For 35 years, we have been studying interactions between mothers and babies. We have documented—in objective observations, from personal records, and on videotape—the times at which mothers report their babies to be “difficult.” These difficult periods are usually accompanied by the three C’s: clinginess, crankiness, and crying. We now know that they are the tell-tale signs of a period in which the child makes a major leap forward in his mental development.

Babies all undergo these fussy phases at around the same ages. During the first 14 months of a baby's life, there are eight developmental leaps with their corresponding clingy periods at onset. The clingy periods come at 5, 8, 12, 15, 23, 34, 42, and 51 weeks. The onsets may vary by a week or two, but you can be sure of their occurrence.

All babies experience clingy periods when big changes in their development occur. Usually calm, easygoing babies will react to these changes just as much as difficult, temperamental babies do. But not surprisingly, temperamental babies will have more difficulty in dealing with them than their calmer counterparts. Mothers of “difficult” babies will also have a harder time as their babies already require more attention and will demand even more when they have to cope with these big changes. These babies will have the greatest need for mommy, the most conflict with their mothers, and the largest appetite for learning.