0 to 6 months

Most babies start making soft vowel sounds like "ooo" and "aah" in their first few months. In a study in four countries, half did so by about 1 month, and a review of earlier studies puts cooing at around 3 months.

Most babies can hold their head steady and upright at around 2 months. In a large study in four countries, about half did so by about 2 months.

Most babies start reaching for toys and holding them on purpose at around 4 months. In a large study in four countries, about half did both by about 4 months.

Most babies start rolling over at around 5 to 6 months. In a study that followed babies in Hong Kong, many rolled from back to tummy first, the opposite of what many books say.

Most babies start passing a toy from one hand to the other at around 5 to 6 months. In a large study in four countries, about half did so by about 5½ months.

Babies start smiling back at the people who care for them in their first weeks. In a four-country study, parents reported half doing so within the first month. Filmed studies found many more babies smiling at their mother at 3 months than at 6 weeks.

Most babies show they know and prefer the people who care for them, by smiling, reaching or studying their faces, at around 3½ to 4 months.

Many babies start reacting when the person caring for them leaves, and settling when they come back, at around 5½ to 6 months.