both of ours were born with straight hair. our son's hair curled up at 2 months. our daughter's hair curled up at about 3 months.
now, her hair is literally to her butt and is wavy because of the length and weight. our son wears his hair about a half-inch to an inch long all over. his hair waves to his scalp when it's short and curls up into a variety of sizes all over his head as it gets longer.