market, and asked if she could say hello to the baby.  We watched with amusement as she spoke to Corvidae, put her finger up for the baby to grab - and offered it to the third hand.  She seemed a bit disappointed when the baby didn't grab her finger, and stepped back.   Finally her eyes widened in shock.  "Your baby has three hands!"

Step 10: Pose for Family Pictures!

Document your nuclear family!



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56 Discussions


5 years ago on Introduction

Amazing! Too bad I saw just now, three days after Halloween. My baby would have such a great mutant! I wander if you could be a mutant on Purim?


7 years ago on Introduction

I liked the fact that nobody realy picked up on it. That's what I call a costume. Cute kid

This is fantastically, beautifully, wonderfully subtle. I love it!

It doesn't hurt that the baby is just a little bit cute.


8 years ago on Introduction

That's gorgeous.
Perhaps next year you could do a beeblebaby. http://www.zz9.org/merchandise/beeblebears.html

That is an awesome costume - what a great idea! Also, your baby is incredibly adorable and lucky to have such creative parents.


8 years ago on Step 10

Genius! I love the family photos


8 years ago on Introduction

No mutant babies were harmed in the construction of this Halloween costume. Can't say the same about dolls, but at least this "Chucky" won't be doing anything left handed... :-)

Great job by the way.


8 years ago on Introduction

Brilliant - I have to make an adult version as my kids are too old!

Jon B3

8 years ago on Introduction


The rest of Australia reckon us Tasmanians have got two heads... this would show them!


8 years ago on Introduction

Excellent!!! When he gets older, this child will probably fully appreciate the humor behind this wonderful gag. This is one of the funniest things I've ever seen-- Hope you'll keep evolving. I love humor.


Reply 8 years ago on Introduction

The limbs have moved from a shelf to a bucket at the back. :-)

The bucket of arms must give a few people a start as they walk by.