Somewhere around four months, the AAP notes, most babies begin rolling from front to back, then back to front. The neck and trunk muscles have caught up with the head, and a new skill appears: propping up on the elbows during tummy time, chest lifted, forearms planted. The social laugh arrives too, a real belly laugh in answer to yours, not the early reflex grin. Vision has sharpened to roughly 20/100, and depth perception is switching on, which is why a dangling toy suddenly gets grabbed.

Around fourteen to nineteen weeks, the leap calendar linked to The Wonder Weeks describes leap four, the world of events. Your baby starts to see the world as smooth, flowing sequences rather than still pictures: a ball rolling, a spoon moving to a mouth, you walking across the room. This is why he can now anticipate. He turns toward the door before you appear, and his hand opens before the rattle arrives. The AAP calls the same change a burst in hand-eye coordination. Expect three fussy weeks as the new wiring settles.

The WHO recommends exclusive breastfeeding or formula until about six months, and the AAP agrees that solids should not start before four months. Readiness is a set of signs, not a date on a calendar. A baby is ready when he holds his head steady, sits propped with little wobble, opens his mouth for a spoon, and has lost the tongue-thrust reflex that pushes food back out. Most babies show all four somewhere between five and six months. Starting too early is linked to more digestive upset, and it does not mean better sleep.

The four-month sleep regression is the first real one, and it is developmental, not a discipline problem. At this age sleep matures: the old newborn pattern of light and deep sleep gives way to adult-like cycles that end in brief awakenings every one to two hours. A baby who used to sleep through now surfaces at each cycle and has not yet learned to resettle. The AAP notes this is a normal, temporary stage. The Wonder Weeks calendar places it alongside leap four. Most families see it ease within two to six weeks.

Pediatricians describe a pain cry as sudden, high-pitched, and hard to soothe, different from hunger or tiredness. Around four months, two common causes show up. The first is teething: gums swell and ache days before a tooth breaks through, though a true fever is not teething. The second is gas or a hair tourniquet, a strand of hair or thread wrapped so tightly around a toe or finger that it cuts circulation. The AAP advises checking toes, fingers, and the penis for a tight loop. A cry that stops as suddenly as it started can also signal a startle.

At four months a fever is a real signal. The AAP defines a fever as 38°C (100.4°F) or higher, measured rectally, and says any fever in a baby under three months needs a same-day call. By four to six months the rule relaxes slightly, but you should still call for any fever above 38°C, for a fever lasting more than twenty-four hours, or for a fever with a rash, poor feeding, or unusual sleepiness. Other red flags: a bulging soft spot, a stiff neck, breathing that pulls the ribs in, or fewer than four wet nappies in a day.

By four months the routines that carry the next year are worth locking in. Confirm the cot is still bare and the sleep bag is the right weight for the season. Ask the pediatrician about the next round of vaccines and about starting vitamin D drops if the baby is breastfed. If you have not already, take over one full night or one full morning so your partner gets an unbroken stretch. Baby-proofing has a first step now: clear small objects, cords, and hot drinks from the floor where he will soon roll.