The Wonder Weeks

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The Wonder Weeks is the English translation of the Dutch book Oei, ik groei! (literal translation: Ai, I'm growing!) by former professor Frans Plooij, originally published in 1992. It has been republished several times, with an updated version published in 2017.[1] It describes the theory that the cognitive development of babies occurs in predictable jumps.[2] However, a follow-up study by Plooij's PhD student, Carolina de Weerth, failed to find any evidence of predictable leaps.[3] Despite this, the book continues to be popular, and the publisher has produced a mobile app based on the book.[4]


According to the book, a baby should go through 10 predictable jumps or "leaps" in its cognitive development during the two years, with 8 in just the first year, counted from the due date if the child was premature.[5] These jumps consist of two phases; A phase where the baby is generally unhappy, followed by a period where the baby is generally happy, due to discovering new things with the newly gained cognitive skills. The "leaps" are predicted to occur at 5, 8, 12, 19, 26, 37, 46, 55, 64 and 75 weeks old.[1]


A follow-up study by Plooij's PhD student, Carolina de Weerth, examined the claims of the book. She tested both behaviour and cortisol levels and failed to find any evidence of greater fussiness or higher cortisol levels corresponding to the leaps.[3][2] Frans X Plooij tried to prevent the British Journal of Developmental Psychology from publishing the study.[2] Van Geert, a coworker, described his behaviour as "very indecent." He also described the claims made by the book as contradicting the greater body of research on child development.[6] The controversy over these results that ensued led to Plooij's firing and departure from academia.[2][7][8]


  1. ^ a b Plooij, Frans X.; Rijt, Hetty van de (September 2017). The Wonder Weeks: How to Stimulate Your Baby's Mental Development and Help Him Turn His 10 Predictable, Great, Fussy Phases Into Magical Leaps Forward. Kiddy World Publishing. ISBN 9789491882166.
  2. ^ a b c d Kok, Annemarie (1997-11-19). "'Oei, ik groei!' leidt tot bittere ruzie van wetenschappers". Trouw. Retrieved 14 June 2015.
  3. ^ a b de Weerth, C.; van Geert, P. (1998-03-01). "Emotional instability as an indicator of strictly timed infantile developmental transitions". British Journal of Developmental Psychology. 16 (1): 15–44. doi:10.1111/j.2044-835X.1998.tb00748.x. ISSN 2044-835X.
  4. ^ "The Wonder Weeks App". The Wonder Weeks. Retrieved 2017-08-27.
  5. ^ Plooij, Frans. "Groeisprongen". oeiikgroei. Retrieved 14 June 2015.
  6. ^ "'Oei, ik groei!' leidt tot bittere ruzie van wetenschappers". Trouw (in Dutch). Retrieved 2017-08-27. zeer onfatsoenlijk
  7. ^ Dirks, Bart (January 13, 1997). "Hoogleraar woedend over weerlegging theorie". Algemeen Dagblad (AD). Retrieved 14 June 2015.
  8. ^ Dirks, Bart (1998-01-14). "Positie Plooij onhoudbaar na openlijke kritiek op oud-promovenda; Auteur 'Oei, ik groei' ontslagen als hoogleraar". Volkskrant. Retrieved 14 June 2015.
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