Why we count your baby's age in weeks

Ask a new parent how old their baby is and you'll get weeks, usually until somewhere near the first birthday. Pediatricians, midwives, and the people who write growth charts answer the same way. Most baby apps round you to a month anyway.

We went with weeks. Here's why, and what it changes about the advice you get.

Months are too big for the first year

Take two babies who are both "two months old." One is six weeks, the other is ten.

• At six weeks, crying is at its peak. The AAP describes daily fussing that "usually peaks at about 3 hours a day by 6 weeks old," then falls to one or two hours by three to four months.
• At ten weeks, the social smile is in, the head is steadier, and for most babies the worst of the evening crying is on its way out.
• At six weeks, feeding is still finding a rhythm. By ten weeks, many babies have stretched the gaps and some are doing a longer stretch at night. The Henderson study of sleep consolidation found the fastest change in self-regulated sleep between one and four months, which is the exact span a single "month 2" label covers.

Give both families the same two-month advice and you're early for one and late for the other.

Clinicians already count this way

• Corrected age. For babies born early, the AAP tells parents to start with "your baby's actual age in weeks (number of weeks since the date of birth)" and subtract the weeks of prematurity. The whole method runs in weeks because months are too coarse to correct.
• Growth charts. The World Health Organization publishes its weight-for-age standards for the earliest period as separate tables from "Birth to 13 weeks," in weekly steps, before switching to the longer-range charts.
• Checkups. In the UK, the NHS schedules a new-baby review "within 10 to 14 days of the birth," a heel-prick test at 5 to 8 days, and a full physical examination at 6 to 8 weeks. In the US, the AAP's schedule starts with a visit at 3 to 5 days, then one month, two months, four months, six months. The early visits are days and weeks apart because that's how fast things change.
• Milestones. The CDC's checklists are organized by month, but each one is a floor: a milestone is something "most children (75% or more) can do by a certain age." When the CDC and AAP revised those checklists in 2022, they moved from the 50th percentile to that 75 percent threshold so a missed milestone would actually mean something. A floor at two months tells you what to bring up at a checkup. It doesn't tell you what to do on a Wednesday at seven weeks.

What it changes about advice

Timing gets tighter. Advice about the umbilical stump belongs in weeks zero to four and nowhere else. A heads-up about the six-week growth spurt is useful at week five and noise at week nine. The reminder to stop swaddling matters from the moment a baby starts pushing to one side, which the AAP says can be as early as two months, and matters less once rolling is old news. Every tip in our library has a start week and an end week. A few span the whole two years; most don't.

Expectations get looser. Weeks also make it easier to say "babies vary." A month-based app that says "your 4-month-old should be rolling" has turned a range into a deadline without quite meaning to. A week-based range, say rolling somewhere between weeks 14 and 28, shows the spread. The windows we use to time tips are wide on purpose, drawn from mainstream AAP, CDC and NHS summaries, and none of them is a verdict on your baby. They decide which sourced tip you see a little earlier. That's all they do.

Preemies, and after the first birthday

If your baby came early, use corrected age for developmental expectations, as the AAP advises, for roughly the first two years. In Coffee & Cradle you enter whatever age you and your pediatrician actually use; if that's a corrected age, enter the corrected age. The app doesn't know or ask for a birth date, so it can't second-guess you.

After the first birthday, weeks get silly out loud ("she's sixty-three weeks") and most families switch to months. We keep counting in weeks under the hood all the way to 104, partly because the arithmetic is cleaner and partly because the windows for things like the twelve-month sleep wobble and first words are still narrower than a month. You don't have to update anything; the age moves on its own.

What weeks don't do

They don't make anyone's baby early or late, and they don't replace a pediatrician who knows your child. A general tip is still a general tip. If something about your baby's development worries you, the right move is the same at week six and week sixty: say it out loud at the next checkup, or call sooner. Pediatricians would much rather hear a question that turns out to be nothing.

The clearest case for timing is the 4-month sleep regression, where a heads-up two weeks early changes the whole experience. The first twelve weeks, mapped the same way, are in the fourth trimester.

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Coffee & Cradle asks one thing about your baby, their age in weeks, and picks six short, sourced tips a day around it. The first week is free, and the age keeps itself current.