By Lauren Bartell Weiss, Ph.D.
Regular exercise is an important lifestyle for many women, for aesthetics, overall health, and mental health benefits. The question still remains as to how much and what type of exercise is beneficial during pregnancy and if too much or the wrong type of exercise can be harmful to the mother or the baby. While light-to-moderate cardiovascular exercise has been shown to be beneficial, the potential impact of heavy weight lifting is still unknown. New, intense exercises involving heavy weight lifting programs, such a CrossFit, have now raised concerns about the possible effect to mom and baby during pregnancy.
Though the research is scarce, it has been suggested that heavy exercise, whether cardiovascular or strength training, during pregnancy can have detrimental effects on both the mom and the baby. When avid exercisers or athletes consult their OBGYN or other health care providers for advice, they are unable to receive evidence-based responses because there is insufficient data and no specific evidence-based recommendations on exercise during pregnancy have been established. Most of the research has been performed on strenuous “cardiovascular” exercise and not on strenuous “strength training” or “weight lifting”. Epidemiologic studies have long suggested a link exists between strenuous physical activities and the development of intrauterine growth restriction, and this is particularly true for pregnant women engaged in physical work (e.g., lifting). A few studies, mostly performed in third world countries where occupational lifting during pregnancy is common, have found that heavy lifting is associated with increased risk of miscarriage in early pregnancy, increased pelvic pain, prematurity, low birth weight/ intrauterine growth restriction, mostly likely resulting from intermittent but prolonged reductions in blood flow to the uterus.
The state of pregnancy is associated with many physiological changes in mom which in turn, create changes to baby during exercise. While there are major changes in cardiovascular and respiratory functions at rest and during exercise in pregnant women, the changes in the musculoskeletal system may place a pregnant women at risk for severe injury with heavy lifting. During pregnancy, there is a progressive increased curving of the spine causing a displacement in the woman’s center of gravity. To compensate for this, pregnant women increase the anterior flexion of the cervical spine and abduct the shoulders. In addition, due to a hormone known as relaxin, there is increased laxity of joints and ligaments, especially the spine, sacrum and ilium joints, pubic bone joints, knees, and ankles, all increasing the risk of serious injuries. Pregnancy is also associated with metabolic changes as exercise causes blood glucose levels to drop, most likely as a result of simultaneous uptake of glucose by the developing baby and the exercising muscles. Most importantly though, are the potential risks to baby, such as changes in heart rate and blood flow while mom is exercising. The main concern about heavy exercise in pregnancy is that reduced blood flow to the uterus may cause hypoxia (deprivation of oxygen) to the baby. Muscular activity alters blood distribution in the body, and circulatory blood flow in the uterus and the placenta decreases in the standing position. Pronounced physical exercise may lead to hormonal disturbances, hyperthermia and nutritional deficits, all of which may have negative effects on the baby. Heavy lifting increases the intra-abdominal pressure and this may provoke uterine contractions or mild abdominal trauma.
The questions most likely to be asked concerning exercise and pregnancy are whether or not exercise affects the growth of the baby or causes the baby to be distressed or starts premature labor. There are 4 potential adverse baby outcomes from maternal exercise in pregnancy:
- Risk of congenital malformations
- Risk of physical injury to the fetus
- Effects of exercise in pregnancy on growth
- Risk of premature labor
For the mother, the potential adverse effects of exercise are few — there may be an increased risk of physical injury from the increased ligament laxity which may affect joint stability and an increased risk of hemorrhage. However, for the growing baby, there may be several unknown risks.
The most recent ACOG guidelines are listed below. These recommendations are made for women who do not have any additional risk factors for adverse maternal or perinatal outcome:
American College of Obstetricians and Gynecologists’ Guidelines for Exercise During Pregnancy and Postpartum
|1. Regular exercise (at least three times per week) is preferable to intermittent activity.|
|2. Avoid exercise in the supine position after the first trimester. This position is associated with decreased cardiac output in most pregnant women, causing a decreased distribution of blood to splanchnic beds including the uterus.|
|3. Pregnant women should stop exercising when fatigued and not exercise to exhaustion.|
|4. Non–weight-bearing exercises such as cycling or swimming will minimize the risk of injury and facilitate the continuation of exercise during pregnancy.|
|5. Adequate diet should be ensured.|
|6. Avoid types of exercise in which loss of balance could be detrimental to maternal or fetal well-being, especially in the third trimester. Further, any type of exercise involving the potential for even mild abdominal trauma should be avoided.|
|7. Adequate hydration, appropriate clothing, and optimal environmental surroundings during exercise should be ensured.|
|8. The physiologic and morphologic changes of pregnancy persist 4–6 weeks postpartum. Thus, prepregnancy exercise routines should be resumed gradually based on a woman’s physical capability.|
Warning signs to stop exercise during pregnancy include: vaginal bleeding, dyspnea prior to exertion, dizziness, headache, chest pain, muscle weakness, calf pain or swelling (need to rule out blood clot), preterm labor, decreased fetal movement, and amniotic fluid leakage.
Many women stop exercising during pregnancy because of worries regarding the well-being of their baby. Although pregnancy is associated with several physiologic changes and response to exercise is different in the pregnant state than in the non-pregnant state, exercise can be beneficial for generally healthy moms-to-be (who are free of obstetric or medical complications). Scientific research suggests that light-to-moderate exercise during pregnancy is safe, but the jury is still out on the potential detrimental effects to the baby after heavy lifting. This is possibly due to a reduction in blood flow from heavy exertion. While exercise can definitely be part of a healthy pregnancy, including even structured programs like CrossFit, caution should be taken with heavy lifting routines that can compromise the baby’s health and have potentially life-long effects on baby.
Expectant mothers should consult with their care provider to help weigh the pros and cons of this type of exercise during pregnancy on their own health and the health of their baby, and use their best judgment and gut-instinct.
Lauren Bartell Weiss, PhD, is a Postdoctoral Research Fellow at UC San Diego’s Center for the Promotion of Maternal Health and Infant Development and former American College of Sports Medicine-certified trainer.
For more information on exercise during pregnancy, visit MotherToBaby.org. MotherToBaby has a library of fact sheets about the risks of various exposures during pregnancy and breastfeeding. For more information or to get a personalized risk assessment about exercise, medications or other exposures, call MotherToBaby toll-free at (866) 626-6847. MotherToBaby is a service of the Organization of Teratology Information Specialists (OTIS). MotherToBaby and OTIS are suggested resources by many agencies including the Centers for Disease Control and Prevention (CDC).
- Exercise during pregnancy and the postpartum period: Number 267, January 2002 Committe on Obstetric Practice, International Journal of Gynecology & Obstetrics, Volume 77, Issue 1, April 2002, Pages 79–81