This Is What You Need to Know About the Fourth Trimester

mother holding newborn baby
Photo by KieferPix/Shutterstock

About 20 years ago I was thrilled to have Random House publish my parent guide, The Happiest Baby on the Block. In it, I shared my belief that one of the most important things for new parents to understand was a pretty unknown idea called the fourth trimester. Today, that obscure name has so thoroughly penetrated the minds of parents and doctors that the celebrated Merriam-Webster dictionary has included it in their newest edition …and I could not be prouder.

What is the fourth trimester?

The fourth trimester is the first three to four months of your baby’s life after birth. The word “trimester” implies that the baby is still a fetus…and that’s on purpose! Your newborn’s brain and nervous system are not fully developed at birth, making them more fetus-like than baby-like.

Where did the term “fourth trimester” come from?

I first heard the term in the late 1970s when I was working on the child abuse team at UCLA. At that time, I saw so many babies come in who were severely injured—even killed—for the simple offense of crying.

At the time, I was deeply curious about what medical doctors had not been able to figure out—the cause of colic (severe persistent crying seen among babies 1-4 months of age). I began to read everything I could about this centuries-old medical puzzle.

While researching, I learned that in some cultures, colic is virtually nonexistent. For instance, babies of Botswana bushmen rarely—if ever—suffer from colic. Those moms hold their infants almost 24 hours a day, constantly feeding, rocking, and jiggling them. In essence, they mimic the womb experience for months. I also realized that, thanks to evolutionary changes, human babies are born about three months before they’re technically “ready.” In fact, they had to be born three months early because a baby’s developing brain and skull have gotten so big by the end of the third trimester that they can barely safely fit through the birth canal.

That’s when it really hit home: Some babies cry so much because they’re born three months too early! From a baby’s POV, the world is both too crazy busy…and much too boring compared to the continual soothing rhythms of the womb. So as caregivers, we need to fill the gap of that missing fourth trimester.

When did the fourth trimester” become well-known?

In 2002 I used The Happiest Baby on the Block to share what I’d learned from my research and in my practice as a pediatrician. I’d discovered that parents could best create that womb-like fourth trimester experience for their babies with what I call the 5 S’s, which consists of swaddling, shushing, swaying, sucking, and holding babies in a side/stomach position (for soothing, not for sleep). In the book, I explained how the 5 S’s help trigger a baby’s calming reflex—a never-before recognized neurological response that develops deep in a baby’s brain months before they are born—basically nature’s “off switch” for fussing.

Since then, millions and millions of parents across the globe have turned to the 5 S’s to calm crying! So, while I did not coin the term “fourth trimester,” I do think that my book helped to bring the notion of the fourth trimester into the consciousness of parents and physicians alike.

How do you feel about “fourth trimester” being added to the dictionary?

I’m so proud to be part of a worldwide cultural awakening to the importance of this stage of life…not only for babies but for new parents, too. The more people know about the fourth trimester, the better off so many families will be.

The exhaustion that often goes hand-in-hand with the fourth trimester isn’t just a nuisance…it can be dangerous, triggering mental health struggles, unsafe sleep practices, accidents, marital stress, and so much more. I can’t help but think back to my time with the child abuse team at UCLA and the parents who hurt their babies. It’s very easy to think that these were mean, terrible mothers and fathers, but for the most part, they were simply sleep-deprived and stressed-out young parents who cracked under the strain of their babies’ unstoppable crying. I also think about the parents who, out of desperation, have put their babies to sleep in risky positions (such as on the sofa with mom or dad, on the stomach, or in unsafe sleepers) that have led to the thousands of infant sleep deaths we still see every year. I hope—and I truly believe—that understanding what babies need during those critical first few months has saved lives.

What do parents need to know about their baby’s fourth trimester?

A little perspective can go a long way. Think about what your newborn is like compared to other newborn mammals. Horses, cows, and camels can literally run the day they are born, but little baby humans can barely do anything. They can’t even lift their heads for several weeks! Your baby’s brain and nervous system are works in progress when they’re born. Your newborn is a fetus…just one that’s outside the womb! It’s your job to remember this—and to ease your baby into their new world gradually.

To do that, it’s important to create a womb-like environment during your newborn’s fourth trimester. For instance, swaddling mimics the warm hug inside the womb, white noise stands in for the constant loud whooshing of blood pulsing through the uterine arteries, and swinging replicates the jiggling sensation of being inside Mom’s belly for every step she took. These are three of the 5 S’s…and they’re also the cornerstone of SNOO. SNOO was specially designed to provide the fourth-trimester care that babies need by automatically responding to fussing with womb-like rocking, white noise, and the snug embrace of a safe swaddle.

What do new moms need to know about their fourth trimester?

The fourth trimester is a period of incredible growth, development, upheaval, and adjustment for the whole family. New moms especially need to take time during the fourth trimester to heal and adjust…and we collectively need to help moms through this big transition. For thousands of years, new parents had the help of an entourage of other adults, but today, that village is often tiny to non-existent.

One way our society could recognize and respect the importance of the fourth trimester is by implementing national paid family leave of at least 12 weeks, which is the entirety of the fourth trimester. We already know that policies supporting new families during this critical period lowers maternal stress, reduces postpartum depression, boosts breastfeeding, and more. Another way would be to ensure that new parents get the mental health support they need during this challenging period (tragically one in four new moms suffer from postpartum depression/anxiety).

I think that acknowledging this special period in a family’s life in such a big way is a good first step…and I’m hopeful it will help push us toward more innovative solutions that will allow new parents and their babies to thrive in the fourth trimester…and beyond!

mom and baby sleeping
More from Dr. Harvey Karp | The Recipe for Better Parenting? Just Add Sleep!

harvey karp

Dr. Harvey Karp is a world-renowned pediatrician and child development expert. His celebrated Happiest Baby/Happiest Toddler books and videos have guided millions of parents and are translated into 30 languages. In 2016, Dr. Karp debuted SNOO Smart Sleeper, a new class of responsive infant bed designed to add 1-2 hours to a baby’s sleep, quickly soothe crying and to improve safety by preventing dangerous rolling. SNOO won the National Sleep Foundation Innovation of the Year award as well as 20 other top national and international honors. Medical studies are underway to evaluate SNOO’s potential to reduce postpartum depression, infant sleep death and to improve the care of infants withdrawing from opiates. Dr. Karp is an Assistant Professor of Pediatrics at the USC School of Medicine and a fellow of the AAP. He is an advocate for children’s environmental health and a board member of EWG, whose mission is to protect our nation’s public health and the environment.

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