A woman during pregnancy undergoes different physiological, psychological, social and emotional challenges . It is indicated that stress and anxiety were occurred frequently during pregnancy and postpartum period of pregnancy, which contributes significantly to prematurity, low birth weight  and unplanned cesareans . There are many therapeutic schemes for the discomfort of pregnant women, mainly including pharmaceutical drugs treatments and non-pharmacological treatments. However, pharmaceutical drugs may be harmful for the fetus, including the risks of side effects and substance dependence. To be specific, in the first and late trimester of pregnancy, inappropriate administration of the drugs can lead to malformation of the fetus. Therefore, non-pharmacological treatments have attracted more attention from researchers in recent years. It is widely acknowledged that mindfulness meditation , relaxation therapy , music , tai chi , qigong  and yoga   could relieve the unpleasant symptoms of pregnant women.
Yoga has drawn more and more attention because it has plentiful forms and the essence of yoga is the coming together of mind, body and spirit, as well as the combination of postures, controlled breathing, meditation and relaxation  . Yoga is a popular health practice for a variety of immunological, neuromuscular, psychological, and pain conditions in developed nations  . Yoga has been shown to play crucial roles in ameliorating a variety of physical and psychological health conditions, involving anxiety, depression, metabolic syndrome, cancer, and cardiovascular, musculoskeletal, and pulmonary disorders  . Cumulating studies have been demonstrated that prenatal yoga can improve the quality of sleep , infant and maternal outcomes , immune function , birth weight , maternal comfort during labor , as well as remission of low-back pain , depression, stress , anxiety  pregnancy-induced hypertension , refractory epilepsy , labor duration and anesthesia requirements  . Nevertheless, there are not standard procedures for practicing yoga in pregnancy. In particular, there is an urgent need to determine which form of yoga is suitable for what kinds of pregnant women or different stages of pregnancy.
In the present study, the latest evidences that yoga could improve the physical and psychological symptoms of women during the prenatal or postpartum period of pregnancy were summarized.
2. Methods and Materials
To be included in the present study the articles have to meet the following criteria: 1) Was published in the English language. 2) Was included in the following databases: EBSCO, PubMed, Wiley and Science Direct online. 3) Was published between January 2015 and June 2020. 4) Was included any form of yoga as an intervention. 5) Was conducted during the prenatal or postpartum period of pregnancy. 6) Were full text clinical studies.
With use of the term (yoga or yogic) and the term (pregnancy or pregnant or prenatal or postnatal or postpartum), 343 articles were selected: 101 from EBSCO, 95 from PubMed, 49 from Wiley, 98 from Science Direct. We included randomized controlled trials (RCTs) that specifically stated that the intervention was yoga or yoga-based exercise during pregnancy or the postnatal period. We excluded 329 articles for the following reasons: not during pregnancy or postpartum, review, non-English, not a clinical study, no abstract available, no yoga intervention, no postures and reduplicative. Finally, the remaining articles (N = 14) satisfied the eligibility criteria (As shown in Figure 1).
3.1. Characteristics of the Included Trials
In this study, 14 trials published between 2015 and 2020 were selected for final analysis. Four studies were originated from the United States    , three from Japan   , one each from India , Taiwan , UK , Turkey , Iran  , and Indonesia . As shown in Table 1, the characteristics of the included studies were summarized. Randomized controlled design was used in ten of fourteen studies, single-blind experiment method was used in two studies, pre-test/post-test designs were applied in one trail. Participants included healthy pregnancy and high-risk pregnant women, with their mean ages ranging from 18 to 45 years. Among them, one study was high-risk pregnant woman, two studies were postpartum women, and five of the subjects were primiparous women. Specifically, participants in three studies experienced symptoms of depression and anxiety, and in two studies experienced labor pains during pregnancy. However, majority of studies have been done for healthy pregnant women. Moreover, most of these researches reported measurements which made with scaling method and questionnaire, and three studies reported measurements made with saliva samples    and blood samples . In addition, the sample size is small in most studies, except the sample size of one study over 100 participants. In fact, the sample size directly or indirectly affects the results of the experiment.
Figure 1. The process of literatures screening.
Table 1. The controlled studies of yoga for pregnancy.
Abbreviations: wk = week; hr = hour; WG = Week of Gestation; PHQ-9 = 9-item Patient Health Questionnaire; SCID-I = Structured Clinical Interview for DSM-IV Axis-I Disorders; HDRS = Hamilton Depression Rating Scale; RM-ANOVA = Repeated Measures ANOVA; SPSS = Statistical Product and Service Solutions; EPDS = Edinburgh Postnatal Depression Scale; STAI = State-Trait Anxiety Inventory; IPAQ = International Physical Activity Questionnaire; CSQ-8 = Client Satisfaction Questionnaire-8; QIDS = Quick Inventory of Depressive Symptomatology; EIA = Salivary Enzyme Immunoassay; PAR-Q = Physical Activity Readiness Questionnaire; ANOVA = Analysis of Variance; PANAS = Positive and Negative Affect Schedules; BDHQ = Brief-type Diet History Questionnaire; VAS = Visual Analogue Scale; PSS = Perceived Stress Scale; PIF = Pregnant Information Form; PPHAS = Pregnant Psychosocial Health Assessment Scale; PAI = Prenatal Attachment Inventory; HARS = Hamilton Anxiety Rating Scale; RMDQ = Roland Morris Disability Questionnaire; PSI = Pregnancy Symptom Inventory; HRQOL = Health Related Quality of life; TAU = Treatment as usual; POMS = Profiles of Mood states.
As shown in Table 1, these researchers found that the yoga group was more effective at reducing depression, frequency of labor induction and stress than that in the routine care group (P < 0.01). Although Davis (2015) found no evidence of greater decline in depression or anxiety for either the yoga group or the routine care group, the practitioners favored yoga . Two studies found that the yoga group improved psychological well-being, decreased α-amylase levels and reduced stress when compared with those in the non-yoga groups (P < 0.05)  . Lisa et al. (2015) compared the yoga group with the group focused on mother’s and baby’s health and found that yoga practice was more effective in reducing depression (P < 0.05) . What’s more, control interventions for some studies included a walking group  and an educational support group .
3.2. Yoga Intervention Program
Four different types of yoga were included in these researches: Dru yoga, Hatha yoga, Prenatal yoga and Ashtanga yoga. Dru yoga was origin from Hatha yoga and included classical yoga postures, pranayama, mudra (hand gestures) and positive affirmations, empowering visualizations and sequences, performing in a flowing, gentle style . Ashtanga yoga is the traditional Ashtanga Vinyasa system of yoga, which relieves the symptom of depressed and anxiety  . However, most of these researches used Hatha yoga and prenatal yoga, including yoga postures, breathing, meditation and relaxation. Most of the yoga interventions in the included studies were ranged from 40 to 75 minutes, 1 to 3 times per week. Most of studies provided participants with materials, including cassettes, booklets, and DVDs, which guide them to practice at home 1 to 3 times per week.
Moreover, many trials have been provided the list of postures, which were used in the yoga intervention (see Table 2). The various components of yoga practice process that were included in the reviewed trials, involving postures (Asana), breathing practices (Pranayama), concentration/meditation (Dharana/Dhyana), deep relaxation (Yoga Nidra), mudra (Hand Gestures), Guided imagery and Chanting. Among them, yoga postures were included in all the reviewed researches. Breathing practices were applied in most of the yoga interventions. However, meditation and deep relaxation were excluded in some studies. Only minority of studies have been applied chanting , hand gestures  and guided imagery  . In fact, the complete pregnancy yoga class should include the most important elements such as meditation, breathing, postures and relaxation . In addition, the maternity yoga class was recommended at least 60 minutes at a time, 2 - 3 times a week, for 8 weeks or more for better pregnancy improvement.
3.3. Outcome Measurement
Outcomes measurements in the present study included psychological well-being, pregnancy-related pain, birth outcomes and so on.
3.3.1. Effect of Yoga on Psychological Well-Being
The results showed that yoga intervention is positively correlated with psychological well-being. Studies suggested that yoga can be effective for enhancing psychological well-being   and increasing confidence  by reductions in stress, negative affect (P = 0.011)  , and dysfunctional coping (P < 0.05) in the perinatal and postnatal period . Eight studies used the psychological-related scales to assess the effects of yoga on mood and quality of life during and after perinatal period. For example, Rukiye et al. (2018) showed that the scores of the mean pregnancy psychosocial health assessment scale (P < 0.001) of the yoga group were higher than those of the control group (P < 0.05) . The scores of the mean pregnancy psychosocial health assessment scale can effectively reflect the level of psychological well-being.
In addition, negative psychological effects of pregnancy include depression, stress, anxiety and so on. Many pregnant women experience prenatal or
Table 2. The components of yoga intervention for each study.
postpartum depression, which is accompanied by anxiety and poor health-related quality of life  , affecting the physical and mental health of both mothers and fetus . Five studies reported that alterations have been found in stress and depression symptoms after the yoga intervention. Yoga plays an important role in decreasing stress and depression symptoms     of pregnant or postpartum women (P < 0.001). Furthermore, Chen & Momoko found that yoga intervention group demonstrated lower salivary cortisol (P < 0.001)   and higher immunoglobulin A (P < 0.001) levels than that in control group, because of the positive relationship between stress and cortisol levels .
Momoko & Mako found the salivary alpha-amylase and salivary a-amylase (a biomarker of stress) levels were decreased significantly after yoga intervention (P < 0.5)  . Salivary a-amylase is often used as a non-invasive and alternative biomarker to norepinephrine . Masayo et al. (2017) showed that the yoga intervention could reduce psychological stress and gain gestational weight when compared with that in control group . Momoko et al. (2016) found that the scores for negative dimensions of mood (Trait-Anxiety, Depression, Anger-Hostility, Fatigue and Confusion) were decreased significantly in the yoga group . The scores for negative dimensions of mood can reflect levels of stress, and the scores are negatively correlated with stress. Moreover, Melissa’s study is the first efficacy trial of yoga for postpartum depression, and extends empirical support for yoga with postpartum women . The women in the yoga group showed greater improvement in their mental health than those in the control group .
Maternal anxiety usually occurs during pregnancy and adversely affects maternal and infant health if inappropriate managed. Shinta et al. (2018) showed that there was a significant difference in the anxiety levels before and after the yoga intervention (P = 0.005), as well as the groups between yoga and control. Ellahe et al. (2020) found that anxiety levels was lower in yoga group than that in control group (P = 0.003). Yoga had an effect on reducing anxiety levels among pregnant women at the third trimester .While most studies have shown that yoga to be useful for depression, one study found both of yoga group and control group reduced the severity of symptoms of depression and anxiety, but they found no evidence of greater decline in depression and anxiety either in yoga group or control group .
In a word, the negative psychological effects of depression, stress and anxiety during or after pregnancy will be reduced by yoga intervention.
3.3.2. Effect of Yoga on Pregnancy-Related Pain
Pregnancy-related pain is a serious health problem affecting a large majority of pregnant women . During pregnancy, many women have experienced labor pain and low back pain, especially labor pain is unique and complex to each mother . Yoga has been an effective non-pharmacological method to relieve labor pain (P = 0.001)  and back pain (P < 0.05)  . Fereshteh et al. (2017) found that yoga can reduce pain of labor, they also showed that prenatal yoga can reduce the percentage of cesarean section (P = 0.002), decrease the frequency of labor induction (P = 0.008) and improve adequacy of childbirth during pregnancy . However, Selma et al. (2019) showed that there was no significant difference in back pain between the yoga group and the control group .
Martins (2008) concluded that a weekly hour of Hatha yoga practice for 10 weeks can significantly reduce lower back and pelvic pain during pregnancy . For pregnant women who were not in the habit of practicing yoga regularly, a yoga intervention which included one-time 20-minute yoga session significantly relieved leg and back pain .
3.3.3. Effect of Yoga on the Labor Outcomes
The labor outcomes directly affects pregnant women’s mood. Chen et al. (2017) found that women in the yoga group gave birth to heavier babies than women in the control group (P < 0.001) . In addition, Abbas et al. (2015) investigated the high-risk pregnancy women and found that yoga can improve the intrauterine fetal growth and the utero-fetal-placental circulation .
Fereshteh et al. (2017) showed there was no difference in birth weight in the yoga group during the first stage of labor (P > 0.5), but the average duration of the second and third stages of labor was shorter than that in the control group (P < 0.05) .
Mako et al. (2018) showed the yoga group had longer night-time sleep duration and activated the parasympathetic nervous system than that in the control group .The immune function of maternal was significantly enhanced in the yoga intervention group when compared with that in control group . The incidence of prenatal disorders was lower in the yoga group than that in the control group (P < 0.05) . Selma et al. (2019) found changes in the biomechanical assessments of pregnant women, including percentage change in gait speed (P < 0.04), double support time (P < 0.01), instrumented timed-up-and-go (P < 0.01), and turn time (P < 0.02) after yoga intervention .
3.3.4. Effect of Yoga on Various Stages of Pregnancy
One of studies for the first time is to target women in various stages of pregnancy for monitoring . Mako et al. (2018) found the yoga group showed longer sleep times at 28 - 31 weeks, the sleep times are longer and the ratio of sleep at night is also higher at 36 - 40 weeks than that in the control group, but is not observed at 20 - 23 weeks’ gestation . One of studies is innovative to practice yoga from 16 to 36 weeks’ gestation, it was longer time for pregnant women, but it was better visual observation, included the effect and the attention of yoga intervention .
What’s more, Pont et al. (2019) found the effectiveness of prenatal yoga in reducing maternal discomfort at third trimester of pregnancy . These studies were looking at the third trimester of pregnancy, because of this stage is important for both the pregnant woman and the fetus. In the third trimester of pregnancy, women should pay more attention to the intensity and time of exercise, to avoid fatigue, but also need appropriate exercise, conducive to natural childbirth.
4. Conclusions and Prospection
In conclusion, the findings showed that the pregnancy yoga was useful for either pregnant or postpartum women. The evidences suggested that yoga intervention could reduce depression, stress, anxiety, labor pain, back pain and the percentage of cesarean section during pregnancy. Moreover, yoga intervention will improve psychological well-being, immune function, quality of sleep and life, adequacy of childbirth, the intrauterine fetal growth and the utero-fetal-placental circulation during pregnancy. Overall, there is evidence that yoga is suitable for both pregnancy and postpartum women.
Additionally, these studies also have some limitations. Most of studies had small sample size, short time and single research object. In addition, there are no standard programs for yoga practice in pregnancy and it is not clear what aspects of yoga cause its impact. Studies have confirmed that yoga interventions have an effect on pregnant women during pregnancy, but none of the studies have identified which components of yoga have an effect on women during pregnancy. Last but not least, the majority of studies were unable to collect all the necessary data because some participants did not complete the entire experiment.
In light of the limitations of existing research, we recommend that further randomized controlled trials are needed to provide more reliable evidence of the effectiveness and practicality of yoga interventions. The existing researches focused primarily on the effects of yoga intervention on the health of pregnant women. Future research should pay more attention to the effects of yoga on different stages of pregnancy and different groups of pregnant women. It is recommended that more trials be devoted to unveil the effects of yoga on childbirth and fetus. In addition, our recommendations allow researchers and yoga practitioners to make standardized programs that are suited for many pregnancy women, and make yoga exercise prescriptions for different groups of pregnancy women.
The present review was supported by Science and Technology Project of Hubei Province Education Department (Q20191305), Social Science Fund of Yangtze University (2019csy06), the Open Project of the Innovation Base for Cultivating Sport Doctoral Students’ Academic Literacy (JDTY2019A04), the National Natural Science Foundation of China (31700736), China Scholarship Council (201908420102), Hubei Medical Youth Tip-Top Talent, Leading Talent Program of Yangtze Talent Project.
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