Pushing limits: the acute effects of a heavy-load resistance protocol and supine exercise on fetal well-being

Abstract

Objectives

To explore the acute effects of a heavy-load resistance protocol and exercise in the supine position on fetal heart rate (FHR) and uteroplacental blood flow.

Method

In this experimental laboratory study, 48 healthy pregnant athletes (elite: n=7; recreational: n=41) completed 3×8 repetitions with one repetition in reserve in sumo deadlift, bench press and incline bench press. FHR and umbilical and uterine artery pulsatility index (PI) were assessed before and after exercise. Symptoms of vena cava syndrome were recorded following the supine exercise.

Results

Pre- and post-exercise FHR remained within the normal range (110–160 bpm), but there were statistically significant increases after sumo deadlifts (p=0.002) and incline bench press (p=0.008), with no significant change after bench press (p=0.122). Umbilical artery PI did not change, while uterine artery PI significantly decreased after the sumo deadlift (p=0.005), bench press (p<0.001) and incline bench press (p<0.001). No fetal bradycardia was observed after the sumo deadlift or incline bench press, but one case occurred after bench press, resolving shortly after 3 min. Fetal tachycardia (>160 bpm) occurred in four cases after sumo deadlift, five after bench press and three after incline bench press, though none exceeded pathological levels (>180 bpm). One recreational athlete experienced symptoms of vena cava syndrome after the supine exercise, coinciding with a slight decrease in umbilical artery PI, but with stable FHR.

Conclusion

The results indicate that highly active women, especially those with resistance training experience, can safely engage in a heavy-load resistance protocol, including supine exercise, without jeopardising fetal well-being.

WHAT IS ALREADY KNOWN ON THIS TOPIC

• Muscle-strengthening activities with light-to-moderate intensity are recommended by global physical activity guidelines to support maternal and fetal health.

• In contrast, heavy-load resistance training and supine exercise are generally discouraged due to potential risks during pregnancy.

WHAT THIS STUDY ADDS

• This is the largest study to date on the acute effects of heavy-load resistance training on fetal heart rate and uteroplacental blood flow.

• The results suggest that engaging in heavy-load resistance training, including supine exercise, does not pose a risk to fetal well-being in healthy, active pregnant women.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

• These results will help athletes, coaches and healthcare professionals to develop safe and effective training strategies during pregnancy.

• The results will inform the development of evidence-based guidelines for the inclusion of heavy-load resistance training in prenatal exercise programmes.

Introduction

Women’s participation in strength-focused sports such as CrossFit, Olympic weightlifting and powerlifting has increased significantly over the last decade,1,3 reflecting a broader cultural shift toward women embracing strength training for performance, health and fitness.⁴

Current exercise guidelines recommend muscle-strengthening activities with light-to-moderate intensity at least 2 days per week during pregnancy,5,9 offering significant maternal and fetal health benefits.¹⁰ However, heavy-load resistance training is generally not advised^{5 8} particularly when combined with the Valsalva manoeuvre, which is a natural breath-holding response to intense exertion.¹¹ This manoeuvre causes significant increases in blood pressure and intra-abdominal pressure,¹² which may reduce maternal cardiac output and hence decrease uteroplacental blood flow, potentially leading to fetal hypoxia.¹³ Notably, this advice is based on expert opinion rather than empirical evidence.¹⁴ Three studies have investigated the effects of the Valsalva manoeuvre during resistance exercise, reporting no adverse maternal or fetal outcomes.^{11 15 16} However, two of the studies used relatively low training loads, with a maximum of 50 lb (22.68 kg) for a one-repetition maximum (RM) in the incline bench press¹⁵ and 40% of a 10 RM in the seated leg press.¹⁶ In contrast, the third study applied higher loads, with participants performing 10 repetitions at 90% of 10RM using the Valsalva manoeuvre during barbell back squats, bench press and deadlifts.¹¹

Supine exercises, such as bench press, are commonly discouraged after 16 weeks of gestation due to the risk of compression of the inferior vena cava by the enlarged uterus while lying on the back, a condition known as vena cava syndrome.^{8 17} This may reduce maternal venous return and cardiac output, potentially decreasing blood flow to the fetus.¹⁸ Supine rest can lead to symptomatic hypotension in up to 10% of women,^{17 19} with symptoms such as maternal dizziness, nausea and light-headedness.¹⁷ Recent reviews indicate that most do not experience hypotensive symptoms despite this compression,²⁰ and current evidence remains insufficient to determine whether supine exercise is safe or should be avoided during pregnancy.¹⁸ To reduce risk, most guidelines advise modifying supine exercises, for example, by lying on the side.⁸

Due to the limited research on heavy-load resistance training during pregnancy, Prevett et al¹⁴ conducted an online survey of pregnant women who lifted at least 80% of their 1 RM and asked about the use of the Valsalva manoeuvre and supine weightlifting. Their findings suggested no negative impact on pregnancy or delivery outcomes such as gestational hypertension, premature labour or delivery mode. However, the study relied on retrospective self-reported data, which are subject to recall bias, and there were no physiological measures. Moolyk et al¹¹ examined maternal and fetal responses to acute resistance training at 70%–90% of 10 RM and concluded that the protocol was well tolerated. With only 20 participants, 10 pregnant and 10 non-pregnant, the study had limited generalisability and statistical power.

Despite this early evidence, empirical data on the safety of heavy-load resistance training during pregnancy remain sparse. Therefore, the aim of this study was to assess fetal well-being and symptoms of vena cava syndrome during a heavy-load resistance protocol, including supine exercise, with a significantly larger sample size than previous research in the field.

Method

Study design

This investigation was part of the Strong Mama project, an experimental study to explore fetal and maternal physiological responses to high-intensity exercise and a heavy-load resistance protocol in elite and recreational athletes. It was conducted in a laboratory setting at the Norwegian School of Sport Sciences in Oslo, Norway, from October 2022 to October 2023.

Participants and recruitment

Participation was limited to elite and recreational athletes between 26 and 35 weeks of gestation, with a singleton pregnancy and with the ability to understand verbal and written Norwegian or English. Following the consensus set by the IOC expert committee,⁹ we define elite athletes as individuals who are part of any national team or other high-level representative teams (eg, elite leagues for team sports such as handball and football) in any sport, as organised by a National Sports Federation. All elite athletes maintained a weekly exercise volume of ≥240 min throughout their pregnancies. Recreational athletes were defined as women who regularly exercised for fitness or competition, completing ≥240 min per week of exercise,²¹ which included high-intensity exercise and/or heavy-load resistance training (≥70% of 1 RM²²) for a minimum of 2 years prior to pregnancy. However, these women were not members of any high-level representative team. Participation in high-intensity exercise and/or heavy-load resistance training was self-reported. Athletes were not required to engage in both types of training but had to participate in at least one type regularly. There were no age or parity-related inclusion criteria. Before inclusion, all participants had a routine ultrasound at around 18 weeks of gestation, and women were excluded if they experienced any medical or obstetric contraindications to exercise.⁷

Elite athletes were recruited in collaboration with the Norwegian Olympic Sports Centre, national team doctors and support teams at the various sports federations. Recreational athletes were recruited through social media, newspaper articles, healthcare clinics in Oslo and word of mouth.

When this study was initiated, no research had been conducted on fetal well-being during heavy-load resistance training. Hence, there was no data to inform power calculations, making it challenging to determine the necessary sample size for statistical analysis. Given this limitation, we aimed for a convenience sample of 60 participants. In total, 212 women (12 elite and 200 recreational athletes) expressed interest in participating and were contacted by the research team. Of these, 28 were excluded as they no longer met the inclusion criteria of engaging in ≥240 min of exercise per week at 26–35 weeks of gestation. Ultimately, 57 women participated in the heavy-load resistance training. However, nine of these participants were part of a pilot study, and their data were excluded from the final analysis, leaving 48 athletes, 7 elite and 41 recreational, in this investigation.

Protocols and data collection

As part of the Strong Mama project, participants came to the laboratory on two separate days. On the first day, they completed a submaximal high-intensity interval session on a treadmill, consisting of 5×5 min intervals at a target intensity of 17 on the Borg RPE scale and 90% of maximal maternal heart rate. This session lasted approximately 2 hours. One or two days later, participants returned to complete a second interval session on a cycle ergometer, followed by the heavy-load resistance protocol.²³ The combined testing lasted about 2½ hours. Baseline measurements of fetal heart rate (FHR) and umbilical and uterine artery pulsatility index (PI) were obtained before cycling. Approximately 20 min after completing the cycle ergometer interval session, participants started the heavy-load resistance protocol. All sessions were supervised by an exercise physiologist, with a medical doctor present to ensure safety. Following the protocol, participants completed an electronic questionnaire that included items on parity, sports participation, weekly exercise duration and the number of hours dedicated to light, moderate and heavy-load resistance training.

Heavy-load resistance protocol

All participants reported being familiar with the exercises and had regularly included them in their training before and/or during pregnancy. Nevertheless, an exercise physiologist demonstrated each movement to ensure correct technique and safety. This was followed by a structured warm-up, during which the load was gradually increased to prepare for the resistance exercises and determine the appropriate weight for maintaining one repetition in reserve (RIR), meaning one repetition away from failure. This intensity was chosen as it exceeds current recommendations for resistance training during pregnancy.5,8

After a structured warm-up, participants completed three sets of eight repetitions in sumo deadlift, bench press and incline bench press (30° bench angle). Loads were individually adjusted to maintain one RIR, ensuring high yet controlled intensity corresponding to approximately 76% of 1 RM.²⁴ Each set was followed by a standardised 90 s rest period to allow partial recovery while sustaining workout intensity, in line with evidence for hypertrophic adaptations using moderate to heavy loads.²⁵ All lifts were closely supervised and spotted by an exercise physiologist. While the working period was not time-restricted, on average, each set of eight repetitions took 30 s. After completing three sets of one exercise, ultrasound assessments were conducted within 30 s and lasted approximately 4 min before participants proceeded directly to the next exercise. This procedure was repeated for all three exercises.

The sumo deadlift, bench press and incline bench press were selected because they are multi-joint, compound exercises that activate multiple muscle groups and enhance muscle engagement.²⁶ These exercises are commonly included in standard training routines and promote functional strength that directly translates to daily activities and sports performance.²⁶ The rationale for including both the bench press and incline bench press was to examine potential differences in the occurrence of symptoms of vena cava syndrome between exercises in the supine and incline positions.¹⁸ This was assessed by asking the open question: “Did the exercise make you feel dizzy, light-headed, nauseous, or otherwise unwell?”.

Ultrasound assessment

An obstetrician-gynaecologist (OBGYN) specialising in maternal-fetal medicine conducted the ultrasound assessments using a colour Doppler ultrasound at baseline and after each exercise. The assessments were completed with a Voluson device (V.E8, V.E10 or V.E22) and an XDclear Probe (GE Healthcare, Oslo, Norway). The athletes were in an inclined supine position (30°) during the ultrasound assessments. Baseline assessments included FHR, fetal growth, amniotic fluid volume, blood flow in the umbilical and uterine arteries, and biophysical profile to confirm that all measures were within normal parameters. A reference mark was placed at the probe location during baseline assessments to ensure consistent positioning for all measurements.

FHR and umbilical artery PI were assessed in a free loop of the umbilical artery within 30 s after each exercise. A normal Doppler waveform was required, with no reversed or absent flow. Normal FHR range for gestational age <32 weeks is 110–160 bpm. Post-exercise FHR was assessed, and if this was <110 bpm (fetal bradycardia) or >160 bpm (fetal tachycardia), this was recorded. The cut-off for stopping the protocol was set at bradycardia persisting >3 min, as durations shorter than this would be considered a deceleration rather than true bradycardia.²⁷ Fetal tachycardia was required to be below pathological levels of 180 bpm before the woman could continue to the next exercise.²⁸ The PI was evaluated based on the methods described by Acharya et al.²⁹

Uterine artery PI was measured bilaterally at the point where it crosses the internal iliac artery immediately after confirming FHR. Colour Doppler was used to identify the uterine arteries, and the insonation angle was kept as close as possible to zero (0°–30°). The uterine artery diameter was measured on both sides before exercise, with virtual callipers positioned on the inner vessel wall. The mean diameter was used throughout the ultrasound assessments.

For the assessment of fetal well-being, the first recorded FHR and umbilical artery PI values, along with the technically best recordings from each uterine artery, were used. After completing all three exercises, a final ultrasound measurement of FHR and umbilical artery PI was conducted following 10 min of rest. If the woman experienced any signs of concern or in the case of abnormal FHR (bradycardia>10 min), she was to be promptly referred to the gynaecology and obstetrics department at her birthing hospital or, if necessary, to the nearest hospital, which is located approximately 5 min away.

Patient and public involvement

This research was designed without direct involvement from the athletes in the initial planning phase. However, nine athletes participated in a pilot study to ensure participant safety, refine the protocol and optimise timing between exercises and assessments. Additionally, the medical team at the Norwegian Olympic Sports Centre provided valuable input on the study design, ensuring its practical and clinical relevance.

Statistical analyses

All statistical analyses were conducted using SPSS Statistical Software V.28 (IBM, Armonk, New York, USA). Descriptive statistics are presented as mean with SD or frequencies with percentage, as appropriate. To compare FHR, umbilical artery PI and uterine artery PI from baseline to after each exercise, paired samples t-tests were used. Mean differences (and 95% CIs of these differences) were assessed. Uterine artery PI is the mean of measurements from the left and right uterine arteries. Independent samples t-tests were used to compare fetal responses between participants who regularly engaged in heavy-load resistance training and those who did not. Due to the small number of elite athletes, statistically meaningful comparisons between elite and recreational athlete groups were not feasible. As a result, these data are presented descriptively without statistical inference.

Results

Participants

48 athletes participated in the study. Three elite athletes were endurance sports athletes, three were ball sports athletes and one competed in CrossFit. The recreational athletes primarily participated in resistance training (n=33) and running (n=31), with cycling (n=17), CrossFit (n=12) and cross-country skiing (n=11) also being common activities. One elite and two recreational athletes were unable to do the sumo deadlift due to pelvic girdle pain, while one recreational athlete did not complete the incline bench press due to fetal bradycardia after the bench press. During pregnancy, the athletes did strength training for an average of 2.8 (SD 1.3) hours per week (range: 0–6 hours), mostly with moderate (1.6 (0.8) hours) and high (0.9 (1.0) hours) loads. Participant characteristics are shown in table 1.

Table 1. Participant characteristics.

	Mean (SD)	Range
Age (years)	32.0 (3.2)	24–44
Weight	73.6 (8.2)	58.9–100.6
Gestational age at testing (weeks)	28.7 (2.4)	25–35
Pre-pregnancy BMI, kg/m2	22.5 (2.2)	18.8–28.3
Exercise hours/week	7.1 (3.2)	4–17
Parity	0.3 (0.6)	0–2
	N (%)
College/university-educated	46 (95.8)
Primiparous	36 (75.0)
Multiparous	12 (25.0)

Results are presented as mean with SD and range or frequencies with percentages (N=48).

BMI, body mass index.

Fetal response

Fetal responses to maternal heavy-load resistance training are shown in table 2. FHR was significantly higher after sumo deadlift than baseline values (p=0.002). The same was observed for incline bench press (p=0.008), but with a non-significant increase after bench press (p=0.122). Uterine artery PI decreased from baseline to after heavy-load resistance training (sumo deadlift, p=0.005; bench press, p<0.001; incline bench press, p<0.001). There were no changes in umbilical artery PI from baseline to post-exercise.

Table 2. Exercise load and fetal responses at baseline and after resistance exercises (sumo deadlift, bench press and incline bench press).

		Sumo deadlift		Bench press		Incline bench press
N		45		48		47
Absolute load (kg)		64.4 (16.6) (range: 33–105)		34.8 (9.0) (range: 20–63)		26.8 (6.7) (range: 15–50)
Relative load (% of body weight)		87.5 (21.4) (range: 47.9–135.8)		47.3 (10.8) (range: 30.3–77.7)		36.5 (8.2) (range: 23.6–61.6)

	Baseline	Post-exercise (mean (SD))	Mean difference (95% CI)	Post-exercise (mean (SD))	Mean difference (95% CI)	Post-exercise (mean (SD))	Mean difference (95% CI)
FHR (bpm)	140.6 (8.8)	147.2 (9.1)	6.7 (2.6 to 10.7)**	143.3 (15.3)	3.1 (−1.0 to 7.2) ns	145.1 (11.8)	4.7 (1.3 to 8.0)**
Uterine artery PI	0.7 (0.2)	0.6 (0.2)	−0.1 (−0.16 to –0.03)**	0.6 (0.2)	−0.1 (−0.17 to –0.05)***	0.6 (0.2)	−0.1 (−0.15 to –0.05)***
Umbilical artery PI	1.0 (0.2)	1.0 (0.2)	0.01 (−0.07 to 0.09) ns	1.0 (0.2)	0.00 (−0.05 to 0.05) ns	1.0 (0.2)	0.02 (−0.08 to 0.04) ns

**p value<0.05.

***p value<0.001.

Values are presented as means with SD, range and mean differences with 95% CIs.

FHR, fetal heart rate; ns, no significant difference; PI, pulsatility index.

FHR was lower during the sumo deadlift among athletes who regularly engaged in heavy-load resistance training (144.7 (SD 8.6), than in those who did not (151.3 (SD 8.7), p=0.02), with no differences during the bench press (143.3 (SD 11.1) vs 143.5 (SD 20.6), p=0.96) or incline bench press (144.6 (SD 12.3) vs 145.8 (SD 10.9), p=0.74). Umbilical artery PI did not differ during sumo deadlift (p=0.92) or bench press (p=0.25) but was lower in the resistance-trained group during the incline bench press (0.9 (SD 0.2) vs 1.1 (SD 0.3), p=0.01). No group differences were observed in uterine artery PI for any exercises (sumo deadlift: p=0.52; bench press: p=0.20; incline bench press: p=0.20).

Analyses of data from elite and recreational athletes found that fetal responses were similar during all exercises: (sumo deadlift FHR: elite: 146.5 (SD 3.4), recreational: 147.2 (SD 9.7); umbilical artery PI: elite: 1.0 (SD 0.2), recreational: 1.0 (SD 0.3); uterine artery PI: elite: 0.7 (SD 0.2), recreational: 0.6 (SD 0.2), bench press FHR: elite: 149.3 (SD 12.5), recreational: 142.3 (SD 15.4); umbilical artery PI: elite: 1.0 (SD 0.1), recreational: 1.0 (SD 0.2); uterine artery PI: elite: 0.6 (SD 0.1), recreational: 0.6 (SD 0.2) and incline bench press FHR: elite: 149.6 (SD 12.6), recreational: 144.3 (SD 11.6); umbilical artery PI: elite: 1.0 (SD 0.1), recreational: 1.0 (SD 0.2); uterine artery PI: elite: 0.6 (SD 0.1), recreational: 0.6 (SD 0.2)).

No cases of fetal bradycardia were observed following the sumo deadlift or incline bench press. However, one case occurred after bench press, where the FHR dropped to 78–91 bpm for 3 min before returning to normal range, and the uterine artery PI value increased from 0.6 to 0.7. As a result, the protocol was stopped. There were four cases of fetal tachycardia after sumo deadlift, five after bench press and three after incline bench press. None of these were above pathological levels (>180 bpm), and umbilical and uterine artery PI remained within the normal range.

Exercising in the supine position

There were no differences in mean FHR or umbilical and uterine artery PI between bench press and incline bench press (table 2). One recreational athlete experienced dizziness after completing the bench press exercise. In this case, FHR remained comparable to baseline (baseline: 148 bpm vs exercise: 146 bpm), while umbilical artery PI showed a slight decrease (baseline: 1.05 vs exercise: 0.86). She reported feeling fine shortly after sitting up. No other participants reported symptoms of vena cava syndrome.

Discussion

Strong Mama is the largest study to date aiming to assess the acute effects of a heavy-load resistance protocol and supine exercise on FHR and uteroplacental blood flow. Overall, the protocol, which consisted of three multi-joint compound exercises, was well tolerated by this sample of pregnant athletes who regularly incorporate high-intensity exercise and/or heavy-load resistance training into their routines. Between sets of heavy-load resistance exercises, FHR remained within the normal range and uterine blood flow was not compromised.

We observed one case of fetal bradycardia that resolved spontaneously shortly after 3 min. As this episode did not meet the clinical definition of bradycardia (FHR<110 bpm for 10 or more min),³⁰ it was not considered clinically significant. Brief FHR decelerations of this kind may reflect a physiologically protective response to maternal exercise or an early sign of transient hypoxia, which does not necessarily indicate fetal compromise.³¹ Given the short duration and spontaneous resolution of this episode, this isolated finding does not raise safety concerns within the context of our study. We also noted some transient fetal tachycardia episodes, all of which remained below pathological levels. Although one woman experienced dizziness after supine exercise, FHR remained stable and umbilical artery PI decreased, indicating improved fetal perfusion.

While participation by women in weight-based sports, for example, CrossFit, Olympic weightlifting and powerlifting, continues to increase, research on heavy-load resistance training during pregnancy remains limited.¹⁴ International guidelines advise against heavy lifting, based on evidence from occupational studies linking such activity to higher risks of preterm delivery and miscarriage.^{5 32} However, occupational lifting differs from recreational weightlifting in load distribution, repetition, recovery and overall physical demands. Occupational lifting often involves repetitive movements with minimal rest, which can lead to physical overload and adverse health effects.³³ In contrast, recreational weightlifting is performed in controlled sessions with adequate recovery between sets and sessions.¹⁴ Thus, extrapolating findings from occupational settings to heavy-load resistance training in pregnancy is problematic.

A recent online survey of 679 pregnant women who lifted a minimum of 80% of their 1 RM during pregnancy found similar or lower rates of pregnancy complications than population-based estimates.¹⁴ Findings from our pregnant athletes who regularly incorporate high-intensity exercise and/or heavy-load resistance training into their routines support this.³⁴ Compared with data from the Medical Birth Registry of Norway, our cohort had similar birthweights, gestational age at delivery and rates of post-term birth, with lower rates of pre-term births and low birth weight.³⁴ These results suggest that recreational heavy-load resistance training is generally well tolerated during pregnancy, with no negative impact on maternal or neonatal outcomes.

Consistent with other research, FHR remained within the normal range for most pregnant athletes between sets of the heavy-load resistance protocol.^{11 35} In contrast with our protocol, which primarily involved upper-body exercises, both Moolyk et al¹¹ and Avery et al³⁵ focused mainly on lower-body movements, like squats, deadlifts and leg extensions, which tend to put greater strain on the cardiovascular system and involve more muscle mass. The observation that FHR remains stable during both upper- and lower-body resistance training further supports the safety of heavy-load resistance exercise for fetal well-being. Notably, fetal responses appeared slightly more stable among those who regularly engaged in heavy-load resistance training, with no notable differences between elite and recreational athletes. This suggests that training experience, rather than performance level, may be a more meaningful predictor of physiological adaptations and tolerance to exercise during pregnancy.

We observed one case of fetal bradycardia, which occurred after bench press. Supine exercise is generally discouraged after 16 weeks of gestation due to vena cava compression, which can prevent blood from returning to the heart and result in decreased fetal blood flow.¹⁸ Although this episode was accompanied by a slight increase in uterine artery PI, the values remained within the normal range,³⁶ suggesting that the overall uterine blood flow was not significantly compromised.

One athlete experienced symptoms of vena cava syndrome, yet the fetus showed no signs of distress. Overall, FHR and both umbilical and uterine artery PI were similar during bench press and incline bench press, indicating maintained fetal blood flow. These findings align with results from another study, which reports no adverse effects of heavy-load resistance training in the supine position.¹¹ However, a single bout of supine exercise, as seen in this and other studies, cannot reliably predict fetal tolerance to repeated sessions throughout pregnancy.

The Valsalva manoeuvre increases blood pressure and intra-abdominal pressure and is commonly used by athletes and resistance-trained individuals to enhance performance during near-maximal to maximal load resistance effort.¹¹ However, it is discouraged during pregnancy due to the potential reductions in fetal blood flow.¹³ In our study, athletes chose their own breathing strategies, and the use of Valsalva was unfortunately not recorded, which precluded evaluation of the safety of the Valsalva manoeuvre. Nevertheless, since more than 60% of the athletes (n=30) regularly engaged in heavy-load resistance training, many likely used the Valsalva manoeuvre instinctively. Given that we observed no change in umbilical artery PI post-exercise and a decrease in uterine artery PI, our findings are consistent with previous research which suggests that heavy lifting and the Valsalva manoeuvre may not compromise fetal blood flow in well-trained persons.^{11 15} Future research should include systematic monitoring of breathing strategies to better assess the potential impact of the Valsalva manoeuvre on maternal and fetal outcomes during heavy-load resistance training.

Clinical implications

Current exercise guidelines recommend light-to-moderate intensity muscle-strengthening activities during pregnancy and discourage heavy-load resistance training due to fetal health concerns.5,9 However, this advice is based on limited research, as most previous studies have focused on light-to-moderate loads.^{15 37} Only one study with 10 pregnant participants has investigated fetal responses to heavy-load, compound, multi-joint movements.¹¹ Compared with earlier studies, our study includes a larger sample, encompasses both elite and recreational athletes, focuses more on upper-body resistance exercises and incorporates additional measures such as uterine artery blood flow. Together, these enhancements contribute to a stronger foundation for evidence-based recommendations and offer more robust guidance for pregnant recreational and elite athletes who wish to continue heavy-load resistance training during pregnancy. Nonetheless, further research is needed to validate and extend our findings.

Limitations

The significantly larger sample size and direct assessments of fetal responses to heavy-load resistance training are strengths of this study. Unlike previous studies that used lighter loads,³⁷ extended rest periods between sets for ultrasound assessments^{35 37} or gradual load increases,¹¹ our protocol was designed to replicate the usual structure of a resistance training session, with multiple sets and rest intervals appropriate for hypertrophy training.²⁵ While our study was less well-controlled than previous research,^{11 15 16} the aim was to reflect real-world training loads, to provide additional insight into the safety of heavy resistance training during pregnancy. Our estimated intensity of 76% 1 RM aligns with standard definitions of heavy lifting. In contrast, Moolyk et al¹¹ used 90% of 10 RM (~67.5% 1 RM), Meah et al¹⁶ used 60% of 10 RM (~45% 1 RM) and Gould et al¹⁵ capped load at 50 lb (22.68 kg) in a supine chest press, which is lower than participants in our study lifted for eight reps at 1 RIR. While definitions vary, these differences suggest our study may better reflect true heavy-load training.

Ultrasound measurements were conducted by an experienced OBGYN. The direct involvement of OBGYNs in data collection was a major strength of the study, particularly given the difficulty of locating FHR via ultrasound immediately after exercise. Furthermore, collecting measurements within minutes of exercise contributed to the reliability of the data.

However, the study has some limitations. Our participants were part of the Strong Mama project, which meant that before the heavy-load resistance protocol, they had already completed two submaximal high-intensity interval sessions, one on a treadmill 1 or 2 days before and another on a cycle ergometer the same day. The aerobic component preceding the heavy-load resistance protocol may have caused maternal fatigue, potentially limiting efforts during training and influencing fetal responses. While perceived exertion may still reflect meaningful physiological stress, variability in maternal effort represents a limitation and should be considered when interpreting the results. Conversely, the combination of high-intensity intervals and resistance training may better reflect real-world training practices for some women, adding ecological validity to the protocol.

Because of the significant load already placed on the participants, we had to limit our protocol to three exercises, and we did not include a separate RM test before the exercise testing. However, most required assistance on their final repetition, which supports that the majority were training at or near maximal effort. It is also important to note that the ability to perform a true 1 RM test can vary between experienced and novice lifters, and using a fixed percentage of 1 RM may not be appropriate across all populations.³⁸ In this context, using RIR offers a more practical and adaptable method for regulating training intensity. Further, since our protocol included few exercises, we cannot rule out that a longer workout session, incorporating additional exercises, more repetitions or shorter rest intervals might have yielded different outcomes, due to increased intensity and stress. Finally, all participants completed the exercises in the same order, and doing them in a randomised order might have produced different results and stronger comparisons.

In our protocol, fetal well-being was assessed in an inclined supine position (30°), which differed from the positions used during the exercises (eg, upright during the deadlift) and may have influenced maternal cardiovascular responses and fetal perfusion. This position was selected to reduce vena cava compression, ensure comfort in late pregnancy and maintain a consistent setup across all conditions, including high-intensity intervals. Due to practical constraints, alternating between multiple postures was not feasible. We acknowledge that this reduces ecological validity, as most individuals would not rest in this position following heavy-load resistance training. Additionally, fetal monitoring began approximately 30 s after exercise, due to transfer time. While brief bradycardic episodes may have gone undetected, such transient events (<30 s) are not typically considered clinically significant in healthy pregnancies.

Our findings are limited to healthy, highly active women with uncomplicated pregnancies, who were experienced with resistance training and residing in Oslo, Norway. Therefore, these results may not be generalisable to the broader population of pregnant women. Further research is needed to explore the safety of heavy-load resistance training in a more diverse group.

Unfortunately, we did not measure maternal heart rate and blood pressure, which could have provided valuable insights into maternal cardiovascular responses to the exercises and enabled comparisons with other studies. Additionally, umbilical artery PI data were missing for the athlete who experienced bradycardia, as continuous FHR monitoring was prioritised until normalisation. Fetal well-being assessments could only be conducted immediately after exercise, as obtaining ultrasound measurements during the activity was not feasible. As a result, we are unable to provide continuous data on fetal well-being throughout the exercise session.

Conclusion

This is the largest study to date to assess fetal responses to a heavy-load resistance protocol and supine exercise. Despite one transient episode of fetal bradycardia that did not meet clinical criteria for concern, the findings suggest that highly active women, especially those with resistance training experience, can safely engage in heavy-load resistance training without apparent adverse effects on fetal well-being. Further research is needed to confirm these findings in a more diverse sample and to assess the effects of a more extensive training programme.

Data availability statement

Data are available upon reasonable request.