October 24, 2019
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Sleeping on back in third trimester linked to reduced birth weight

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Women who fall asleep in a supine position during the third trimester of pregnancy are more likely to have infants with reduced birth weight, according to findings published in JAMA Network Open.

“Women who go to sleep supine in the third trimester are, on average, 3 times more likely to have a baby who is small for gestational age, with an average decrease in birth weight of 144 g — an equivalent magnitude as smoking 10 cigarettes per day in pregnancy,” Ngaire H. Anderson, MBChB, PhD, FRANZCOG, senior lecturer in the department of obstetrics and gynecology at the University of Auckland, New Zealand, told Healio Primary Care.

Researchers explained that while it is common to switch positions while sleeping, pregnant women stay in the position they fall asleep in longest.

Anderson and colleagues explored the association between sleep position and birth weight by analyzing findings from four case-controlled studies that were conducted in Australia, New Zealand and the United Kingdom.

Black Pregnant Woman 
Women who fall asleep in a supine position during the third trimester of pregnancy are more likely to have infants with reduced birth weight, according to findings published in JAMA Network Open.
Source: Adobe Stock

As there is no international consensus on the most appropriate method to define normal birth weight, researchers evaluated outcomes using two common standards. The first, INTERGROWTH-21st centiles, is a birth weight standard developed from a low-risk birth cohort and adjusted for infant sex and gestation at birth. The second, customized centiles, is based on fetal growth standard and adjusted for maternal height, weight, parity and ethnicity in addition to infant gestation and sex. Lower birth weight infants were categorized into lower centiles.

A total of 1,760 women were evaluated in the study, 3.2% of whom reported that they usually went to sleep on their back in the last week to 4 weeks.

Within the cohort, the adjusted mean birth weight of infants with mothers who fell asleep on their back was 3,410 g, compared with 3,554 g in those who fell asleep in another position (adjusted mean difference = 144 g; 95% CI, 253 to 36). Compared with falling asleep in another position, falling asleep in the supine position was associated with lower mean INTERGROWTH-21st centile (48.5 vs. 58.6; adjusted mean difference = 10.1; 95% CI, 17.1 to 3.1) and lower mean customized centile (40.7 vs. 49.7; adjusted mean difference= 9; 95% CI, 16.6 to 1.4).

Researchers found that falling asleep in the supine position was associated with a nonsignificant increase in the odds having an infant in less than the 50th INTERGROWTH-21st centile. However, they identified the supine sleep position with doubled odds of infant birth weight below the 50th customized centile (adjusted OR = 2.12; 95% CI, 1.2-3.76) compared with those who slept in other positions.

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In addition, there was a 3-fold increase in the odds having an infant small for gestational age among those who slept on their back compared with those who did not based on INTERGROWTH-21st standards, with a nonsignificant increase based on the customized standards.

Anderson noted that a possible explanation for the association between reduced birth weight and mothers who fell asleep on their back in the third trimester of pregnancy is that their uterus presses against major blood vessels and reduces the blood flow to the womb.

“We believe education is key to encouraging women to fall asleep on their side,” Anderson said. “Informing women that side sleeping is best for baby's growth and well-being is a straightforward message and should be a routine part of antenatal education.” – by Erin Michael

Disclosures: Anderson report no relevant financial disclosures. Please see study for all other authors’ relevant financial disclosures.