The World Health Organization endorses a two-year period between birth and a woman's next conception.
Still, one third of all repeat pregnancies in the U.S. occur within 18 months of the previous child's birth. And a growing body of evidence shows this close timing increases the risk a baby will be born early or at a low birth weight.
The time between pregnancies "cannot be explained only by the mother's preferences," Heike Thiel de Bocanegra said.
She and her colleagues from the University of California, San Francisco investigated the link between access to birth control or family planning services and pregnancy spacing.
In the current study of 117,644 California women who'd had at least two children, 64 percent waited 18 months or more between pregnancies and the rest did not.
All women included in the study filed claims through the state's Medicaid program for the poor, called Medi-Cal, or through health providers offering state-funded family planning services.
The researchers matched data on claims for contraceptives to California's birth registry.
"We assumed that access to contraception . . . would improve birth spacing," Dr. Anitra Beasley wrote in an email to Reuters Health.
"This study actually examines this assumption," she said.
Beasley, who studies family planning at Baylor College of Medicine in Houston, was not part of the current research.
Women who used long-acting reversible contraception, including IUDs or implants, were four times more likely to wait at least 18 months to conceive again, compared to those who only used "barrier" contraceptives like condoms or spermicide.
More than half of women started using birth control pills, the ring or the patch after giving birth. They were twice as likely to wait at least 18 months between pregnancies as condom users.
Those relationships stood firm even when the researchers looked at possible influences like the mother's race, education, age and whether she was born in the U.S., according to the report published in the American Journal of Obstetrics and Gynecology.
Image: Birth control words, via Shutterstock
Find out when you're most likely to conceive with our Fertility Maximizer tool.