In prior decades, pregnant women were counseled to limit all physical activity during pregnancy. Over time, this recommendation has fallen by the wayside as more and more evidence has found that exercise and physical activity during pregnancy have a host of positive effects on maternal health (both during pregnancy and post-partum), with minimal risk (and likely benefits) for the fetus. This has been a gradual change, with each decade bringing new guidelines for pregnant women that allow (or even promote) more exercise, more types of exercise, and higher intensities of exercise throughout each stage of the pregnancy (2). However, current recommendations still caution against very heavy resistance training, supine exercise, and the use of the valsalva maneuver during pregnancy.
Heavy resistance training and the use of the valsalva maneuver are cautioned against due to concerns related to extremely large (but transient) increases in blood pressure, with fears that these acute increases in blood pressure may harm the fetus. Heavy resistance training is further cautioned against due to concerns about injury risk – during pregnancy, hormonal changes increase the laxity of tendons and ligaments to prepare the birth canal for delivery, and high joint laxity is likely a risk factor for ligament injuries (sprains and tears; 3). Furthermore, supine exercise is cautioned against, because the weight of the fetus may compress major blood vessels in the abdomen (the inferior vena cava and aorta), which may result in hypotension (concerningly low blood pressure) for the mother, and reduced fetal oxygenation.
It’s noteworthy that these guidelines aren’t based on research conclusively demonstrating that heavy resistance training, the use of the valsalva maneuver, and supine exercise are definitely dangerous during pregnancy, and result in worse outcomes for the mother or the fetus. Rather, medical advice is (very understandably) conservative by default. If there’s reason to think something might be dangerous, and there’s insufficient evidence to allay those fears, the activity, behavior, or treatment in question is cautioned against. This general orientation is in keeping with the maxim “First, do no harm” (4).
When researchers or doctors think that a current medical guideline might be too conservative, there’s generally a two-step process toward changing it. You generally can’t start with randomized controlled trials, because ethics boards are also inherently risk-averse, and understandably so. If the conventional wisdom is that a particular behavior or treatment is likely to increase the risk of serious health consequences, but you think the behavior or treatment is likely safe, an RCT would have one of two outcomes: either you’re right, and the subjects retrieving the treatment are alright, or you’re wrong, and your study results in serious (avoidable) health consequences for your subjects. Ethics boards exist to protect the rights and safety of research subjects, and to minimize legal liability for research institutions, so they’re unlikely to greenlight a controlled study on a treatment that most experts expect to result in serious harm.
So, the first step toward overturning an excessively conservative recommendation generally involves uncontrolled cohort studies. In these studies, you find people who already intend to engage in behaviors that are believed to be dangerous, and observe the effects of their volitional actions. With this type of study, researchers aren’t instructing subjects to partake in allegedly dangerous behavior, thus limiting the liability of the researchers and the university or hospital approving the study. However, the researchers can still observe whether people engaging in allegedly dangerous behaviors (of their own volition) have worse outcomes than people not engaging in those behaviors. If researchers conduct a few observational studies on a particular topic, and they consistently observe that an allegedly dangerous behavior doesn’t appear to result in the predicted deleterious outcomes, then an ethics board will generally be a bit more comfortable with greenlighting controlled studies on the topic.
So, with all of this in mind, a recent study by Prevett and colleagues aimed to assess the risk of continued heavy weight training during pregnancy (1). The final paragraph of the introduction does a great job of summarizing the purpose of the study: “Given the distinct lack of empirical evidence supporting (or not) the safety of heavy weightlifting (including supine and Olympic weightlifting) and the Valsalva maneuver, the aim of the present study was to examine whether women who continued to participate in heavy resistance training during pregnancy (>80% 1 repetition maximum [RM]) were at a heightened risk for adverse birth outcomes, adverse fetal outcomes, or pelvic floor dysfunction.”
To this end, the researchers recruited lifters to complete a questionnaire. To complete the questionnaire, subjects needed to have engaged in resistance training during pregnancy with loads exceeding 80% of 1RM, and to be at least 18 years old. The researchers ultimately recruited 679 lifters via word of mouth, social media, and their own personal networks. The questionnaire consisted of 60 questions about “maternal demographics, sport history, reproductive history, pregnancy health outcomes, and pre-pregnancy/pregnancy/postpartum aerobic and resistance training characteristics,” taking 20-30 minutes to complete.
Within this sample, some subjects continued lifting heavy throughout pregnancy, and others dialed back their training. Some subjects used the Valsalva maneuver, and others didn’t. Some continued weightlifting during pregnancy, and others didn’t. Some engaged in supine weightlifting during pregnancy, and others didn’t. The researchers performed a number of analyses to see whether these training characteristics were associated with differing maternal and fetal outcomes during and following pregnancy (for instance, analyses designed to determine whether the women who performed supine weight training during pregnancy had higher rates of pregnancy complications than the women who didn’t).
There are literally hundreds of individual outcomes in the present study, so I’m just going to report the findings in broad strokes, much like the authors did in the paper. All of the granular findings are presented in a digital appendix, if you’d like to dig in.
The respondents were mostly in the late 20s or early 30s (age = 29.8 ± 3.9 years), with the majority training for CrossFit (61%) and/or weightlifting (49%; 5), with plenty of crossover. 13% of the respondents didn’t train for either CrossFit or weightlifting. Overall, 85% of subjects continued engaging in heavy training during pregnancy, with 24% maintaining their training levels until delivery (6). Furthermore, 72% continued weightlifting training during pregnancy, 71% did supine resistance training during pregnancy, and 34% used the Valsalva maneuver during pregnancy.
General rates of pregnancy, delivery, and postpartum complications were generally at or below rates observed in the general population. Specifically, the subjects in this study had lower rates of gestational hypertension (3% versus ~7%; 7), preeclampsia (3%, versus ~5%; 8), gestational diabetes (1%, versus ~8%; 9), and postpartum depression and anxiety (~7% versus ~17%; 10) than the general population of pregnant women. However, rates of postpartum urinary incontinence were higher than those observed in the general population (57% versus ~33%; 11), though rates of incontinence didn’t seem to be predicted by the exercise-related variables in this study. You can see the reported rates of all pregnancy, delivery, and post-partum outcomes in the digital appendix.
Olympic weightlifting training, use of the Valsalva maneuver, and supine training didn’t seem to meaningfully affect rates of pregnancy and delivery complications. However, women who maintained their pre-pregnancy levels of training up to delivery had ~50% lower rates of pregnancy and delivery complications than women who scaled back their training (OR = 0.49; 95% CI = 0.29-0.81).
There are two things to keep in mind when interpreting this study, since this is observational research.
First, women who do heavy resistance training are a self-selected subset of all women, women who continue performing heavy resistance training during pregnancy are a self-selected subset of women who perform heavy resistance training generally, and women who completed this questionnaire are a self-selected subset of women who continued performing heavy resistance training during pregnancy. At all three levels of self-selection, there are factors in play that could influence rates of pregnancy, delivery, and postpartum complications. Performing heavy resistance training is likely associated with other health-promoting behaviors (that would predispose women toward having a healthy pregnancy). Maintaining exercise habits during pregnancy may be related to maintaining other behaviors that would promote a healthy pregnancy and delivery. There may also be some participation bias – for example, women who didn’t follow their doctors’ advice about avoiding the Valsalva maneuver, and who had serious pregnancy complications, may have been less likely to participate in the survey than women who didn’t follow their doctors’ advice while still having a healthy pregnancy and delivery. In short, this is observational research, so it can’t be used to draw causal inferences. We can observe the pregnancy outcomes of the women who participated in this study, but we can’t directly attribute those outcomes to the training-related decisions these women made.
Second, if there are causal relationships in play, we can’t determine the direction of causation. For example, we don’t know if women who maintained their level of training through pregnancy had lower rates of complications than women who scaled back their training because they maintained their level of training. It’s also possible that some women scaled back their level of training because they had pregnancy complications. Similarly, we don’t know if rates of postpartum depression were lower in these respondents than the general population because they (mostly) continued exercising through pregnancy. It’s also possible that women who are more predisposed to postpartum depression are less likely to engage in heavy resistance training due to a shared causal factor.
With those caveats out of the way, here’s the major takeaway: this study suggests that heavy training, supine training, and use of the Valsalva maneuver may not be quite as dangerous during pregnancy as is commonly believed. However, since this is observational research, we need randomized controlled trials to verify and validate these observations. As mentioned in the intro, moving past excessively conservative medical recommendations is a two-step process (observational research, followed by experimental research). The presently reviewed study is (part of) the first step of this process, but the second step is also crucially important – observational findings frequently fail to be confirmed by experimental research. So, don’t expect the present study to result in sweeping changes to OB/GYN’s exercise recommendations overnight. That would be wildly premature. But do expect to see experimental research examining the longitudinal effects of more strenuous resistance training during pregnancy during the next few years. If those studies have similarly positive results, medical experts may start changing exercise guidelines to allow for more strenuous resistance training during pregnancy. Until we have that experimental data, however, we simply don’t know if the current recommendations are excessively conservative, or appropriately conservative.
Before wrapping up, I just want to make a quick note about urinary incontinence. Female lifters experience rates of urinary incontinence that exceed those observed in the general population, female lifters who’ve given birth experience higher rates of urinary incontinence than female lifters who haven’t given birth, and female lifters who’ve had more kids experience higher rates of urinary incontinence than female lifters who’ve had fewer kids (12). However, it’s not entirely clear that heavy training is actually a risk factor for developing urinary incontinence. In the present study, type and intensity of training weren’t predictive of rates of postpartum urinary incontinence. Furthermore, in a prior study, training age and absolute strength levels weren’t predictive of rates of urinary incontinence in female powerlifters (12). Thus, it seems likely that lifters report higher rates of urinary incontinence than the general population because many people in the general population rarely (if ever) strain hard enough to have an incident of incontinence. In other words, if 30% of women occasionally experience a bit of urinary incontinence in day-to-day life, and an additional 20% of women (who don’t experience incontinence in day-to-day life) would also experience a bit of urinary incontinence when physically straining to their absolute limits, then about 30% of women in the general population would report urinary incontinence, and 50% of serious female lifters would report urinary incontinence. However, that wouldn’t mean that lifting had any underlying effect on the physiological mechanisms of urinary incontinence – it would just mean that anyone is more likely to experience a bit of incontinence when they strain really hard. In other words, the higher reported prevalence of urinary incontinence in serious lifters doesn’t necessarily imply that lifting contributes to the new development of urinary incontinence in activities where you wouldn’t experience incontinence otherwise.
I’ll close with an important disclaimer: I’m not a doctor, and I’m certainly not an obstetrician. I don’t believe I’ve made any statements in this article that would amount to advice for pregnant women, nor would I claim to be qualified to give advice to pregnant women that conflicts with anything their doctor has told them, or guidelines published by The American Council of Obstetricians and Gynecologists (ACOG). If you’d like to learn more about exercise during pregnancy, I’d highly recommend checking out ACOG’s recommendations and guidelines (2).
Note: This article was published in partnership with MASS Research Review. Full versions of Research Spotlight breakdowns are originally published in MASS Research Review. Subscribe to MASS to get a monthly publication with breakdowns of recent exercise and nutrition studies.
- Prevett C, Kimber ML, Forner L, de Vivo M, Davenport MH. Impact of heavy resistance training on pregnancy and postpartum health outcomes. Int Urogynecol J. 2022 Nov 4. doi: 10.1007/s00192-022-05393-1. Epub ahead of print. PMID: 36331580.
- Physical Activity and Exercise During Pregnancy and the Postpartum Period: ACOG Committee Opinion, Number 804. Obstet Gynecol. 2020 Apr;135(4):e178-e188. doi: 10.1097/AOG.0000000000003772. PMID: 32217980.
- Myer GD, Ford KR, Paterno MV, Nick TG, Hewett TE. The effects of generalized joint laxity on risk of anterior cruciate ligament injury in young female athletes. Am J Sports Med. 2008 Jun;36(6):1073-80. doi: 10.1177/0363546507313572. Epub 2008 Mar 7. PMID: 18326833; PMCID: PMC3407802.
- This phrase is often assumed to be a part of the Hippocratic oath. Though the sentiment is certainly present, this precise phrase is not. Instead, it likely originated in a speech by physician Thomas Inman in the 1800s.
- In this article, “weightlifting” refers to the sport of weightlifting, consisting of the snatch and clean & jerk. “Weightlifting” is not used as a catch-all term for resistance training.
- I may have missed it, but I don’t think the phrase “training level” is operationally defined in the study or the supplementary material. I assume it refers to the general volume, intensity, and consistency of someone’s training, but I’m not entirely sure.
- Butwick AJ, Druzin ML, Shaw GM, Guo N. Evaluation of US State-Level Variation in Hypertensive Disorders of Pregnancy. JAMA Netw Open. 2020 Oct 1;3(10):e2018741. doi: 10.1001/jamanetworkopen.2020.18741. PMID: 33001203; PMCID: PMC7530635.
- Wang W, Xie X, Yuan T, Wang Y, Zhao F, Zhou Z, Zhang H. Epidemiological trends of maternal hypertensive disorders of pregnancy at the global, regional, and national levels: a population-based study. BMC Pregnancy Childbirth. 2021 May 8;21(1):364. doi: 10.1186/s12884-021-03809-2. PMID: 33964896; PMCID: PMC8106862.
- Zhou T, Du S, Sun D, Li X, Heianza Y, Hu G, Sun L, Pei X, Shang X, Qi L. Prevalence and Trends in Gestational Diabetes Mellitus Among Women in the United States, 2006-2017: A Population-Based Study. Front Endocrinol (Lausanne). 2022 Jun 6;13:868094. doi: 10.3389/fendo.2022.868094. PMID: 35733768; PMCID: PMC9207520.
- Wang Z, Liu J, Shuai H, Cai Z, Fu X, Liu Y, Xiao X, Zhang W, Krabbendam E, Liu S, Liu Z, Li Z, Yang BX. Mapping global prevalence of depression among postpartum women. Transl Psychiatry. 2021 Oct 20;11(1):543. doi: 10.1038/s41398-021-01663-6.
- Thom DH, Rortveit G. Prevalence of postpartum urinary incontinence: a systematic review. Acta Obstet Gynecol Scand. 2010 Dec;89(12):1511-22. doi: 10.3109/00016349.2010.526188. Epub 2010 Nov 5. PMID: 21050146.
- Wikander L, Kirshbaum MN, Waheed N, Gahreman DE. Urinary Incontinence in Competitive Women Powerlifters: A Cross-Sectional Survey. Sports Med Open. 2021 Dec 7;7(1):89. doi: 10.1186/s40798-021-00387-7. PMID: 34874496; PMCID: PMC8651931.