On This Page

Time never zips by faster than when you’re watching your little one grow. And those weeks and months are jam-packed early on. In fact, did you know your baby’s brain will double in size in the first year?! That’s a lot of learning! With it, comes a bunch of both mental and physical milestones that equip your baby with the skills they need in toddlerhood and beyond. Wondering which baby development milestones to have on your radar? Check out our guide to baby development guide by week and month for the first two years.

Baby Development by Week: Weeks 1 to 12

You’ve probably heard of the first, second, and third trimesters of pregnancy The first 12 or so weeks of your baby’s life on earth are known as the Fourth Trimester. The Fourth Trimester is a period coined by Dr. Harvey Karp to describe the first three months of a baby’s life after birth. While you may have been eager to meet your baby, your little probably would have opted for a little more time in the cozy womb, where they were hugged by the uterine walls, serenaded by the whooshing sound of blood flow, and gently rocked with every step. When they join our world, the stark, still bassinet and quiet nursery can be quite a shock to the system! That’s why they respond so well to swinging, shushing, and swaddling (as well as the other 5 S’s). 

Additionally, even though they don’t seem up to much, there’s lots of growth and activity going on in their little brains. Here’s a week-by-week guide to the first 12 weeks of your baby’s development:

Baby Development by Month: Months 4 to 12

Once your little one passes three or four months, they’ve really woken up to the world around them! You may notice they’re eagerly taking everything in. Soon, your once tiny, stationary baby will be moving, cooing, and interacting with their surroundings in all sorts of fun new ways! Here’s what to expect in months 4 through 12: 

Baby Development by Month: Months 13 to 24

So technically once your bub is north of 12 months, they’re a toddler. But the truth is, your little one might look and sound and act a lot more like a baby for a while longer! While months 0 through 12 are marked by an explosion of development, there’s plenty happening in months 13 and beyond too (including some pretty literal explosions…at least when it comes to their emotions!). In the second year, look for these toddler developmental milestones:

Baby Development: Milestones to Watch

As your tot grows, they’ll hit a whole bunch of milestones along the way. Our guide to the stages of baby development will give you an overview of when to expect your bub to hit these marks. 

And then you can dig into some common milestones to expect—including when to expect them, how to encourage them, and what to do if they appear to be delayed.

Social, Cognitive & Language Baby Milestones

Physical Baby Milestones

More on How Babies’ Brains Develop

View more posts tagged, milestones

Have questions about a Happiest Baby product? Our consultants would be happy to help! Submit your questions here.

Disclaimer: The information on our site is NOT medical advice for any specific person or condition. It is only meant as general information. If you have any medical questions and concerns about your child or yourself, please contact your health provider.