An analysis of births in California, the largest state in the United States, indicates that pregnant women who are diagnosed with sleep disorders appear to be at risk of delivering their babies before reaching full term.

Published this week in Obstetrics & Gynecology, the study found the prevalence of preterm birth, defined as delivery before 37 weeks' gestation, was 14.6 percent for women diagnosed with a sleep disorder during pregnancy, compared to 10.9 percent for women who were not.

In addition, according to researchers at the University of California, San Francisco, including Jennifer Felder, a postdoctoral fellow in the UCSF Department of Psychiatry and the lead author of the study, the odds of early preterm birth, namely before 34 weeks, was more than double for women with sleep apnea and nearly double for women with insomnia.

To separate the effects of poor sleep from other factors that also contribute to a risk of preterm birth, the researchers used a case-control design: 2,265 women with a sleep disorder diagnosis during pregnancy were matched to controls who did not have such a diagnosis, but had identical maternal risk factors for preterm birth, such as a previous preterm birth, smoking during pregnancy, or hypertension.

In contrast to the normal sleep changes that typically occur during pregnancy, the study focused on major disruptions likely to result in impairment.

Because of the large sample size, the authors claimed they were able to examine the relationship between different types of sleep disorders and subtypes of preterm birth.

The researchers were surprised by how few women in the dataset - well below 1 percent - had a sleep disorder diagnosis, and suspected that only the most serious cases were identified. They noted that the true prevalence of these disorders is unknown because pregnant women often go undiagnosed.

Treating sleep disorders during pregnancy could be a way to reduce the preterm rate, which is about 10 percent in the United States, more than most other highly developed countries. The researchers suggest that cognitive behavioral therapy has been shown to be effective in the general population and does not require taking medications that many pregnant women prefer to avoid.