The Ultimate Guide to Baby Clothing Sizes ♡

(so you never have to guess again, mama)

Hi sweet friend, it’s Anne — currently remembering holding up a 9–12M romper to one of my girls like “please still fit for one more month?” while they just giggle and try to eat the tag. If you’ve ever stared at a size chart feeling completely lost, welcome to the club. Babies grow at warp speed, brands size completely differently, and one wrong choice means another package headed back to the post office.

I’ve been there more times than I can count. That’s why I wrote this guide — the one I wish someone had handed me when my girls were brand new and I was panic-buying everything in every size “just in case.” Here’s what I’ve learned (the hard way) so you can shop with confidence, keep your little love cozy, and spend less time on returns and more time on snuggles. Because I get it, mama, you’re not just buying clothes — you’re choosing comfort, softness, and those perfect-fitting memories.

Toddler girl dressed in Gabby Lou Baby smocked angel wing dress

Baby Sizes Are More Suggestion Than Rule

Most brands (including us at Gabby Lou Baby) label by age:

• Newborn / NB — up to 8–9 lbs

• 0–3M — 8–12 lbs

• 3–6M — 12–16 lbs

• 6–9M — 16–19 lbs

• 9–12M — 19–22 lbs

• 12–18M — 22–26 lbs

• 18–24M / 2T — 26–30 lbs

  
  

But here’s the real truth: weight and height matter way more than months on a calendar. Some babies are tall and lean at 3 months and already busting out of 0–3M. Others are petite and stay in newborn sizes for what seems like forever. Always check weight + height + chest/waist measurements.

How Gabby Lou Baby Sizes Work

We design for real babies, not runway models.

• Generous growing room — every piece has a little extra length and width so it doesn’t become too snug overnight

• True-to-size with stretch — our Peruvian pima cotton has gentle natural give, so even a “just right” fit feels soft and cozy

• Size-up friendly — if your baby is between sizes, growing fast, or you want longevity, go up one size

  
  

Quick Gabby Lou cheat sheet:

Size	Approx Weight	Approx Height	Typical Age Range	When to Choose It
0–3M	8–12 lbs	21–24"	0–3 months	Most newborns live here 2–4 months
3–6M	12–16 lbs	24–27"	3–6 months	Sweet spot for 4–7 month babies
6–9M	16–19 lbs	27–28"	6–9 months	Size up if tall or chunky
9–12M	19–22 lbs	28–30"	9–12 months	Crawlers & early walkers
12–18M	22–26 lbs	30–33"	12–18 months	Our current bestseller right now!
2T	26–30 lbs	33–35"	18–24 months	Toddlers

Baby boy dressed in Gabby Lou Baby Short Playsuit

When to Size Up (my personal cheat codes)

• Baby is tall, long-torso’d, or growing like a weed

• You want the piece to last 4+ months

• It’s for an upcoming milestone (birthday photos, holiday outfits)

• You love that “room to grow” cozy look (especially in rompers & overalls)

  
  

When to Stick to Size

• Baby is petite or on the slower-to-grow side

• You want a fitted, picture-perfect look for photos

• It’s a special occasion piece (smocked dresses, heirloom blankets)

How to Measure Your Baby in 2 Minutes

1. Weight — Step on the bathroom scale alone, then with baby in your arms. Subtract.

2. Height — Lay baby flat on a blanket against a wall. Gently mark head and heels.

3. Chest & Waist — Use a soft tape (or string + ruler) around the fullest part of the chest and natural waist.

Keep these numbers handy — they’re your secret weapon for online shopping.

Quick Seasonal Fit Tips

• Summer — Lightweight pima, slightly looser for airflow

• Winter — Layer under pima pieces; size up for bulky jackets or footed layers

Still not sure? DM me or email gabbyloubaby@gmail.com with your baby’s age, weight, height, and what you’re shopping for — I’ll personally help you pick the perfect size.

Here’s to outfits that fit like they were made for them (because they basically were), babies who stay cozy, and mamas who don’t have to guess ♡

P.S. All current collections follow this exact sizing guide — shop with confidence here → gabbyloubaby.com