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CHARLOTTE, N.C. -- More time spent supine during sleep was correlated with worse maternal breathing during pregnancy, a finding that may explain the previously described excess risk of stillbirth in people who favor this position.

In obese and overweight pregnant women with suspected sleep disordered breathing (SDB), level III sleep apnea monitoring revealed significant worsening of both the oxygen desaturation index (ODI) and the apnea-hypopnea index (AHI) with more time spent in supine sleep during the first trimester of pregnancy.

By the third trimester, there was still a significant correlation between ODI and maternal sleeping position, but not AHI, said Juliana Katz, BA, a recent graduate of Brown University in Providence, Rhode Island, who presented her 126-person study during the SLEEP meeting hosted jointly by the American Academy of Sleep Medicine and the Sleep Research Society.

Supine sleep is associated with unfavorable airway geometry and reduced lung volumes, she explained. With airway dilator muscles unable to compensate, this may lead to upper airway collapse.

Some data suggest a link between supine sleep position during pregnancy and stillbirths. However, this association has not been consistent across studies, which are limited by retrospective design and recall bias, as well as subjective reporting of sleep position.

For this study, Katz and colleagues did not use subjective reporting and instead opted for objective measurement, using Noxturnal T3 devices to track supine, right lateral, left lateral, prone, and upright positions over the course of one night's sleep.

The number of position changes per night stayed stable, around 14, from the first to the third trimester. There was a significant correlation between sleep-onset position and predominant sleep position in both the first and third trimesters, study authors found.

However, supine going-to-bed position predicted predominant supine sleep in only 47% of women. Moreover, from early to late pregnancy, there was a reduction in supine sleep (from 51.6% in the first trimester to 30.2% in the third trimester) and an increase in left lateral sleep (24.6% to 37.3%).

This may throw into question the validity of using questionnaires for sleep position research.

Even so, Katz cautioned that the devices used in the study may underestimate breathing events. Indeed, even though study participants had been recruited because of suspected SDB, fewer than 40% actually met criteria for obstructive sleep apnea. ODI stayed in the 5-12 range and AHI was generally under 10 over the course of the study.

Session moderator Dennis Hwang, MD, of Kaiser Permanente Southern California's Fontana Medical Center, said he did not share the same concern of underestimation by the Noxturnal devices, noting from personal experience that they produce higher AHIs than polysomnography.

The current study was a secondary analysis of a sleep apnea monitoring study in two cohorts of high-risk pregnant women. Participants were free to use any positional devices during sleep, and there was no education provided to them on how to sleep.

Average age was 30.5 years and average BMI was 34. The most represented races were white (55.6%), Hispanic (27.0%), and Black (7.1%).

Katz acknowledged that going-to-bed position may not be equivalent to sleep-onset position, and that there was no accounting for sleep changes that may have occurred from one night to another in her study.

"More prospective studies are needed to evaluate the potential for sleep position changes over time as a potentially modifiable risk factor for maternal and neonatal health outcomes," the authors noted.

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