What is the fourth trimester?

The fourth trimester is a period coined by Dr. Harvey Karp as the first 3 months of a baby’s life after birth. Dr. Karp explains that babies are born too soon! Yes, you read that right. If given the choice, babies would definitely have voted for a few extra months inside the womb. So, the first three months post-birth –the Fourth Trimester – you should think of babies as fetuses outside the womb.

What does your baby want in the fourth trimester?

A fourth trimester (the first 3 months of a baby's life) of cuddling is the birthday present your baby really wants! You may think your peaceful nursery offers your new baby the perfect environment, but from her point of view, your home feels like it's part wild Las Vegas casino...and part dark closet!

I always tell my patients that babies are born too soon. I know what you’re thinking: Are you kidding? Too short?! For many moms, the last month of pregnancy seemed interminable. Heartburn, puffy ankles, stretch marks and peeing every 2 hours can take all the shine off that pregnancy glow.

But while you couldn’t wait to finally hold your baby in your arms, your baby would definitely have voted for a few extra months inside if you had given her the choice.

Remember—your baby’s brain was so big that you had to “evict” her after 9 months, even though she was still smushy, mushy and very immature. As a result, she isn’t quite ready for the big, bad outside world.

So, for the first months, it can help to think of her like a fetus…outside the womb.

In fact, grandmas, nurses and nannies who are gifted baby calmers all have one talent in common: they’re really good at mimicking a baby’s life in the womb.

To be a good womb impersonator, you first need to know: “What was it like in there?” Warm? Sure. Dark? Actually, fetuses see soft red light as the rays of the sun pass through your outer skin and muscle. Quiet and still? No way!

What Can I Expect in the 4th Trimester?

During the fourth trimester, you can expect fussiness and crying from your baby and very likely exhaustion for you. Newborn babies are learning to adjust to life outside the womb where it was warm and squishy! They will push their new vocal cords to the limits with screams, naps might feel few and far between and only happen on you, and you’ll find yourself on an erratic sleep schedule.

How to Soothe a Baby During the 4th Trimester

Before birth, fetuses are lavished with rhythmic sensations: the caress of velvet-soft walls, lots of jiggly motion and loud whooshing from blood pulsing through the uterine arteries (BTW, they don’t hear your heartbeat). To give your little one "fourth trimester" care, you'll want to give her plenty of snug wrapping/swaddling, shushing, swaying motion, holding in the side/stomach position and opportunities to suck. These techniques are what I call the 5 S's. They will make your baby feel back at home and trigger her calming reflex.


Fun fact: Happiest Baby's SNOO Smart Sleeperuses fourth trimester sound and motion to calm crying and boost sleep for both babies and parents. Learn more.

About Dr. Harvey Karp

Dr. Harvey Karp, one of America’s most trusted pediatricians, is the founder of Happiest Baby and the inventor of the groundbreaking SNOO Smart Sleeper. After years of treating patients in Los Angeles, Dr. Karp vaulted to global prominence with the release of the bestselling Happiest Baby on the Block and Happiest Toddler on the Block. His celebrated books and videos have since become standard pediatric practice, translated into more than 20 languages and have helped millions of parents. Dr. Karp’s landmark methods, including the 5 S’s for soothing babies, guide parents to understand and nurture their children and relieve stressful issues, like new-parent exhaustion, infant crying, and toddler tantrums.

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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.