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The physical effect of exercise in pregnancy on pre-eclampsia, gestational diabetes, birthweight and type of delivery: a structu

28 May, 2013

The physical effect of exercise in pregnancy on pre-eclampsia, gestational diabetes, birthweight and type of delivery: a structu

The aim of this literature review was to investigate the association between physical exercise in pregnancy and the development of hypertensive disorders, gestational diabetes, birthweight and types of delivery. This review hypothesised that modifiable behaviour – physical activity – may reduce the risk of these conditions.

Evidence Based Midwifery: June 2013

Andrée Dignon1 PhD, M.Phil, BSc.
Amanda Reddington2 RN.

1. Senior lecturer, Department of Nursing and Health Studies, Faculty of Health and Life Sciences, Coventry University, Coventry CV1 5FB UK England.
Email: hsx155@coventry.ac.uk
2. Community development health team leader, Nuneaton and Bedworth Healthy Living Network, St Nicholas Park Clinic, Nuneaton CV11 6HH England.
Email: amanda.reddington@warwickshire.nhs.uk



Abstract
Background. Previous research suggests that the risk of developing some of the more common complications in pregnancy, such as hypertension, gestational diabetes, low birthweight and complicated labour, may be lowered by regular physical activity. This review examined research into the effects of physical activity on health in pregnancy, with particular reference to these areas.
Aims. The aim of this literature review was to investigate the association between physical exercise in pregnancy and the development of hypertensive disorders, gestational diabetes, birthweight and types of delivery. This review hypothesised that modifiable behaviour – physical activity – may reduce the risk of these conditions.
Method. The method deployed in this paper was that of a structured review of the literature, selected through a systematic search of the Medline, CINAHL and SPORTDiscus databases.
Findings. The findings indicate that physical activity in pregnancy may be associated with lower risk of pre-eclampsia, gestational diabetes, very low birthweight and caesarean birth.
Implications. Physical activity has positive effects on physical health in pregnancy. Inactive women should be encouraged to take up exercise, while women who exercised in pre-pregnancy should be encouraged to continue.

Key words: Pregnancy, health, physical activity, exercise, evidence-based midwifery

Introduction
Pre-eclampsia and gestational hypertension are high blood pressure conditions presenting from the 20th week of gestation during pregnancy (NICE, 2010a). Hypertensive disorders in pregnancy are prime causes of maternal and perinatal morbidity and mortality across the world, caused by the defect in the formation or arrangement of the placenta in early pregnancy (Khan et al, 2006). Pre-eclampsia describes hypertension where there is a concentration of protein in the urine greater than 300mg (Saftlas et al, 2004).

These conditions are potential complications that can occur in pregnancy. A further potential complication is gestational diabetes. A total of 7% of American women are affected by gestational diabetes each year and have a 30% to 84% chance of developing the condition in future pregnancies (Dabelea et al, 2005). Also, children born to mothers with gestational diabetes may be at greater risk of obesity and diabetes themselves (Pettitt et al, 1993).

At the moment of birth certain outcomes are preferred to others: natural vaginal births are preferred to caesarean sections (CS), as they lower the risk of bowel dysfunction; infection; excess blood loss; longer recovery time; and extended hospital stay (Villar et al, 2006). As well as avoiding the necessity of a CS, it is also highly preferable to avoid pre-term delivery (Olesen et al, 2003). Leiferman and Evenson (2003) identified that 17% of birth outcomes were pre-term births (less than 37 weeks’ gestation), which may result in lower birthweight babies.

Research suggests that exercise during pregnancy may be beneficial in the above four areas (Hegaard et al, 2007; Hu and Manson, 2003; Artal and Sherman, 1999; Sternfeld et al, 1995; Mittlemark et al, 1991). Clearly there are other areas in which exercise can also be beneficial (Siega-Riz et al 2006), but these four areas form the focus of this review.

Background
Patterns of physical exercise
Patterns of physical exercise indicate that most pregnant women (Evensen et al, 2004) fail to take exercise according to recommended guidelines (for example: ACSM, 2012; ACOG, 2011; RCOG, 2006). These women frequently report barriers to taking exercise, such as tiredness, sickness, work, lack of time and facilities (Weir et al, 2010; Halksworth, 1993; Wells, 1991). Physical activity thus appears to decline – or even cease – during pregnancy (Borodulin et al, 2009), but NICE guidelines (2008) recommend that women who exercised regularly before pregnancy should be encouraged to continue for as long as they feel comfortable and for as long as the type of exercise is suitable.

Guidelines and safety issues
Established guidelines suggest a mixture of moderate-intensity aerobic and muscle-strengthening activity during pregnancy (NICE, 2010b). Women should also be clearly informed that physical activity and moderate exercise is safe (ACSM, 2012). From the 16th week of pregnancy, however, it is not advisable to exercise while lying on one’s back as the vena cava can become compressed by the baby causing light-headedness and possible fainting (RCOG, 2004). In addition the hormone relaxin increases during pregnancy causing increased laxity of ligaments, which can lead to tissue and skeletal injury (Dumas and Reid, 1997; Kramer, 2000). However Clapp’s aerobic study (2000) showed no evidence of any increased injury due to increased susceptibility at this time. Nevertheless, contact sports where there is a likelihood of being hit, a risk of injury to the abdomen or a potential fall, are not advisable (NICE, 2008).

Benefits
Exercise can improve common pregnancy symptoms. Vaughan (1951), for example, identified that sedentary women prolonged their confinement after giving birth, in comparison to active pregnant women, due to experiencing difficult childbirths. Numerous other studies, including Rice and Forte (1991), have shown aerobic exercise to be beneficial to pregnant women without compromising fetal growth or development or hindering the progression of pregnancy, labour or delivery outcomes.

Rationale for the review
Our review investigates whether exercise reduces the risk of: pre-eclampsia; gestational diabetes; CS; and low birthweight. The rationale for this review was to identify whether exercise was beneficial in each of these areas in light of some previous conflicting findings, particularly with reference to pre-eclampsia (Vollebregt, 2010; Sorensen et al, 2003). The authors established the effectiveness of exercise by referring to studies with the largest samples, and, where conflicting findings were present, by referring to several studies to establish the ‘majority verdict’.

Method
Aims and objectives
A search strategy was carried out to identify primary research articles relevant to the question posed (Gash, 1989). From this initial retrieval, sub-headings such as ‘labour’ and ‘physiological complications’ were selected to identify specific articles including those subject words. The method adopted was that of a systematic search of three databases: CINAHL, Medline and SPORTDiscus. The first two databases were accessed as they contain the largest pool of articles related to midwifery, nursing and medicine. The database SPORTDiscus was also accessed as it contains references to studies in the area of sport and sports medicine, in particular studies which describe the benefits of exercise. The search terms entered into the above databases included ‘pregnancy’, ‘pregnant women’, ‘female’, ‘physical exercise’, ‘physical activity’, ‘fitness’, ‘labour’ and ‘physiological complications’. These terms were combined using the Boolean operator ‘AND’ to ensure any articles retrieved would contain the above terms. The following inclusion criteria were specified: studies of women aged 16 to 49; studies in the English language; studies conducted between the years 2000 to 2012.

This search was performed on two occasions, firstly by AR, then six months later by AD (to achieve search triangulation). In addition, the reference lists of the articles retrieved were read to identify further papers. AR’s search using the above strategy uncovered 24 articles. These articles were read and those with comparatively small sample sizes (as well as studies making no mention of ethical approval, or of the four conditions which formed the focus of this review) were discarded. This left four articles of relevance to the study. The subsequent search (by AD) retrieved a further two articles of relevance, making six articles in total.

The process of the review
The review took the form of identifying papers from a series of studies retrieved that addressed the four areas in which we were interested. We specifically selected studies with large sample sizes (see Table 1, below). The review consisted of appraising the methodology in each study and assessing whether these studies demonstrated that exercise in pregnancy was beneficial.

Table 1. Papers selected for the review Table 1. Papers selected for the review


Analysis 

All of the studies retrieved were cohort studies. In a cohort study, outcomes for one group of patients ‘exposed’ to a condition – for example a group of pregnant women ‘exposed’ to physical activity – are compared to a similar group not exposed – those not undertaking physical activity. We analysed six cohort studies by consulting the Critical Appraisal Skills Programme (CASP) framework (2011). CASP suggests applying a range of questions when appraising cohort studies, including the questions:
• Did the authors use an appropriate method to answer their question?
• Was the exposure accurately measured to minimise bias?
• Have the authors identified all of the important confounding factors?
Six articles were analysed (by asking the above questions) in order to present critical findings from each study. Benefits of physical exercise were analysed in terms of hypertension in late pregnancy, gestational diabetes, type of delivery and birthweight.

Findings
All six investigations reviewed were prospective cohort studies and used quantitative methods. The studies were of survey design, using a pre-validated questionnaire. The first study, conducted by Vollebregt et al (2010), aimed to analyse the association between leisure time physical activities in early pregnancy and the effect this had on hypertension later on in pregnancy. Postal questionnaires were sent to a large sample of nulliparous women from diverse backgrounds. Data were captured through a self-reporting questionnaire, while medical and birth data were obtained through an obstetric register.

Two groups were identified from the sample, one being a non-exercising group of women (n=1956) and the other being the exercising group (n=1645). The authors defined exercise as including: walking, cycling and playing sports, and classified the women’s participation in these activities as either ‘low’, ‘moderate’ or ‘high’. Women in the exercising group undertook one or more of these activities at a moderate or a high level. Both groups had their medical records accessed to eliminate any pre-existing medical conditions, such as diabetes. Levels of exercise duration, intensity and frequency were measured through self-reporting. Vollebregt et al’s findings showed that leisure-time physical activity (LTPA) in early pregnancy did not reduce gestational hypertension (based on duration or intensity of exercise).

While Vollebregt et al (2010) found no association between physical activity and gestational hypertension, an earlier study by Saftlas et al (2004) produced different results. Saftlas et al’s investigation is the second study appraised in this review. These scholars assessed the effect of physical activity on gestational hypertension and pre-eclampsia. ‘Few studies of pre-eclampsia’, they say, ‘have assessed physical activity level yet recent evidence suggests that the pathologic mechanisms in pre-eclampsia are similar to those in cardiovascular disease in which physical activity is shown to be protective’ (2004: 758). In view of this, Saftlas et al investigated the effects of physical activity in leisure time among nulliparous and multiparous women, (44 with pre-eclampsia and 172 with gestational hypertension). The sample was recruited (from 2967 referrals to obstetric practices), and each participant was administered a one-hour face-to-face interview before the 16th week of pregnancy, where information on LTPA and on elevated blood pressure was recorded. Further information on pregnancy-related high blood pressure was obtained from medical notes. Maternal age at delivery, cigarette smoking, body mass index and years of education were also recorded as confounding variables. Participants were asked whether they performed any physical exercise or sport – along with type and frequency of exercise – at least once a week in the year before their pregnancy.

While the study found no association between physical activity and hypertension, it observed a marked effect of physical activity in relation to pre-eclampsia. The study found that those participants who were active before pregnancy and engaged in weekly exercise during pregnancy had a 41% reduced risk of pre-eclampsia. However, this was not statistically significant.

The third study appraised in this review was conducted by Sorenson et al in 2003. Like Saftlas et al, this study examined recreational physical activity and its association with pre-eclampsia risk. A total of 201 women with pre-eclampsia were recruited as the study group along with 383 normal controls. The study administered a structured questionnaire investigating leisure time physical activity both during and before the woman’s pregnancy. These activities included light activities such as gardening and golf, at one end of the scale, and, at the other end, intense activities such as swimming laps and aerobic dancing. Women supplied data concerning ‘the type, intensity, frequency and duration of physical activity’ during the first 20 weeks of pregnancy and during the year before.

A systematic method encompassing the calorific energy expenditure associated with the activity was used to classify the exercise taken and to divide women into active and non-active groups. The social demographic profiles of participants, along with their medical history, were also examined (as potential confounding variables). The study found that women who took part in any recreational physical activity in their first 20 weeks of pregnancy were at a 34% reduced risk for pre-eclampsia, compared to women who did not (OR 0.66 95% CI 0.47 to 0.94). Even when confounding variables, such as age and smoking, were taken into account, this difference remained constant.

Although there were limitations in Sorensen et al’s study, for example, the response rate among controls was higher than that for the pre-eclampsia group – 80% versus 50%, their findings, nevertheless, provide a useful indication of the possible benefits of exercise in reducing pre-eclampsia risk. Also, one of the study’s limitations, that of relying on self-reports of physical activity, was minimised through the use of standardised questions and a highly skilled interviewer.

In relation to hypertensive disorders, such as pre-eclampsia, therefore, it appears that physical activity may help reduce risk. This review, however, also examines the effect of exercise on other pregnancy complications, such as gestational diabetes. The fourth study appraised in this review was conducted by Liu et al (2008). This study examined whether physical activity during pregnancy reduced the risk of gestational diabetes among previously inactive women. National Maternal and Infant Health Survey data (Sanderson et al, 1988) and postal and telephone surveys were used, which recruited a large sample of 9953 sedentary women (nulliparous and multiparous). The overall results of Liu et al’s study implied that women who started exercise during pregnancy had a 57% lower risk of developing gestational diabetes, compared with non-active women. Those who exercised at a higher level than the average had an even lower risk (67%).

As well as investigating the effect of physical activity on gestational diabetes, this review also examines the effect of such activity on type of delivery (vaginal versus CS). The fifth study appraised in this review was conducted by Bungum et al (2000) to identify the association of aerobic exercise during the first two trimesters of pregnancy with type of delivery in nulliparous women.

The sample (n=137) was recruited from antenatal education classes. While participation in such classes may have influenced participants’ behaviour, and thus created bias, such error was minimal since the researchers were not involved with the classes and attended only one to recruit participants. Group allocation was identified by self-reporting the number of times women engaged in at least 20 minutes of exercise each week, while a checklist was used to note the type of exercise they engaged in. The active group was comprised of women who had exercised for at least 20 minutes a session, in excess of twice a week, while women who did not meet this criterion were identified as the inactive group.

A written survey was mailed two weeks prior to delivery. A total of 28% of the non-active women had CS births compared to 16% of the active group. As the authors explain: ‘The unadjusted odds ratio indicated that the risk of CS delivery was 2.05 times greater among sedentary than among active women’ (Bungum et al, 2000: 261). After applying logistic regression (using the data from the surveys), the study found inactive women were four and a half times more at risk of CS deliveries. However, there were no significant differences between the groups regarding length of labour, birthweight or maternal weight gain. There were also no significant differences regarding labour intervention, such as forceps. The data thus indicated that non-active women were twice at risk of CS births than those women who were active: ‘The odds of a CS delivery were 4.48 (95% CI=1.2-16.2; p=.023) times greater among sedentary mothers than among active mothers’ (Bungum et al, 2000: 261).

The final study reviewed in our report was conducted by Leiferman and Evenson in 2003. As well as examining the effect of physical activity on type of delivery, the authors also examined its effect on birth outcomes in relation to birthweight. Leiferman and Evenson investigated birth outcomes among 9089 women (74% response rate from 13,417 women mailed a questionnaire – all but live singleton births excluded). Medical data were captured through systematic sampling of the national maternal and infant survey and post-birth self-response questionnaires, both of which had achieved an exceptional response rate (74%). Four ‘exposure groups’ were formulated, depending on how physically fit the women were. Each group had the same measurement methods applied, consisting of two questions: ‘Did you exercise or play sports at least three times a week before you found you were pregnant?’ and ‘Did you exercise or play sports at least three times a week after you found you were pregnant?’. Women answering ‘yes’ to both questions were identified as ‘conditioned exercisers’, while those affirming only the first were deemed ‘conditioned non-exercisers’. Significantly, the results showed ‘unfit’ women, who were sedentary prior to pregnancy, and fit non-exercising groups, women who exercised pre-pregnancy but stopped during pregnancy, gave birth to very low birthweight babies (16.7% and 18.1%, respectively, compared to 10.6% for fit exercisers and 11.1% for unfit exercisers).

Notably, women who exercised prior to pregnancy and stopped during pregnancy doubled their risk of giving birth to babies of low weight (with an odds ratio of 2.05, 95% CI 1.69-2.48). In conclusion, women who did not exercise in pregnancy were at a higher risk of giving birth to very low birthweight or low birthweight babies. Physical exercise in pre-pregnancy and during pregnancy should therefore be actively promoted to benefit birth outcomes.

Discussion
The overall findings in this review indicate that the risk of developing certain conditions in pregnancy among previously sedentary women can be reduced by exercising. For example, women who exercise during their first two trimesters have a greater likelihood of giving birth naturally and in relation to pre-eclampsia, time appears to be of benefit. In addition, such women appear to be more likely to avoid gestational diabetes. Therefore women who continually exercise prior to pregnancy through to delivery have the greatest health benefit. If the promotion of physical exercise could reduce the incidence of pre-eclampsia, then the first half of pregnancy is likely to be the most effective period, especially for first-time mothers. A further cohort study may help to assess the potential effectiveness of such a programme.

Our study extends the findings of previous studies (in relation to pre-eclampsia) by identifying controversy and comparing conflicting findings. Vollebregt et al (2010), for example, found no relation between pre-eclampsia and physical activity, but other studies such as Saftlas et al (2004) and Sorensen et al (2003) do observe such an association. The majority of research in this area would thus appear to show that physical activity lowers the risk of pre-eclampsia and this concurs with findings (Marcoux et al, 1989) from early research. Further research in this area, using an RCT design, is clearly needed, but for now we contend that exercise may be beneficial in reducing pre-eclampsia risk.

Similarly, exercise during pregnancy has also been shown to reduce the risk of gestational diabetes among previously inactive women. This can be further enhanced if exercise continues longer than the first trimester while leading an inactive lifestyle during pregnancy can raise the risk of gestational diabetes (Liu et al, 2008). A limitation of Liu et al’s study was that the study analysed survey data from 20 years previously when the prevalence of gestational diabetes was half its current level. However, Liu et al confirm that there is no biological evidence to suggest that the association between gestational diabetes and physical activity in the present population is any different to previous ones. Liu et al also suggest that women starting to exercise during pregnancy were at significantly lower risk (57%) of developing gestational diabetes. These findings correlate with Alcazar et al’s theory (2007) that exercising muscles on a regular basis replenishes glycogen stores and increases glucose protein in muscles, which assists the reduction of insulin secretion by the pancreas and decreases the liver’s glucose production. It would appear from Liu et al’s study that taking up exercise in pregnancy does seem to reduce the risk of developing gestational diabetes and the risk may be lowered further if exercise continues after the first trimester. Conversely, leading a sedentary lifestyle during pregnancy raises the risk gestational diabetes (Dempsey et al, 2004).

As well as in the area of gestational diabetes, physical activity may also be beneficial to the type of delivery. Our review has highlighted that women who exercise for 20 minutes a day, three times a week during the first two trimesters of pregnancy are more likely to have a natural vaginal birth (Bungum et al, 2000). In support of Bungum et al’s study, numerous other prospective studies, including that of Hall and Kaufmann (1987), have observed that more women who exercised in pregnancy had vaginal birth outcomes compared with less active women. Also, exercise may have an effect in relation to birthweight. The relationship between physical exercise and birthweight observed by Leiferman and Evenson (2003) seems to suggest that women who do not exercise during pregnancy are more at risk of giving birth to low birthweight babies, while exercise does not have any negative effect on the timeliness of delivery. These findings suggest exercise in pregnancy enables mothers to better manage the rigours of childbirth and confirms that women who exercised prior to pregnancy should be encouraged to continue, while those inactive could be motivated to begin exercising through the promotion of the importance of exercising to achieve positive labour and birth outcomes (subject to medical or obstetric complications).

Solely providing information, however, may not be enough to persuade women to take exercise as barriers, such as financial and time constraints, lack of facilities and transport, may exist. This may be especially so for lower income women, given the additional financial, housing, occupational and psychological pressures that many such women face (Graham, 1984). These pressures may prevent them from taking control of their lives and organising a programme of exercise, making health promotion advice in this area hard to follow. Nevertheless, the provision of such advice is a starting point and promoting the benefits of exercise would be valuable to pregnant women. However, if the professional’s knowledge is limited as to whether exercise is beneficial during pregnancy, then a significant health promotion opportunity with regard to the wellbeing of pregnant women may be lost.
The results from this review suggest that physical exercise can improve physical health outcomes in pregnancy. The common theme in this review is that exercise needs to start during the first trimester – if not pre-pregnancy – to gain the greatest benefit. Promoting physical exercise during the first half of pregnancy appears to be highly beneficial, especially to first time-mothers.

Strengths and weaknesses
The strengths of the papers in this review are that they showed validity with significant response rates in each study generating large sample sizes. Also the studies created ‘exposure groups’ to highlight comparable findings. In addition, studies used valid measures and subjects were followed up through their pregnancy until after the birth of their child. Areas where the papers showed weaknesses were in the limited recording of the intensity, duration and, in some cases, the frequency of physical activity, which may have influenced the robustness of the review. In addition, the main method used in these studies was self-reporting and this can distort the findings. However, one study reviewed (Vollebregt et al, 2010) captured information in early pregnancy to minimise such errors, while another (Sorensen et al, 2003) addressed this bias by using a trained interviewer to administer study questions.

The authors acknowledge the major limitation of this review was that no meta-analysis was performed of the statistics in each individual study. In a future review it would be desirable to address this limitation. Regardless of its limitations, the current paper has significant strengths. It identifies very important findings, in particular that women who exercised prior to pregnancy should be encouraged to continue, while those inactive could be motivated to start. By undertaking physical activity, such women may be able to limit the onset of gestational diabetes and pre-eclampsia, give birth to healthy weight babies and increase their chances of a natural birth. Therefore, women who exercise prior to pregnancy through to delivery have the greatest health benefit.

Conclusion
This paper set out to identify the association between physical activity and four of the more common complications in pregnancy: pre-eclampsia, gestational diabetes, type of delivery and low birthweight. On reviewing pre-eclampsia, gestational diabetes, type of delivery and birthweight, all have been shown to improve when exercise is taken. Physical exercise has thus been shown to have positive effects on physical health in pregnancy. Women already exercising should be encouraged to remain active at the same level for as long as it is comfortable. Nonetheless, all pregnant women should take individual medical advice to offset any possible adverse complications.

Informing women of the benefits of physical exercise could facilitate informed decision-making regarding exercise in pregnancy. Such information provision alone may not be enough to change behaviour as many barriers to taking exercise, such as lack of time, money, facilities and transport, may remain, especially for lower income women. Although information provision is not always sufficient to change behaviour, it is a step in the right direction and providing such information, while attempting to address other barriers to physical activity, must remain a priority.

 

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