While birth is the end of your pregnancy journey, many medical professionals and experienced parents acknowledge that a new mom’s physical and emotional experience is just beginning.

Likewise, your newborn is encountering unfamiliar territory, too. The big wide world they’ve unwittingly entered is nothing like the warm and cozy womb they’ve called home for the last few months.

The first 12 weeks of life on the other side of pregnancy will be a whirlwind, but you and your baby will navigate this uncharted territory together. Welcome to your new reality — the fourth trimester.

The fourth trimester is the idea of a transitional period between birth and 12 weeks postpartum during which your baby is adjusting to the world and you’re adjusting to your baby.

While there’s often much to be celebrated, it can also be a physically and mentally taxing time for parents and a period of major developmental changes for your baby.

Dr. Harvey Karp, renowned pediatrician and author of “The Happiest Baby on the Block,” is credited for popularizing the concept of the fourth trimester.

According to Karp, even full-term human babies are born “too soon,” and he encourages parents to think of their little ones as fetuses outside the womb for the first 3 months of their lives.

Parents also experience major transition during the first 12 weeks. The learning curve is real; it takes time to master those swaddling skills and distinguish cries of hunger from those of discomfort.

Additionally, birth parents may be contending with postpartum pain, breastfeeding challenges, and fluctuating hormones.

Throw in some sleep deprivation and it’s fair to say that new parents have a whole lot on their proverbial plates.

The first 3 months of your baby’s life may seem like a blur of poop and spit-up, but there is an abundance of activity occurring on a cellular level, and you get a front-row seat for all the developmental changes.

By the time a newborn hits the 3-month milestone, they’ve become little people with budding personalities, curious minds, and basic motor skills. In the meantime, there’s a lot you’ll be doing to support that development.

Why this time is important

There’s a compelling reason Karp believes babies are born too soon — a newborn’s nervous system and brain aren’t entirely developed at birth. It takes time for a baby to create those important synapses that help them master skills like smiling.

Fortunately, you can encourage this brain-cell connectivity by interacting with your newborn — holding, rocking, and talking to them fosters activity in a baby’s blossoming brain.

Additionally, while a baby is born with all five senses, some need additional time to mature. A newborn sees light and dark items within an 8- to 10-inch radius most distinctly. By the end of the fourth trimester, however, many babies are better able to focus on smaller items and to notice colors.

Of course, the fourth trimester also lays the foundation for your baby’s continued physical growth and muscular development.

At birth, a newborn has an array of reflexes — they innately startle, grasp, suck, and root for food. However, throughout the first 3 months of life, a baby’s responses will become less automatic and more controlled.

While a newborn tends to resemble a bobble-head doll in the first couple of weeks, early tummy time work will help them gain the ability to lift their head, push up with their arms, and stretch out those scrawny little legs. It’s fascinating how quickly they can master these all-important moves and gain muscular strength.

Sometime in the fourth trimester, a baby might also learn to bring their hands together, grab a toy, and track a moving item. While all of these are important developmental advances, in the meantime you’ll be doing a lot of the same things to care for your fourth trimester baby.

Lots of feeding

Newborns eat often. Whether you’re breastfeeding, expressing milk, or formula feeding, you’ll likely be offering the breast or bottle 8 to 12 times per day or every 2 to 3 hours.

A newborn will initially consume about an ounce per feeding, graduating to 2 to 3 ounces by 2 weeks of age and 4 to 6 ounces by 3 months.

Babies go through sudden growth spurts, so you might find your little one sometimes requires more frequent feedings and/or additional ounces. Cluster feeds can have a breastfeeding mom nursing around the clock — so trust your instinct and watch for hunger cues.

If your baby is steadily gaining weight and consistently wetting diapers, you can feel confident they’re getting what they need.

Lots of soothing to sleep

On average a brand new baby will snooze for 14 to 17 hours in a 24-hour span. Unfortunately, this sleep schedule is quite erratic. New babies have shorter sleep cycles and more frequent wakings. Moreover, many babies start off with their days and nights confused, further fueling the exhaustive routine.

Fortunately, around 6 to 8 weeks, babies begin to sleep less during the day and more in the evening hours. While most infants won’t sleep through the night for another few months (many stop needing nighttime feedings around the 4- to 6-month mark), it’s encouraging to know that longer stretches will come as you approach the end of the fourth trimester.

Lots of interpreting crying

A newborn cries as a means of communication. It’s their way of letting you know they’re wet, distressed, tired, uncomfortable, or hungry.

It can be disheartening listening to a baby’s incessant wails; but, rest assured, that periods of fussing are completely normal, and crying usually peaks around 6 weeks of age — so there is a light at the end of the fourth-trimester tunnel.

If a healthy baby cries for 3 or more hours a day for 3 weeks, they may be suffering from colic. While many people believe colic may be connected to tummy troubles, the underlying causes are actually unknown.

Holding and comforting your newborn is key during these ornery hours, but it might not completely quell the crying. It can be trying while it lasts, but colic is temporary and typically ends in tandem with the fourth trimester.

Babies seem to have it made, but life on the outside is harder than it looks, and your wee one may need constant comforting and care during these first weeks.

The good news: You can’t spoil a newborn. Holding them for extended periods of time will not make them dependent, so feel free to snuggle to your heart’s content and your baby’s satisfaction. They’ll thrive with your close attention and affection.

There are some additional tactics you can try:

The 5 S’s

The stark and bright disruptions of a baby’s new normal can be intimidating at first. Part of Karp’s theory of the fourth trimester involves helping your baby to slowly adjust to the change of leaving the womb for the world. Recreate a serene gestation-like scene, and help them feel like they’re back in the womb — safe, secure, and snug.

The 5 S’s, as coined by Karp, will help you find what works best for your baby.

Swaddle

Bundling a baby and restricting the free movement of their arms and legs can have an instantly calming effect on a fussy newborn. It mimics the snugness they experienced in the womb and reduces the startle reflex.

Swaddling may also work well to help your baby sleep. Keep in mind that — like the fourth trimester — swaddling is temporary and should be stopped once your baby starts trying to roll over.

Side or stomach

While a baby should always be placed on their back for sleep, you can soothe a fussing newborn by holding them on their side or by placing them over your shoulder and gently putting pressure on their tummy.

Shush

The perpetual sound of blood rushing around your body helped lull your baby into a state of relaxation while in utero. White noise machines can help create comforting acoustics during naps and bedtime.

Swing

For 9 months, you were your baby’s on-the-go swing. Your perpetual movements would rock your little one to sleep inside the womb.

Whether you cradle your baby and gently sway, sit in a glider, or use a fancy swing, experiment with different motions and speed to find a rhythm that soothes your baby.

Suck

Sucking is a reflex and an innately reassuring action, and pacifiers can help a newborn self-soothe. Note that if you are breastfeeding, you might want to wait a few weeks before introducing the binky to avoid potential nipple confusion.

Other tactics

Some newborns respond well to water and are soothed by a warm bath. Others enjoy a gentle massage. Wearing a baby in a sling or carrier can also be very effective; they free your arms but give your sweetie the physical closeness they crave.

Remember that a newborn can easily become overstimulated, so keep things dim and quiet whenever possible.

Becoming a parent is transformative. In a split second, you become responsible for a tiny and helpless human being (no pressure).

The early days of parenthood will be rewarding and stressful — full of exciting firsts and tremendous trials. These challenging 12 weeks will test your patience and exhaust you beyond measure.

It’s a push and pull; you’ll want to relish every moment while eagerly awaiting a more predictable phase.

The emotional and physical toll

It’s normal to feel a range of emotions as a new parent. One moment you’ll be elated, the next you’ll question your ability to raise a child. The fourth trimester is a bumpy ride full of highs and lows.

One of the challenges is feeling on your own. In contrast to the regular doctor visits and checkups you experienced at the end of your pregnancy, after delivery you may not see your own caregiver again for 4 to 6 weeks.

During those first few weeks, many birth parents will experience a fleeting case of the “baby blues.” Postpartum depression, on the other hand, sticks around and can have a completely oppressive presence in a new parent’s life.

If you are feeling helpless, hopeless, or unable to take care of yourself and your baby, seek professional help.

Postpartum Support International (PSI) offers a phone crisis line (800-944-4773) and text support (503-894-9453), as well as referrals to local providers.

Healthline

In the first 6 to 8 weeks, a birth parent is also recovering from the very real trauma of childbirth, be it a vaginal delivery or C-section.

Vaginal soreness from delivery can make just about any level of activity uncomfortable, and bleeding and cramping may continue for weeks. And if you had a C-section, you will need even more downtime as your body recovers from major surgery.

Most birth parents will have their first postpartum checkup 6 weeks after giving birth, but that wait can feel interminable when you are hurting physically or suffering emotionally — so never hesitate to reach out to your doctor.

No two recoveries are completely alike, and you need to listen to your body. It can be hard to strike balance between taking care of yourself and tending to your baby, but a healthy, happy parent is more equipped for the journey of parenthood, so be sure to prioritize your own needs too.

The fourth trimester is what you’ve been waiting for — your baby has arrived and you are officially a parent! Enjoy this fleeting time. It will be frustrating, draining, and so incredibly rewarding.

Your baby might struggle to adjust to life outside the womb in those first 12 weeks, too, but they will find comfort and contentment in your loving arms. You’ve got this.