NB to 18-24m: A Complete Sizing Guide for Baby Clothing at Kate Quinn

Shopping for baby clothes is one of the most exciting parts of preparing for a new arrival, but it can also be surprisingly tricky. With sizes ranging from newborn (NB) all the way up to 18-24 months, it’s easy to feel overwhelmed. At Kate Quinn, we understand that every baby grows at their own pace, which is why we’ve put together this comprehensive sizing guide to help you navigate the journey from those first tiny onesies to toddler-sized adventures.

Whether you’re stocking up on layette essentials or planning ahead for the next growth spurt, knowing how our sizes run and what to expect at each stage will save you time, money, and the frustration of ill-fitting outfits. In this guide, we’ll break down each size range, share tips on how to choose between sizes, and highlight a few of our favorite pieces that are perfect for every stage.

Understanding Kate Quinn’s Size Ranges

At Kate Quinn, our baby clothing sizes are designed to align with typical infant growth patterns, but we also know that babies come in all shapes and sizes. Our size ranges are: NB (newborn), 0-3m, 3-6m, 6-12m, 12-18m, and 18-24m. These ranges are based on average weight and length, but we always recommend checking the specific product measurements, as different styles may fit differently.

For example, our bamboo fabrics tend to be more stretchy and forgiving, while organic cotton jersey pieces often have a more classic fit. This means a 3-6m in one style might fit like a 6-12m in another. That’s why we’ve created this guide—to help you understand not just the numbers, but how our clothes actually fit on real babies.

• NB (newborn): up to 8 lbs, 18-21 inches
• 0-3m: 8-12 lbs, 21-24 inches
• 3-6m: 12-17 lbs, 24-27 inches
• 6-12m: 17-22 lbs, 27-30 inches
• 12-18m: 22-27 lbs, 30-32 inches
• 18-24m: 27-30 lbs, 32-35 inches

Newborn (NB) to 3 Months: The First Few Weeks

The newborn stage is all about softness, convenience, and easy access for diaper changes. Our layette collection is specifically designed with these needs in mind. For those first few weeks, look for pieces with lap neck or envelope necklines, like our long sleeve lap neck footie | jadeite | bamboo, which makes dressing a wiggly newborn a breeze. The bamboo fabric is incredibly gentle on sensitive skin and provides just the right amount of stretch.

As your baby grows into the 0-3m range, you’ll want to start introducing a few more pieces that allow for movement and play. Our cuffed pocket short | diamond | bamboo is a great choice for warmer days, as the cuffs keep the shorts in place while your little one kicks and explores. Remember, babies grow quickly, so don’t overbuy in the smaller sizes—you’ll likely be moving up before you know it.

• Opt for footies and one-pieces for easy diaper changes.
• Choose bamboo or organic cotton for breathability and softness.
• Buy only a few NB items, as babies outgrow this size fast.

3 to 12 Months: Rapid Growth and Increased Mobility

Between 3 and 12 months, your baby will go through major growth spurts and start rolling, sitting, and eventually crawling. This is the time to prioritize comfort and flexibility. Our short sleeve union shortie | hot fudge | organic cotton single rib is a versatile piece that works well for both play and nap time. The single rib fabric stretches with your baby, and the shortie design keeps them cool during active play.

For cooler months, layering is key. A long sleeve two way zipper footie | papa bear | organic cotton jersey is a must-have for easy nighttime changes without fully undressing your baby. The two-way zipper allows for quick diaper access, and the organic cotton is gentle on sensitive skin. As your baby becomes more mobile, you’ll appreciate pieces that move with them, like our cuffed jogger | tapestry | bamboo—perfect for crawling and early steps.

• Look for stretchy fabrics like bamboo and ribbed cotton.
• Two-way zippers are a lifesaver for nighttime changes.
• Cuffed pants and joggers help keep socks on and legs covered.

12 to 24 Months: Toddlerhood Approaches

As your baby approaches their first birthday and beyond, they’ll be walking, talking, and asserting their independence. Clothing needs to be durable, easy to put on, and still adorable. The 12-18m and 18-24m sizes are designed to accommodate toddlers who are on the move. Our short sleeve peter pan dress | retro diamond floral | organic cotton jersey is a charming choice for little girls, offering a classic silhouette that’s both comfortable and stylish.

For boys, a long sleeve sport union suit | rust | organic cotton multi rib provides full coverage and flexibility for active play. This size range is also when you might start investing in sets, like our short sleeve peter pan skirted top + bubble short set | arctic leaf | modal, which takes the guesswork out of matching outfits. Remember, toddlers grow at their own pace, so when in doubt, size up for a little extra room to grow.

• Choose styles with easy closures for wiggly toddlers.
• Sets are a convenient way to ensure a coordinated look.
• Size up if your toddler is between sizes for longer wear.

Tips for Choosing the Right Size

One of the most common questions we hear is, 'Should I size up or down?' The answer depends on the fabric and the fit you prefer. For bamboo pieces, which have a lot of stretch, you can often stick with your baby’s current size for a snugger fit or size up for a roomier feel. Organic cotton jersey tends to have a more classic fit, so if you’re between sizes, we recommend going up.

Another tip is to consider the season. If your baby is between sizes and the next size up will be appropriate for the upcoming weather, it’s wise to size up. For example, if your 6-month-old is about to outgrow 3-6m and it’s entering fall, choosing 6-12m in long sleeves will give them room to grow into the cooler months. Always check the product measurements on the product page, as we provide detailed dimensions for each item.

• Check the product measurements before ordering.
• Consider the season and how long you want the item to last.
• When in doubt, size up—babies grow fast!

Choosing the right size for your baby doesn’t have to be a guessing game. With this guide and our detailed product measurements, you can shop with confidence, knowing you’re picking pieces that will fit well and last through those precious early years. Ready to start shopping? Explore our full range of baby clothing, from newborn layette to toddler sets, and find the perfect fit for your little one.