How safe is aerobic exercise?
Current research suggests that healthy pregnant women can begin or maintain moderate intensity aerobic exercise programs with little fear of adverse effects on their unborn foetus.
Numerous studies investigating the effects of exercise on pregnancy, the foetus, and the mother have been performed in the last ten years. In a normal healthy pregnancy, no study has found any negative effect of moderate intensity aerobic training on the development of the foetus or the outcome of pregnancy. In fact, it appears that the benefits of exercise during pregnancy clearly outweigh the potential risks.
Many forms of moderate intensity aerobic exercise such as swimming, running, aerobics and cycling (stationary, later in pregnancy) appear to be safe throughout pregnancy. Limited research and some anecdotal data have also shown that it may be possible to perform higher intensity exercise during pregnancy, but this should only be considered with close monitoring from the treating doctor. The safe upper limit of exercise in pregnancy is unknown.
In a large prospective controlled trial of women performing aerobic exercise, Clapp (1991) found no association between exercise and risk of miscarriage, congenital malformations, ectopic pregnancies, pre-term rupture of membranes, placental insufficiency, retarded intrauterine growth, or unexplained foetal deaths. These findings have been confirmed in a number of subsequent studies. (Webb 1994, Sternfeld 1995, Bell 1995, Wolfe 1994)
The evidence from many studies also shows that aerobic exercise has no effect on the course or outcome of labour (Bell 1995, Horns 1996, Kardel 1998, Lokey 1991, Magann 1996, Sternfeld 1995). However, one study has shown that physically fit women who ran or performed aerobics continually throughout their pregnancy had fewer medical interventions during labour than a control group (Clapp ‘96)
How safe is resistance (weight) training?
Although general muscular conditioning, in the form of weight or resistance training, is probably safe during pregnancy, there have been few studies of this type of exercise during pregnancy.
From the few studies which have been conducted, no adverse findings have been reported when exercise involved light to moderate weight training with free weights, weight machines or a combination of free weights and stretching. These studies have shown that strength and flexibility are improved, that injury risk is not increased, and that there are no obvious positive or negative effects on weight gain, pregnancy complications, course of labour, or birth weight. (Avery, 1999; Clapp 2000).
As resistance training strengthens muscles, it may help pregnant women to tolerate their heavier body weight and altered centre of gravity more easily. In particular, there may be less low back discomfort due to stronger truncal musculature. (Shangold 1994) Concerns regarding possible injury from resistance training due to increased laxity of ligamentous structures during pregnancy have not been borne out.
It is generally recommended that resistance training during pregnancy involve only light to moderate weights, and that heavy lifts, in particular, maximal isometric muscle contractions, are avoided, due to the cardiovascular responses to this type of activity, and the added pressure on the musculo-skeletal system. It is also important to ensure that beginners are supervised to ensure safe technique, and that proper breathing techniques are used (avoiding breath holding and the valsalva manoeuvre). Because venous return of blood to the heart can be compromised by the developing foetus, exercises in the supine position should be avoided after the first trimester.
How much exercise is 'safe'?
The American College of Obstetricians and Gynecologists used the findings of a meta-analysis to develop their guidelines on exercise during pregnancy (Lokey et al 1991). The meta-analysis concluded that an exercise program comprising any of a variety of exercise modes that is performed for an average of 43 minutes, three times a week, at a heart rate of up to 144 bpm does not appear to be associated with adverse effects to the mother or foetus in a normal pregnancy (Lokey at al 1991). It should be noted however that at that time few studies were considered to be sufficiently methodologically sound to be included in this meta-analysis, and none of these included women exercising at the highest levels (equivalent to elite level training). In light of more recent findings that highly trained women athletes have continued to exercise throughout pregnancy without problems, current opinion is that these guidelines may be somewhat conservative for highly trained athletes.
In general, healthy women who have uncomplicated pregnancies can continue their preconceptual regular aerobic exercise during pregnancy, after consulting with their treating doctor. Whilst there are some more conservative opinions (Hartman 1999), many authors believe that more sedentary women can begin a light to moderate exercise programme during pregnancy, in consultation with their doctor (Stevenson 1997, Clapp 2000, Collings 1983, Sibley 1981, Webb 1994). This was confirmed by Clapp (1998), who commenced a large group of previously sedentary women on a moderate exercise programme at the beginning of their pregnancy, with no ill effects.
