NIHR Signal Moderate exercise does not increase risk of preterm birth in healthy pregnant women

Published on 27 September 2016

Women with an uncomplicated pregnancy who exercise are no more likely to have a preterm birth than those who don’t exercise.

This systematic review pooled the findings of nine trials including 2059 healthy pregnant women who were assigned to exercise for 35 to 90 minutes, three to four times a week, or to no exercise. Despite previous concerns that exercise may increase risk of preterm birth, the review found no evidence for this. It also found that exercise might increase a woman’s chance of a vaginal delivery and reduce risk of pregnancy complications such as diabetes and high blood pressure.

The findings support current guideline recommendations on exercise during pregnancy. Women who have an uncomplicated pregnancy may be advised to take part in moderate aerobic and strength-conditioning exercises during their pregnancy as this can improve their overall health and will not put the baby at risk.

Moderate exercise does not increase risk of preterm birth in healthy pregnant women

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Why was this study needed?

Preterm birth, where a baby is born before 37 weeks of pregnancy, is the leading cause of new born deaths globally. In the UK 60,000 babies are born prematurely and in 40% of these cases the cause is not known.

Women who have no medical complications in their pregnancy are currently advised that moderate, aerobic, non-contact exercise benefits their health and is unlikely to harm their unborn baby. However, some studies have suggested that exercise could cause preterm delivery, possibly by stimulating the womb muscle or altering blood flow in the placenta. Therefore the topic remains controversial.

This systematic review and meta-analysis aimed to shed more light on the safety of exercise in pregnancy, and see whether women with an uncomplicated pregnancy who exercise regularly are more likely to give birth prematurely.

What did this study do?

Nine randomised clinical trials were identified, including 2059 women pregnant with one baby, who had no complications and were of normal body mass index.

The trials compared the rate of preterm births in women who were assigned to regular aerobic activity before 23 weeks of pregnancy with those that weren’t. The type of exercise differed between trials, but all sessions lasted between 35 to 90 minutes and were given three to four times per week for ten weeks or until delivery.

Other outcomes of interest included type of delivery (vaginal or caesarean) and pregnancy diabetes or high blood pressure.

The trials had low risk of bias in terms of randomisation method and data collection, although it is unclear whether assessors were aware of group allocation. The studies had similar methods and could be reliably pooled in meta-analysis. None were UK based.

What did it find?

  • The preterm birth rate was no different in women who exercised during pregnancy compared with women who didn’t exercise (4.5% vs 4.4%; relative risk [RR] 1.01, 95% confidence interval [CI] 0.68 to 1.50).
  • Vaginal delivery was significantly more common among women who exercised (73.6% vs. 67.5%; RR 1.09, 95% CI 1.04 to 1.15), fewer of whom needed a caesarean section (17.9% vs. 22%; RR 0.82, 95% CI, 0.69 to 0.97).
  • Compared to women who didn’t exercise, fewer women who exercised developed pregnancy diabetes (2.9% vs. 5.6%; RR 0.51, 95% CI 0.31 to 0.82) or high blood pressure disorders (1.0% vs. 5.6%; RR 0.21, 95% CI, 0.09 to 0.45). However, the rates of each complication were still low in both groups.

What does current guidance say on this issue?

NICE antenatal care guidelines for uncomplicated pregnancies state that moderate exercise during pregnancy is not associated with adverse outcomes. It highlights the potential dangers of contact sports and high-impact sports which could cause joint injuries or could damage the abdomen, and therefore an unborn child. It advises against diving, which could cause foetal birth defects.

Royal College of Obstetricians and Gynaecologists’ guidance similarly supports aerobic and strength-conditioning exercises as part of a healthy pregnancy, but to avoid anything that may risk falls or foetal trauma. They also advise avoiding exercising on the back after 16 weeks of pregnancy.

What are the implications?

This high quality review supports current guideline recommendations on exercise during pregnancy. Healthcare professionals may advise pregnant women with no medical complications to take part in safe, moderate exercise on a regular basis and it won’t increase their risk of a preterm birth.

Furthermore, exercise might benefit both mother and baby, making a vaginal delivery more likely and possibly reducing the chance of developing some pregnancy-related complications.

Further studies may help to clarify the best type of activity and its frequency and intensity, as well as assessing the effect of exercise in the different trimesters of pregnancy.

Citation and Funding

Di Mascio D, Magro-Malosso ER, Saccone G et al. Exercise during pregnancy in normal-weight women and risk of preterm birth: a systematic review and meta-analysis of randomized controlled trials. Am J Obstet Gynecol. 2016. [Epub ahead of print].

No funding information was provided for this study.

Bibliography

Kramer MS, McDonald SW. Aerobic exercise for women during pregnancy. Cochrane Databse Syst Rev 2006: CD000180.

National Statistics. Regional Profile. East Midlands: selected key statistics. Newport: Office for National Statistics; updated 2008.

NICE. Antenatal care for uncomplicated pregnancies. CG62 London: National Institute for Health and Care Excellence; 2016.

RCOG. Exercise in Pregnancy. Statement Number 4. Royal College of Obstetricians and Gynaecology. London; 2015.

Tommy’s. Premature birth statistics. London: Tommy’s; 2016.

WHO. Fact Sheet Number 363. Preterm birth. Geneva: World Health Organization; 2015.

Wolfe L, Hall P, Webb K, et al. Prescription of aerobic exercise during pregnancy. Sports Med 1989;8:273-301.

Why was this study needed?

Preterm birth, where a baby is born before 37 weeks of pregnancy, is the leading cause of new born deaths globally. In the UK 60,000 babies are born prematurely and in 40% of these cases the cause is not known.

Women who have no medical complications in their pregnancy are currently advised that moderate, aerobic, non-contact exercise benefits their health and is unlikely to harm their unborn baby. However, some studies have suggested that exercise could cause preterm delivery, possibly by stimulating the womb muscle or altering blood flow in the placenta. Therefore the topic remains controversial.

This systematic review and meta-analysis aimed to shed more light on the safety of exercise in pregnancy, and see whether women with an uncomplicated pregnancy who exercise regularly are more likely to give birth prematurely.

What did this study do?

Nine randomised clinical trials were identified, including 2059 women pregnant with one baby, who had no complications and were of normal body mass index.

The trials compared the rate of preterm births in women who were assigned to regular aerobic activity before 23 weeks of pregnancy with those that weren’t. The type of exercise differed between trials, but all sessions lasted between 35 to 90 minutes and were given three to four times per week for ten weeks or until delivery.

Other outcomes of interest included type of delivery (vaginal or caesarean) and pregnancy diabetes or high blood pressure.

The trials had low risk of bias in terms of randomisation method and data collection, although it is unclear whether assessors were aware of group allocation. The studies had similar methods and could be reliably pooled in meta-analysis. None were UK based.

What did it find?

  • The preterm birth rate was no different in women who exercised during pregnancy compared with women who didn’t exercise (4.5% vs 4.4%; relative risk [RR] 1.01, 95% confidence interval [CI] 0.68 to 1.50).
  • Vaginal delivery was significantly more common among women who exercised (73.6% vs. 67.5%; RR 1.09, 95% CI 1.04 to 1.15), fewer of whom needed a caesarean section (17.9% vs. 22%; RR 0.82, 95% CI, 0.69 to 0.97).
  • Compared to women who didn’t exercise, fewer women who exercised developed pregnancy diabetes (2.9% vs. 5.6%; RR 0.51, 95% CI 0.31 to 0.82) or high blood pressure disorders (1.0% vs. 5.6%; RR 0.21, 95% CI, 0.09 to 0.45). However, the rates of each complication were still low in both groups.

What does current guidance say on this issue?

NICE antenatal care guidelines for uncomplicated pregnancies state that moderate exercise during pregnancy is not associated with adverse outcomes. It highlights the potential dangers of contact sports and high-impact sports which could cause joint injuries or could damage the abdomen, and therefore an unborn child. It advises against diving, which could cause foetal birth defects.

Royal College of Obstetricians and Gynaecologists’ guidance similarly supports aerobic and strength-conditioning exercises as part of a healthy pregnancy, but to avoid anything that may risk falls or foetal trauma. They also advise avoiding exercising on the back after 16 weeks of pregnancy.

What are the implications?

This high quality review supports current guideline recommendations on exercise during pregnancy. Healthcare professionals may advise pregnant women with no medical complications to take part in safe, moderate exercise on a regular basis and it won’t increase their risk of a preterm birth.

Furthermore, exercise might benefit both mother and baby, making a vaginal delivery more likely and possibly reducing the chance of developing some pregnancy-related complications.

Further studies may help to clarify the best type of activity and its frequency and intensity, as well as assessing the effect of exercise in the different trimesters of pregnancy.

Citation and Funding

Di Mascio D, Magro-Malosso ER, Saccone G et al. Exercise during pregnancy in normal-weight women and risk of preterm birth: a systematic review and meta-analysis of randomized controlled trials. Am J Obstet Gynecol. 2016. [Epub ahead of print].

No funding information was provided for this study.

Bibliography

Kramer MS, McDonald SW. Aerobic exercise for women during pregnancy. Cochrane Databse Syst Rev 2006: CD000180.

National Statistics. Regional Profile. East Midlands: selected key statistics. Newport: Office for National Statistics; updated 2008.

NICE. Antenatal care for uncomplicated pregnancies. CG62 London: National Institute for Health and Care Excellence; 2016.

RCOG. Exercise in Pregnancy. Statement Number 4. Royal College of Obstetricians and Gynaecology. London; 2015.

Tommy’s. Premature birth statistics. London: Tommy’s; 2016.

WHO. Fact Sheet Number 363. Preterm birth. Geneva: World Health Organization; 2015.

Wolfe L, Hall P, Webb K, et al. Prescription of aerobic exercise during pregnancy. Sports Med 1989;8:273-301.

Exercise during pregnancy in normal-weight women and risk of preterm birth: a systematic review and meta-analysis of randomized controlled trials

Published on 21 June 2016

Di Mascio, D.,Magro-Malosso, E. R.,Saccone, G.,Marhefka, G. D.,Berghella, V.

Am J Obstet Gynecol , 2016

BACKGROUND: Preterm birth (PTB) is the major cause of perinatal mortality in the United States. In the past, pregnant women have been recommended against exercise because of presumed risks of PTB. Physical activity has been theoretically related to PTB as it increases the release catecholamines, especially norepinephrine, which might stimulate myometrial activity. Conversely, exercise may reduce the risk of PTB by other mechanisms such as decreased oxidative stress or improved placenta vascularization. Therefore, the safety of exercise regarding PTB and its effects on gestational age at delivery remain controversial. OBJECTIVE: To evaluate the effects of exercise during pregnancy on the risk of PTB. DATA SOURCES: MEDLINE, EMBASE, Web of Sciences, Scopus, ClinicalTrial.gov, OVID and Cochrane Library were searched from the inception of each database to April 2016. METHODS OF STUDY SELECTION: Selection criteria included only randomized clinical trials (RCTs) of pregnant women randomized before 23 weeks to an aerobic exercise regimen or not. Types of participants included women mainly of normal weight with uncomplicated, singleton pregnancies without any obstetric contraindication to physical activity. The summary measures were reported as relative risk (RR) or as mean difference (MD) with 95% confidence intervals (CI). The primary outcome was the incidence of PTB <37weeks. TABULATION, INTEGRATION, AND RESULTS: Of the 2,059 women included in the meta-analysis, 1,022 (49.6%) were randomized to the exercise group and 1,037 (50.4%) to the control group. Aerobic exercise lasted about 35-90 minutes 3-4 times per week. Women who were randomized to aerobic exercise had a similar incidence of PTB<37 weeks (4.5% vs 4.4%; RR 1.01, 95% CI 0.68-1.50) and a similar mean gestational age at delivery (MD 0.05 week, 95% CI - 0.07 to 0.17) compared to controls. Women in the exercise group had a significantly higher incidence of vaginal delivery (73.6% vs 67.5%; RR 1.09, 95% CI 1.04-1.15) and significantly lower incidence of cesarean delivery (17.9% vs 22%; RR 0.82, 95% CI 0.69-0.97) compared to controls. The incidence of operative vaginal delivery (12.9% vs 16.5%; RR 0.78, 95% CI 0.61-1.01) was similar in both groups. Women in the exercise group had a significantly lower incidence of gestational diabetes mellitus (2.4% vs 5.9%; RR 0.41, 95% CI 0.24-0.68) and significantly lower incidence of hypertensive disorders (1.9% vs 5.1%; RR 0.36, 95% CI 0.19-0.69) compared to controls. No differences in low birth weight (5.2% vs 4.7%; RR 1.11, 95% CI 0.72-1.73) and mean birth weight (MD -10.46 grams, 95% CI -47.10 to 26.21) between exercise group and controls were found. CONCLUSION: Aerobic exercise for 35-90 minutes 3-4 times per week during pregnancy can be safely performed by normal-weight women with singleton, uncomplicated gestations, as this is not associated with an increased risk of PTB or with a reduction in mean gestational age at delivery. Exercise was associated with a significantly higher incidence of vaginal delivery and a significantly lower incidence of cesarean delivery, with a significantly lower incidence of gestational diabetes mellitus and hypertensive disorders, and therefore should be encouraged.

Expert commentary

This publication has a very positive message. It should be used by midwives and obstetricians to encourage all normal-weight women to take aerobic exercise regularly in pregnancy in order to reduce obstetric complications and to increase their chances of a vaginal birth.

Women who already exercise regularly will welcome this message as it reinforces the fact that they should continue to enjoy all the recognised benefits of exercise. Women who are not in the habit of regular exercise may start exercising during pregnancy as a result of this message, with resultant life-long benefits.

Dr Rhona Hughes, Clinical Director of Obstetrics, Royal Infirmary, Edinburgh

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