Strengths and limitations
This study was a prospective cohort study with a large sample size.
Sleep information was collected before birth outcomes, so the results
were credible because of small recall bias. In this study, the PSQI with
high reliability and validity was used to assess the effects of sleep at
different stages of pregnancy and sleep changes on birth outcomes,
adding to the relevant research field. The study has some limitations.
First, although this is a prospective cohort study, the sleep
information of pregnant women over the past week was obtained from
subjective reports, which may have had recall bias. Some studies have
shown that pregnant women actually sleep about 30 minutes less than they
subjectively report, so sleep duration during pregnancy may be
overestimated. However, it was found that subjectively reported sleep
data had a more significant effect on adverse birth outcomes than was
assessed by objective methods41. When conditions
permit, future studies can combine subjective reports with sleep
assessed by more reliable devices such as polysomnography and wrist
motion detectors. Secondly, although we adjusted socio-demographic
characteristics, living habits, and health status, other residual
confounding may still exist, such as fatigue, restless leg syndrome,
sleep apnea, etc., which can be considered for inclusion in future
studies. Moreover, the subjects included in this study only included
pregnant women who went to Shuangliu Maternal and Child Health Hospital
in Chengdu, China, and could not be extended to people in other areas,
which could be further discussed by conducting multi-center studies in
the future. This study did not collect sleep information during the
third trimester of pregnancy, so it could not assess the relationship
between sleep during the whole pregnancy and birth outcome. Future
studies can add the third trimester to make the study results more
comprehensive.
Interpretation, the
association between sleep during pregnancy and adverse birth outcomes
This study found that short sleep duration during the second trimester
was associated with PTB. Consistent with recent
meta-study39.
Some studies have found the same
results in different pregnancies. Micheli et al. assessed sleep in the
third trimester (28-32 weeks) of 1091 singleton pregnancies and found
that women who slept less than 5 hours had an increased risk of preterm
delivery39.
Similarly, Li et al. assessed
sleep duration in 1082 healthy women with
single fetal pregnancies at 8-12,
24-28, and 32-36 weeks of gestation and found that participants with
short sleep duration (≤7 h) at 32-36 weeks were more likely to report
PTB23. However,
other studies have reported different results.
Previous case-control studies by
Guendelman et al. also found no link between short sleep duration and
PTB40. Two other large-sample prospective cohort
studies assessing the relationship between sleep duration and poor birth
outcomes in late pregnancy in Chinese women and throughout pregnancy in
Japanese women also found no association between short sleep duration
and the risk of
PTB20,24.
Different definitions of sleep duration, gestational age of concern,
corrected covariates, and sample size may explain this controversial
result. The mechanisms underlying
the current association between short sleep duration and PTB are not
clear, and some mechanisms may explain the association between lack of
sleep and PTB. One possibility is the effect of excessive inflammatory
reaction. Sleep deprivation will promote the increase of inflammatory
cytokines such as interleukin-6 (IL-6) and IL-8, thereby stimulating the
production of prostaglandins in pregnancy tissue, leading to cervical
maturation and uterine contraction23,41.
Studies have reported that longer sleep duration (>9 hours
or >10 hours) is significantly associated with impaired
glucose tolerance, coronary heart disease, cardiovascular events,
stroke, and mortality30,42. A study by Yang et al.
reported an increased incidence of PTB in pregnant women who slept
longer43. But, Kajeepeta et al. showed that women who
reported long sleep duration and fatigue in the first 6 months of
pregnancy had an increased risk of
PTB, while women who reported long
sleep duration (≥ 9 hours) and no fatigue had no statistically
significant risk of PTB, and fatigue may be a new risk factor for
PTB43. Notably, this study found that longer sleep
duration during the second trimester was associated with a lower risk of
PTB, and differences in study design and definition of long sleep
duration may lead to conflicting findings. We did not find a mechanism
to explain the protective effect of longer sleep duration on pregnant
women. It may be that longer sleep duration counteracts the effects of
fatigue. In conclusion, the results of this study need further
verification. In addition, this study did not find an association
between sleep duration in early pregnancy and PTB, which is consistent
with the results of Li and Nakahara et al20,23. Data
analysis in this study showed that, compared with the second trimester,
women in the first trimester subjectively reported longer sleep
duration. The difference in sleep duration between the first trimester
and the second trimester may explain the relationship between sleep
duration in different stages of pregnancy and
PTB, and the lack of sleep
information in some subjects may also be one of the reasons.
Analysis of the data in this study found that sleep quality during
pregnancy was not associated with PTB. A study by Du et al. in China
also reported consistent results20,23. Other findings
suggest that poor sleep quality during pregnancy may be a risk factor
for PTB. A small cohort study in the United States found that
PSQI> 5 in early pregnancy was associated with PTB, with a
25% increase in the chance of PTB for every percentage point increase
(OR: 1.25, 95% CI: 1.04, 1.50)44. However, this study
only corrected for obstetric risk, income, and stress. A Chinese cohort
with a sample size of 688 Li et al., after adjusting for prepregnancy
weight and birth weight, found that women with poor sleep quality in the
second and third trimesters had a four-fold (OR: 5.35, 95%CI: 2.10,
13.63) and two-fold (OR: 3.01, 95%CI: 1.26, 7.19) increased risk of
PTB, respectively45.
The
results were inconsistent, possibly due to sample size, pregnancy, and
adjusted confounding factors.
The study also did not find an association between sleep during
pregnancy and LBW and
SGA. Consistent with some research
findings14,18,18,26.
However, two large-sample cohort studies in China during the third
trimester found that poor sleep quality
(PSQI>5) was associated
with LBW (OR: 1.50, 95%CI: 1.08, 2.08) 26 and sleep
duration ≤ 7h was a risk factor for SGA (OR: 2.67, 95%CI: 1.18, 6.54)
2317.
Another prospective study also
reported that women with poor sleep quality or sleep deprivation
(<7 hours vs. >9 hours) at 30 weeks gestation had
lower baby weight47. We speculate that the
controversial findings may be related to the study environment, sleep
classification, and the focus on differences in sleep gestational age.
The relationship between sleep during pregnancy and birth weight of
newborn remains to be further verified.
To the best of our knowledge, there have been relatively few studies on
nap duration and poor birth outcomes, focusing on birth weight. A
Chinese birth cohort study recruiting 10,111 women found that women who
reported napping >1 hour had a lower risk of LBW delivery
compared to women who reported no napping (OR: 0.61, 95%CI: 0.44,
0.83)27. In contrast, the present study found that
women who napped for more than 90 minutes had an increased risk of
giving birth small for gestational age, but that association disappeared
after controlling for potential risk factors. Differences in gestational
age and number of outcomes may explain the difference in results. A
similar study in Brazil also examined the relationship and found no
correlation between nap duration during pregnancy and birth weight,
although the sample size was only 176 and the outcome was birth weight
z-score18. This study found no association between
lunchtime sleep during pregnancy and other birth outcomes, and more
evidence on the effect of napping on birth outcomes is needed.
Overall, the available research results are inconsistent, but some of
the findings suggest that poor sleep leads to the possibility of adverse
birth outcome cannot be ignored. Adverse birth outcomes are not only bad
for the short-term physical health of newborns harmful effects and
increased susceptibility to disease in adulthood. Identification of
possible risk factors is helpful for pregnancy preparation, prevention,
screening and early intervention during pregnancy, and will have a
positive impact on the reduction of the incidence of adverse birth
outcomes and good birth and good upbringing.