One-third of U.S. pregnancies occur within 18 months of a previous birth, according to "Short Interpregnancy Intervals in the United States," by Alison Gemmill of the University of California, Berkeley, and Laura Lindberg of the Guttmacher Institute, while 50% occurred within 18–59 months, and 16% occurred at 60 or more months. Short birth spacing, which was measured as 18 months or less, was found to be strongly linked to unintended pregnancies, and being between 15 and 19 years old at the time of conception.
Previous research has shown that short spacing between pregnancies can lead to harmful outcomes for mothers, such as preeclampsia, and for newborns, such as being born preterm or with low birth weight. Additionally, the federal Healthy People 2020 initiative aims to reduce by 10% the number of pregnancies that occur within 18 months of a previous birth. Lindberg explains that preventing this short spacing is thus a public health priority in the United States, and estimates that reducing unintended pregnancies could reduce shortly spaced births from 35% to 23%, a feat that would benefit the health of both the mother and the newborn.
"Pregnancy intervals of more than 18 months are considered optimal birth spacing, as recommended by the Centers for Disease Control and others," said Lindberg. "Helping women plan and space their pregnancies through greater contraceptive access can lead to better outcomes for both mother and infant." The researchers identify long-acting reversible contraceptives, such as IUDs, as particularly well-suited to increasing the space between pregnancies.
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 (interpregnancy intervals) as the time between when a woman conceived her most recent successful pregnancy and when she delivered the one immediately before.
Solely preventing unintended pregnancies, however, will not entirely eliminate short spacing, as the study identified some women for whom this spacing is intentional. This includes women who started childbearing after age 30. Not only were these women significantly more likely to have short spacing between pregnancies than those of other ages, three out of four shortly spaced pregnancies among women who began childbearing after 30 were intended.
"For older women in particular, short birth spacing seems to be part of an intended family building strategy," explains Lindberg.
In these cases, the study notes, women must weigh the health benefits of longer spacing between pregnancies against the health risks and decreased fecundity linked to women giving birth at later ages.
"Short Interpregnancy Intervals in the United States" is currently available online and will appear in a forthcoming issue of Obstetrics & Gynecology.