The institute's Laura Lindberg and colleagues said 50 percent of the second pregnancies occurred within 18-59 months of the first and 16 percent occurred at 60 or more months.
Short birth spacing, which was measured as 18 months or less, was strongly linked to unintended pregnancies and involving those ages 15-19 years at the time of conception, the study said.
Previous research showed short spacing between pregnancies can lead to harmful outcomes for mothers, such as pre-eclampsia in women and for newborns, being born preterm or with low birth weight.
"Pregnancy intervals of more than 18 months are considered optimal birth spacing, as recommended by the Centers for Disease Control and Prevention and others," Lindberg said in a statement. "Helping women plan and space their pregnancies through greater contraceptive access can lead to better outcomes for both mother and infant."
The study analyzed data from the National Survey of Family Growth on second or higher-order births within five years of the interview. The study measured spacing between pregnancies as the time between when a woman conceived her most recent successful pregnancy and when she delivered the one immediately before.
However, women who start having children after age 30 are significantly more likely to quickly have another child on purpose; 3-of-4 shortly spaced pregnancies among women who began childbearing after 30 were intended, the researchers found.
"For older women in particular, short birth spacing seems to be part of an intended family building strategy," Lindberg said.
In these cases, the study notes, women must weigh the health benefits of longer spacing between pregnancies against the health risks and decreased fertility linked to older women giving birth.