Happiest Baby on the Block Swaddle: Sizes, How To Use & More

The Happiest Baby Swaddle, also known as the “5-second swaddle,” was designed by pediatrician Dr. Harvey Karp for its effectiveness and ease of use. It is highly praised for its ability to calm cranky babies and induce a deep slumber through its snug and comforting womb-like wrapping.

Impressive reviews aside, is this swaddle product the best option for your little one?

Is Happiest Baby swaddle safe? Yes, this simple design features a smart inner-arm band to keep infants protected from the startle reflex, and the swaddle is made from breathable all-natural material with mesh ventilation panels to prevent overheating. The spacious leg room also permits the frog position to prevent hip dysplasia.

To learn more about how this sleep swaddle works, stick with us – we’ll look at what makes the Happiest Baby Swaddle stand out from its competitors and steps on how to use it plus tips on helping your baby transition from the swaddle and more.

How Does the Happiest Baby Swaddle Work?

The Happiest Baby Swaddle sleep sack is designed to make the baby feel they’re wrapped snugly and safely in their sleep, mimicking the snug embrace felt in the womb.

An inner band keeps their arms secure to prevent them from becoming startled by whacks to the nose and face in the middle of the night, reducing cries for a better night’s sleep, but you have the option to leave babies’ arms free through the shoulder arm holes once they are ready to transition to arms-up sleeping.

The sleep sack is also made with breathable mesh panels at the legs and shoulders to avoid overheating and a 2-way zipper at the front of the swaddle for convenient diaper changes in the middle of the night.

Are Swaddle Sacks Safe for Newborns?

Yes, swaddle sacks are typically designed for infants from newborn age up to age 6 months.

According to the Minnesota Department of Health, sleep swaddle sacks “are an acceptable and preferred alternative to swaddle blankets as they reduce babies’ risk of suffocation and entrapment.”

The MN Dept. also stresses that appropriate sleep sacks are those that “have not been recalled and allow for an infant’s arms and legs to move freely.”

Is Happiest Baby Swaddle AAP Approved?

Yes. The Happiest Baby Sleepea Swaddle was invented by Dr. Harvey Neil Karp, a fellow of the American Academy of Pediatrics (AAP) and Assistant Professor of pediatrics at the University of Southern California.

To use safely use Happiest Baby Swaddle or any baby swaddle sack, the AAP asks parents/guardians to keep to the following guidelines:

  • Ensure baby is placed on their back when swaddled
  • Never use weighted swaddles
  • Avoid wrapping the swaddle too tightly to the point that baby cannot move their hips or could find it hard to breathe
  • STOP swaddling your baby altogether when it looks like they’re trying to roll over

What Makes Happiest Baby Swaddle Better Than Others?

According to a New York Times review of Happiest Baby Swaddle, the “unique interior velcro panel and compressive pouch make it harder for babies to break out of compared to other swaddles, which rely on just one layer to keep arms down.” The swaddle’s “roomy” sack was also praised for assisting with the frog position for growing hips.

John Doht of the blog Fathercraft has also noted that while Happiest Baby Swaddle shares some similarities with other sleep sacks, a noticeable difference lies in the knee and shoulder ventilation panels:

“Other sleep sacks focus on the fact they’re made of a warmer material, but these mesh panels are designed to prevent overheating – which is a common SIDS (Sudden Infant Death Syndrome) trigger.”

Happiest Baby Swaddle Age Range

The Happiest Baby Swaddles do not expressly categorize their swaddles by age range but rather by size (Small, Medium, and Large).

According to Happiest Baby customer care on their Amazon page: “products are typically used by infants up to 6 months of age, and you should stop swaddling when your baby is able to roll.”

You can determine the expected age for each size category using the weight range (found in the following section) as a guide.

Happiest Baby Swaddle Weight Range

Happiest Baby Swaddle comes in 3 sizes, according to weight:

  • Small: 5-12 pounds
  • Medium: 12-18 pounds
  • Large: 18-26 pounds

If we use this chart of average baby weights as recommended by the AAP, we can determine that, according to the 50th percentile weight for male and female babies, a Small Happiest Baby Sleep swaddle will fit a baby from birth to 2.5 months (with a snugger fit for baby boys).

Babies typically weigh between 16 and 18 pounds by 6 months of age, and as babies can begin to roll at around 6 months (and as early as 4 months), the Small and Medium Swaddle sizes will accommodate average/expected weight babies for the duration of the safe-swaddling window.

Happiest Baby Swaddle Materials

The Happiest Baby Swaddle is made from soft and stretchy 100% organic cotton, which is ideal for your baby’s delicate skin as it means that it’s made without the use of pesticides and no harmful chemicals have been left behind during the processing step.

It is also made with breathable mesh panels and ultra-quiet Velcro on the interior panel as opposed to regular, noisy Velcro straps!

Happiest Baby Swaddle Options

Happiest Baby Swaddle comes in two main styles, both with 2-way zippers, dubbing them “5-second swaddles” for easy removal during the night.

The Sleepea Swaddle is roomier in the legs and comes with more colors/designs while the similar SNOO Sleep Sack is designed with wings to keep babies on their back.

Happiest Baby Sleepea® Swaddle

Happiest Baby Sleepea® Swaddle on Amazon

The Sleepea is made from soft and breathable material to provide ventilation and an inner band to provide womb-like security and a snug top fit with a relaxed fit below. It also allows for safe arms-out swaddling when baby is ready to transition.


  • Small (20.5 x 12 inches) for babies 5-12 pounds
  • Medium (23 x 13 inches) for babies 12-18 pounds
  • Large (25 x 13 inches) for babies 18-26 pounds


The Sleepea comes in Star, Planet, Pine Tree, and Snowflake designs in the colors Graphite, Teal, Ivory, Midnight Blue, Rose, and Sky Blue.

Happiest Baby SNOO® Sleep Sack

Happiest Baby SNOO® Sleep Sack on Amazon

Made with inner-arm bands and a leg flap, the SNOO sleep sack keeps mini Houdinis safe and snug. It is also designed with mesh panels to prevent overheating and features specialized wings that attach to the bassinet to ensure baby stays on his/her back.

Arm openings can also unsnap for when baby transitions to arms-out sleep.


In general, sizes relating to baby weight:

  • Small for babies 5-12 pounds
  • Medium for babies 12-18 pounds
  • Large for babies 18-26 pounds


The SNOO comes in Teal, Rose, Midnight Blue, Ivory, Graphite, and White in mix-and-match Planet and Star designs (for the main body and wings).

Happiest Baby Swaddle Pros and Cons

As with most baby products, there are benefits and drawbacks to the Happiest Baby Swaddle that you will need to consider.


  • Parents have praised the “right kind of snugness” – neither too loose nor too tight
  • The closure doesn’t get too close to the baby’s neck or head
  • Smart inner band allows for dual sleeping position – arms tucked in or out
  • 2-way zipper makes late night/early morning diaper changes less of a hassle
  • Breathable 100% organic cotton material


  • Parents have commented that the Velcro wears quickly, making fastening difficult after a while
  • Material not as stretchy as advertised
  • May take longer than the advertised “5 seconds” to get your baby into the swaddle

How To Use Happiest Baby Swaddle

If you’ve ever used a sleep sack of any kind, then the Happiest Baby Swaddle should feel familiar to you. If not, the simple structure of Dr. Karp’s design will be straightforward to follow.

Here’s how to get your little one nice and snug in their swaddle:

1. First, place your baby onto the zipped-open Sleepea Swaddle, making sure their shoulders are lying about 2 inches above the neckline of the swaddle.

2. Next, gently straighten baby’s right arm, and using the smart inner band, carefully pull the band across their arm and chest. Hold in place, and pull the other band across, and attach it to the Velcro strap.

3. With baby’s arms tucked up tight, place their legs inside the bottom half of the sack, and carefully zip it up all the way to their neck. The top of the swaddle zipper should rest just under baby’s chin.

Here’s a visual guide to using the Baby Swaddle.

Happiest Baby Swaddle Arms Out

The snug, secure fit around the arms of the Happiest Baby Swaddle ensures that your baby won’t startle themselves awake in the night with a rogue hand hitting their face, but as they develop, infants like to sleep with their arms free.

The Swaddle is made with arm openings at the shoulder that can unsnap individually for the baby to let one or both arms out during sleep, helping them transition safely to a new way of sleeping without risking their arms becoming trapped or stuck in the neck hole.

How Should Happiest Baby Swaddle Fit?

The Happiest Baby Swaddle should have a snug fit across the baby’s torso and arms with a looser fit at their hips and legs to promote a frog-like sleeping position.

A good test that the swaddle is not fitted too tightly is if you can fit two fingers at the top of the swaddle between the baby and the material.

Happiest Baby Swaddle Care & Cleaning

The Happiest Baby team advises attaching the Velcro bands to each other and zipping the sack completely closed before washing to prevent the Velcro from catching on other clothes in the load.

Wash the Swaddle in cool/cold water with similar colors (never in hot water or chlorine/bleach), and tumble dry on a low heat setting. Lastly – allow the Swaddle sack to air dry completely before using it again.

How Do I Transition My Baby From the Happiest Swaddle?

Ideally, you should start to transition your baby from using the Happiest Swaddle sack once they start to attempt to roll over in their sleep. This can occur somewhere between 3 and 6 months but can be earlier in some babies.

To help with the transition out of swaddling, Sleepea designer Dr. Harvey Karp recommends the following:

Phase 1

Begin wrapping baby with one arm out of the swaddle by simply unsnapping the shoulder snaps on one side and gently pulling their arm through. It’s best to use the arm your baby tends to favor most for this.

Phase 2

If they sleep well this way for a few nights, then you can move on to leaving both arms out. If their sleep is disturbed by the one-arm-out method, then retry it again in a couple of weeks.

Phase 3

If baby is sleeping successfully with both arms out regularly, then you can keep them in their Sleepea Swaddle in this arms-out position until they begin to show signs of rolling over.

At this point, swaddling must stop to prevent the risk of SIDS. You can now begin to help your little one drift off to sleep in other ways using “sleep cues” at night and for every nap.

Things like using white noise and gentle rocking motions can help.

Where To Buy Happiest Baby Sleepea® Swaddle

Closing Thoughts

All in all, the Happiest Baby Sleepea Swaddle is a well-designed swaddle for your little one.

Having been invented by trusted pediatrician and fellow of the American Academy of Pediatrics Dr. Harvey Karp, you can be sure it is a safe option for your baby.

Whichever swaddling product you choose for your infant, ensure your chosen swaddling product and practices meet the AAP guidelines, i.e., providing enough room in the leg and hip area to promote healthy development, choosing a swaddle without weighted features, and ensuring baby is always placed on their back.