Is The Wonder Weeks scientifically proven?

Partly. Fussy phases in the first year are real and were described in a 1992 study. But the strict week-by-week 'leap' timing did not hold up when independent researchers tested it. Fussy phases are real; the precise calendar is not proven.

What is the difference between Peanutbean and The Wonder Weeks?

Peanutbean uses broad development windows instead of fixed leap weeks, cites every source, pairs each window with official CDC milestone lists, uses adjusted age for premature babies, and the core forecast is free with no ads and no account.

An honest comparison

Peanutbean vs The Wonder Weeks

Fair is fair: The Wonder Weeks got something important right. Babies do go through fussier stretches, and telling exhausted parents "this is normal and temporary" was a genuine service. Where we part ways is the calendar — and what happens when the science gets checked.

The short answer first

The Wonder Weeks says babies make 10 mental leaps at fixed weeks. The idea comes from a real 1992 study, but when independent researchers tested the strict timing, it did not replicate: babies had fussy stretches, just not on the book's schedule. Peanutbean builds on what did hold up — fussy phases exist, development runs on the due-date clock — and drops what didn't: the precise calendar. The full fact-check, with every source, is in The Wonder Weeks, fact-checked.

Side by side

The Wonder WeeksPeanutbean

The calendar 10 "leaps" at fixed weeks 5 broad windows — "windows, not deadlines"

Sources Based on the authors' own 1992 research Every claim linked to CDC, AAP, NHS or the study itself — including the studies that contradict us

Milestones The book's own descriptions Official CDC milestone checklists (2022 revision) per window

Premature babies Counts from due date (correctly), same fixed weeks Adjusted age built into the forecast, plus a dedicated preemie hub

Tone Framed around not missing a leap No alarmism: fussy weeks are normal, not appointments with misery

Price Paid app + book Forecast and all guides free, no ads, no account; printable extras from $7

What we tell you that an app selling leaps can't

That the schedule is uncertain. Our forecast gives windows precisely because the evidence supports windows, not dates. If week 30 is calm and week 33 is chaos, your baby is not "late for a leap" — the calendar was never that precise for anyone. When a product's business model is the calendar, saying this out loud gets expensive. Ours isn't, so we can.

See your baby's windows — with the sources

Two dates, twenty seconds. Free, no signup, adjusted age included, and every claim cited.

Try the forecast

We link to the replication studies on every relevant page — check them yourself. That is the whole point.

Peanutbean provides general information for educational purposes only. It is not medical advice. Every baby develops at their own pace. Always talk to your pediatrician about your child's health.