Is it safe to exercise when you’re pregnant?

Practical recommendations for pregnant women who wish to exercise

Exercise enthusiasts claim that women who are fit have shorter labours and fewer complications during pregnancy and birth. Those who advocate caution warn that exercise may cause distress to the foetus, place the mother at increased risk of injury, and lead to lower birth weight in babies.It’s difficult to come up with standardised, objective guidelines, however, as there’s a dearth of good research information. A large number of animal studies have been done, but relatively few on humans. The animal research can indicate areas of interest to pursue, but it’s not really possible to make straight extrapolations and assume that what’s true for a pregnant rat will be the same for a human female! A recent ‘meta-analysis‘ (where results of numerous studies are pooled and analysed) found only 18 relevant human studies to include.

Meanwhile, women who are keen athletes tend to continue exercising despite an increasing bump – and not just gentle ante-natal classes, either. Pregnant women have competed in the Olympic Games, and there are numerous instances of pregnant women competing in marathons – for example, one American woman ran a 3-hour marathon without realising that she was four months pregnant!

At the other end of the scale, women who don’t usually take much exercise may feel that they should try to increase their fitness, as the process of antenatal preparation brings their focus to their body and health-related topics. Although on the evidence available so far it’s not really possible to prescribe ‘optimum’ levels and types of exercise for pregnant women, general guidelines and areas for caution can be proposed. This article will consider the theoretical risks and benefits, and present the findings from the research evidence currently available

Physiological changes during pregnancy

Research gives us an indication of changes likely to occur when a woman is pregnant. As pregnancy progresses, ventilation at a given work rate (this can also be referred to as exercise economy) tends to increase. For example, the quantity of oxygen consumed per minute has been found to increase by 10 per cent for a given walking speed; this is probably due to increased body weight. For non-weight bearing exercise, the results are more inconsistent, although there is still a general tendency for oxygen taken in per given workload to increase. Significant increases tend not to be seen until the 21st week.

Several studies have found an increase in resting heart rate as pregnancy continues; although it appears that maximal cardiac output stays the same. Resting metabolic rate is also increased. Cardiac output starts to increase in the first trimester (first three months) and reaches an extra 30 to 50 per cent by the second trimester, mainly as a result of an increase in the size of the heart and volume pumped out with each beat, but also resulting from an increase in pulse rate from 70 to 85 per minute. Generally, by the third trimester (the final 3 months of pregnancy), there is no longer a difference between cardiac output of athletically trained and untrained women.

Potential risks for the foetus

Decrease in blood flow to the uterus

Uterine blood flow has been found to decrease during exercise. This used to be a factor quoted when trying to discourage pregnant women from exercising. However, it appears that there are compensatory measures such as increased oxygen extraction by the foetus, and redistributed blood flow, that keep the foetal oxygen supply relatively constant

Reduced glucose supply to the foetus

During pregnancy, maternal blood glucose is used by the growing baby as a major energy source for growth and development. Concern has been expressed that exercise may lower levels of circulating blood glucose. There’s some evidence for this, but also indications that the mother has an enhanced capacity to burn alternative fuel sources during exercise. Although circulating glucose may well dip during exercise, this is a transient effect, which would not be expected to cause problems. There is however, some evidence that prolonged, intense physical work can inhibit the baby’s growth


Increased body core temperature is believed to carry a risk for the unborn baby. For example, a study of maternal heat exposure (hot tub, sauna or fever) during the first three months of pregnancy found an increased incidence of neural tube defects such as spina bifida. Over-heating in the second half of pregnancy has been associated with lower birth weight in animals. Present guidelines suggest that a core body temperature of 39 degrees C or above is risky.

There is therefore concern that exercise could result in a heightened body core temperature, although in the study cited above, exercise was not included as a potential risk factor. Other research has focussed on women performing exercise at their normal pace. No increase of temperature into the risk zone has been seen during such experiments. For example, one study found that for women exercising at moderate to high intensity for 20 minutes, temperature increased by .5 degrees C to 37.3 degrees C in late pregnancy. Some researchers suggest that physiological adaptations such as a tendency towards increased blood pooling at the skin and an increased blood volume lead to more efficient heat loss. There is no evidence that exercise raises body core temperature to a level likely to cause problems.

Other work in this area has suggested that water at standard swimming-pool temperature may be a better medium for exercise than land, as this blunts any rise in core temperature. In general, change in core body temperature with exercise depends on both the intensity of exercise and its duration. Of course, common sense measures should be taken – if the weather is hot and humid, there’s more risk of over-heating; similarly, care should be taken to avoid becoming dehydrated, and loose clothes should be worn.

Effects of exercise on pregnancy symptoms

Suggestions have been made that women who exercise suffer less from typical pregnancy-related symptoms such as nausea, heartburn, leg cramps, etc. In most research, it has been difficult to tease out cause from effect – do the women suffering more symptoms exercise less because they feel lousy, or does the exercise reduce the symptoms? A recent study tried to cut through this ambiguity by including information on women’s levels of exercise before becoming pregnant, followed by taking detailed information on pregnancy symptoms and levels of exercise at different times throughout the pregnancy (‘Exercise during pregnancy and pregnancy outcome’, Sternfeld et al, Medicine and Science in Sports and Exercise, Vol 27, pp634-640).

Nearly 400 women were enrolled in the study, which was carried out in San Francisco. Four different levels of exercise were defined, the most active being Level 1, where women were participating in aerobic exercise at or above the level required for an aerobic training effect – ie, three times a week for at least 20 minutes. Level 4 included the least active women – those doing no aerobic exercise at all, or doing it less than once a week.

The percentage of women exercising at Level 1 before pregnancy decreased substantially from 41 per cent prior to conception to 14 per cent by the last three months before birth. The intermediate Levels 2 and 3 remained relatively constant throughout. The mode of exercise also tended to change – for example, at the beginning, almost a fifth (57) were runners prior to pregnancy, but by the final three months only one was still running. Swimming became a lot more popular.

A pregnancy symptoms score was calculated for each trimester (three-month period of pregnancy). This was calculated from responses to a questionnaire asking about the frequency of 22 physical discomforts – eg, nausea, heartburn, leg cramps. Women who exercised at levels 1 and 2 during the first trimester reported fewer symptoms in that time period than woman exercising at levels 3 and 4. The same pattern was found in the third trimester – those exercising at higher levels had fewer symptoms of physical discomfort.

Also, more exercise earlier in pregnancy was related to fewer symptoms later in pregnancy. But perhaps most interesting of all was the discovery that women who were exercising at Level 1 prior to pregnancy tended to report fewer symptoms in their first trimester. This provides good evidence for a positive effect of exercise on symptoms. When women who maintained a high level of exercise (Level 1) throughout their pregnancy were compared with those who decreased their activity levels, it was discovered that the decrease in activity preceded differences in symptoms. The researchers therefore concluded that the findings suggest that the women were not exercising because they felt better, but felt better because they were exercising. One possible mechanism for this effect is that exercise could increase endorphin (natural pain-killer) levels, and thereby reduce perceptions of pain and discomfort. One study has in fact found increased natural endorphin levels during labour in a group of women randomly assigned to an exercise programme during labour.One reason that may prompt women to take up or maintain exercise during pregnancy is to try and minimise their weight gain. Fat stores increase steadily during pregnancy, until the third trimester. This is partly an effect of a chronic state of elevated blood insulin, which gives the body the message that it needs to lay down energy stores. Two major studies indicate that moderate aerobic conditioning of previously inactive women during the second and third trimesters does not significantly halt the typical pregnancy-induced increase in fat storage. It therefore appears that the hormonal changes found in pregnancy override the usual reduction in body fat that would normally occur with aerobic conditioning.

Effect of exercise on pregnancy outcome

Factors which have been suggested to be linked with exercise include: risk of pre-term delivery, length of labour, birth-weight, and Apgar scores (initial assessment of the baby), and complications during labour. A recent analysis combined the results of 18 studies, representing 2,214 pregnant women in total (‘Effects of physical exercise on pregnancy outcomes: a meta-analytic review’, Lokey et al, Medicine and Science in Sport and Exercise vol 23, pp1234-1239).

This analysis found that none of the variables investigated differed significantly between exercising and control women. The variables included maternal weight gain, length of pregnancy, length of labour, complications of labour, type of delivery, infant birth weight, and Apgar scores. All scores for these variables were within the healthy normal range for both groups. The researchers noted that although various studies have found one or more of the specific outcomes to be more favourable for active compared to inactive women, there was no consistent pattern across studies, and sample sizes were small.

These results give ‘averaged’ outcomes. In looking at specific individual studies, there is some indication that strenuous work, inadequate nutrition and prolonged standing can adversely affect foetal growth. Similarly, data from women working late into the third trimester show an increase in premature deliveries for women performing strenuous work and heavy lifting.

The writers of the meta-analytic review concluded that a pregnant woman can exercise up to three times a week for 43 minutes per session at a heart rate of 144 bpm without appearing to harm herself or her unborn child. The effect of intensities greater that this could not be determined. The results did not indicate any adverse effects of jogging or weight-bearing exercise.

We are still a long way from knowing what an optimum level and type of exercise is for the pregnant woman. As a team of American maternal and paediatric health experts recently concluded, the current public health message is not that pregnant women should exercise, but rather that they may exercise.

Practical recommendations for pregnant women

  1. Taking up a fitness programme before conceiving could reduce pregnancy symptoms. Women interested in a programme of ‘preconceptional preparation’ are often counselled on dietary factors (eg recommended to take folate supplements). It may also be appropriate to encourage inactive women to take up moderate aerobic exercise at this stage
  2. Don’t start an exercise programme for the first time in pregnancy. Ideally, women should be aiming to maintain a fitness level already achieved. For those wishing to take up exercise, walking, or non-weight bearing exercise such as swimming or cycling started very gradually would avoid the extra strain of weight gain during pregnancy. Exercise to exhaustion should be avoided
  3. Limber down gradually after vigorous exercise. Hormonal and physiological effects of pregnancy mean that it is more likely that blood will pool in the leg muscles if exercise is stopped suddenly. A warm-down should be included after aerobic exercise
  4. Take great care with activities requiring balancing skills. Pregnancy hormones have the effect of loosening ligaments; joints therefore become less stable and more prone to sprain. Of particular importance are the spinal joints, hip joints, knees and ankles
  5. Anyone suffering from heart disease, hypertension or anaemia should be discouraged from rigorous exercise. The heart will have an extra load to bear as pregnancy progresses
  6. Take care to avoid over-heating. If exercising in hot or humid conditions, shorten the duration or intensity of activity, and ensure plenty of fluid is drunk. Don’t take saunas or long hot baths after exercise
  7. Measurements commonly used to monitor and prescribe exercise regimes may need to be altered. Heart rate is a less sensitive and reliable method for monitoring exercise intensity during pregnancy. In contrast, perceived exertion has been found to be unaffected by pregnancy – thus some sports scientists recommend using reported perceived exertion in conjunction with heart rate for monitoring and prescribing exercise

Janet Pidcock