A 4-week-old baby is entering the steepest climb in normal infant fussiness, a period that typically peaks around the second month of life and doesn’t taper off until around five months. At this age, it’s not uncommon for babies to cry for five hours a day or longer. That doesn’t make it easy to live through, but it does mean your baby is most likely behaving exactly as newborns do. Several specific things happening in your baby’s body right now can explain the crying.
The Peak Fussiness Window
Pediatric researchers describe the weeks between birth and five months as the Period of PURPLE Crying, a normal developmental phase where crying increases, resists soothing, and often clusters in the late afternoon and evening. At four weeks, your baby is right at the start of the upswing. The crying can look alarming because it seems to come from nowhere, your baby’s face may turn red, and nothing you try seems to help. This is the hardest part: the fussiness often has no clear cause and no reliable fix.
One reason for this pattern is that your baby doesn’t yet have an internal clock. Babies don’t begin producing melatonin (the hormone that regulates sleep) until around nine weeks of age. A cortisol rhythm, which helps organize wake and rest cycles, doesn’t develop until about eight weeks. Without these signals, your baby’s nervous system has no way to sort day from night or to self-regulate through calm and alert states. That biological immaturity shows up as unpredictable fussiness, especially in the evening hours.
Growth Spurts and Hunger
Babies commonly go through a growth spurt between two and three weeks, and another around six weeks. Your 4-week-old may be finishing one spurt or gearing up for the next. During a spurt, babies express their needs through fussiness and increased hunger. Sleep patterns often shift too. These episodes tend to be short, lasting up to about three days, and then settling back to a more predictable rhythm.
You may also notice cluster feeding, where your baby wants to nurse or take a bottle every hour instead of every two to three hours. Cluster feeding in the evening is normal and doesn’t automatically mean your milk supply is low. However, if your baby is cluster feeding all day long for multiple days in a row, that can signal a latch problem or a genuine dip in milk supply. In those cases, the baby isn’t getting enough at each feeding and stays hungry. If your baby seems unsatisfied after every feed, not just during an evening stretch, a lactation consultant can check the latch and help troubleshoot.
Digestive Discomfort and Straining
Four-week-olds frequently grunt, turn red, and cry while trying to have a bowel movement. This looks painful, but it usually isn’t. The medical term is infant dyschezia, and it’s a coordination problem, not a pain problem. Your baby is still learning how to relax the pelvic floor muscles while simultaneously bearing down with the abdomen. Pediatricians believe these babies cry to build up the abdominal pressure they need to push, not because something hurts.
You can tell the difference between dyschezia and actual constipation by looking at what comes out. If the stool is soft and normal-looking after all that straining, your baby’s digestive system is fine. If the stool is hard, pellet-like, or has blood in it, that points to constipation and is worth bringing up with your pediatrician. Episodes of straining typically last 10 to 30 minutes and resolve on their own as the baby’s muscle coordination matures over the coming weeks.
Overstimulation
A newborn’s nervous system processes the world with very little filtering. By the end of the day, ordinary household activity (voices, lights, being passed between visitors) can overwhelm a 4-week-old. Signs of overstimulation include turning the head away from you, withdrawing from touch, clenching fists, waving arms and legs frantically, and jerky or disorganized movements. These cues often appear before the full meltdown starts.
If you notice these signals, moving to a dim, quiet room and holding your baby skin-to-skin or swaddled against your chest can help. The goal is to reduce sensory input, not to add more of it. Bouncing, shushing, and rocking all at once can sometimes make overstimulation worse rather than better. Try one soothing strategy at a time.
When Fussiness Becomes Colic
Colic is essentially the extreme end of normal infant fussiness. The classic diagnostic guideline, known as the Rule of Threes, defines colic as crying that lasts at least three hours a day, happens three or more days a week, and continues for more than three weeks. If your baby’s crying fits that pattern, you’re dealing with colic. It’s not a disease or a sign of something wrong with your baby’s health. It’s a label for the most intense version of the same developmental fussiness all babies go through, and it follows the same timeline, peaking around six to eight weeks and resolving by four to five months.
The distinction matters mainly because it helps set expectations. Colic doesn’t require treatment in most cases. It requires endurance. If you’re hitting the wall with a baby who won’t stop crying, putting your baby down in a safe sleep space (flat on their back in a crib with no loose bedding) and stepping away for a few minutes to collect yourself is not neglect. It’s a safety strategy.
Signs That Something Else Is Going On
Most fussiness at four weeks is normal, but a few signals warrant prompt attention. A rectal temperature above 100.4°F (38°C) in a baby younger than two months is an emergency and requires an immediate trip to the ER. Fever at this age can indicate a serious infection that needs rapid evaluation.
Other signs to watch for: refusing to eat for multiple feedings in a row, fewer than six wet diapers a day after the first week (which can signal dehydration), a swollen or tense abdomen, blood in the stool, or a cry that sounds distinctly different from your baby’s usual fussing, particularly a high-pitched or weak cry. Projectile vomiting after feeds (not just spitting up, but forceful vomiting that shoots across a surface) also deserves a call to your pediatrician, as it can point to a narrowing of the stomach outlet that sometimes develops around this age.
If your baby is gaining weight, producing plenty of wet and dirty diapers, and has periods of calm alertness between crying spells, the fussiness is almost certainly the normal, temporary kind. It will get worse before it gets better, peaking in the next few weeks, but it does get better.