Between week two and week four a healthy baby gains roughly 150 to 200 grams a week, or about 20 to 30 grams a day, and is usually back above birth weight by day fourteen. The American Academy of Pediatrics (AAP) tracks this at the two-week and one-month visits. Head control is starting to emerge: in tummy time he lifts his head briefly and turns it side to side, but the neck still wobbles and needs support. Regaining birth weight is one of the clearest signs that feeding is working.

Around week five, most babies enter the first developmental leap on the calendar associated with The Wonder Weeks, called the world of sensations. For about a week he becomes fussy, clings more, and feeds restlessly, then settles into a new skill: he suddenly notices that light, sound, and touch are separate things. The Dutch researchers behind the leap calendar describe this as a fussy window followed by a brighter alert period. It is not a growth spurt and not an illness, it is his brain sorting raw input into patterns.

Growth spurts cluster around weeks two, three, six, and twelve, and during them a baby wants to feed far more often, sometimes every hour for a few hours, a pattern called cluster feeding. The AAP explains this is the baby telling the breast to make more milk, not a sign that supply has failed. Cluster feeds often land in the evening. They last a day or two, then the frequent feeding and the fussiness drop away as quickly as they came.

Newborns do not yet carry a working body clock, and the light-sensitive hormone melatonin takes until about three months to settle into a rhythm. The AAP advises that day and night are learned cues, not instincts. A baby who sleeps long stretches in the afternoon and parties at two in the morning is confused, not spoiled. Morning light on the face and a dim, boring night help the internal clock reset. Most babies show a clearer day-night pattern between six and twelve weeks.

By the third week a father can often tell hunger from tiredness by sound. A hunger cry is short, low, and rhythmic, and it stops when he finds the breast or bottle. An overtired cry is fussy and breaks into a whine. Pain is sudden, high-pitched, and holds. Pediatricians say you cannot reliably decode every cry, and you do not need to. Look at the context first: time since the last feed, the nappy, and how long he has been awake. The Dunstan Baby Language study found some sound patterns repeat, but it is a guide, not a rule.

Call the pediatrician for a newborn under three months with a rectal temperature of 38 Celsius or higher, which is a fever at this age. Other red flags: refusing most feeds, fewer than six wet nappies a day, no dirty nappy for two days, yellow skin or eyes getting darker, a high-pitched continuous cry, unusual floppiness, or trouble breathing with grunting or ribs pulling in. The AAP treats fever in a young infant as a same-day call, not a wait-and-see. A single vomit after a feed is normal; forceful repeated vomiting is not.

Weeks two to four are the build-your-rhythm stretch. Confirm the two-week weight check happened and the growth line is rising. Learn the difference between spit-up and vomit. Set up the night shift in writing so nobody negotiates at 3 a.m. Know your pediatrician's after-hours number and your area's emergency number. If the family plans a gathering or a naming ceremony, decide now who holds the baby and when he leaves the room. Keep the car seat installed and checked.