Obstructive sleep apnea and perinatal outcomes in a Japanese pregnancy cohort
Lead. Obstructive sleep apnea may be more common among pregnant women with lower body mass index than previously assumed, according to a new prospective study of a Japanese cohort. The findings suggest that screening thresholds developed largely in Western populations may miss cases in leaner Asian groups.
What they found. Researchers led by Manaka Moro and Jun Hosomichi at the Institute of Science Tokyo enrolled 185 pregnant women at or beyond 28 weeks of gestation at Toranomon Hospital in Tokyo. Each participant underwent overnight sleep evaluation using a WatchPAT device, a validated home sleep apnea test. The team defined obstructive sleep apnea as an apnea-hypopnea index of 5 or higher.
Overall, 29 of the 185 women (15.7 percent) met the threshold for OSA. The vast majority of cases were mild: 23 women had mild OSA, 5 had moderate disease, and only 1 had severe OSA. Women in the OSA group had higher pre-pregnancy weight and body mass index compared with those without OSA. However, the OSA group showed a lower rate of BMI change during pregnancy, a pattern that may reflect differing metabolic or physiological adaptations.
Despite these differences in maternal characteristics, the study found no significant differences in neonatal outcomes between the two groups. Birth weight, birth length, and gestational age at delivery were comparable for women with and without OSA.
Why it matters. The central finding of this study is not simply that OSA occurs during pregnancy, but that it does so at unexpectedly low BMI levels in a Japanese population. Previous research on pregnancy-related OSA has come largely from Western cohorts, where higher BMI is a dominant risk factor. The Japanese women in this study had a mean pre-pregnancy BMI well below the typical obesity cutoff, yet the OSA prevalence of 15.7 percent aligns with estimates reported in heavier Western populations.
This pattern suggests that the relationship between body weight and sleep-disordered breathing may differ across ethnic groups. Craniofacial anatomy, airway collapsibility, and fat distribution all vary among populations and may contribute to elevated OSA risk at lower BMI in Asian individuals. For clinicians caring for pregnant women of East Asian descent, reliance on BMI-based risk stratification alone may lead to underdiagnosis of OSA.
The study was funded by JSPS KAKENHI grants and used prospective enrollment with objective sleep testing, which strengthens confidence in the prevalence estimate.
Limits. The study has several important limitations. The sample size of 185 women is modest, and the single-center design at a Tokyo hospital limits generalizability. Most of the identified OSA cases were mild, so the study was not powered to detect differences in rare adverse perinatal outcomes. The WatchPAT device, while validated, estimates AHI based on peripheral arterial tone rather than direct airflow measurement, which may introduce classification differences. The authors also note that the study did not assess symptoms such as snoring or daytime sleepiness, which complicates the clinical picture of which women might benefit from screening.
Bottom line. OSA was present in roughly one in six late-pregnancy Japanese women in this cohort, predominantly mild, and was not associated with measurable differences in neonatal outcomes. The more notable finding is that this prevalence occurred at BMI levels where OSA is often not suspected in clinical practice, reinforcing the need for population-specific screening approaches in pregnancy.