Empirically based lifting criteria established by the National Institute for Occupational Safety and Health (NIOSH) to reduce the risk of overexertion injuries in the general US working population were evaluated for application to pregnant workers. This report proposes criteria to guide decisions by medical providers about permissible weights for lifting tasks performed at work over the course of an uncomplicated pregnancy. Our evaluation included an extensive review of the literature linking occupational lifting to maternal and fetal health. Although it has been 29 years since the American Medical Association’s Council on Scientific Affairs published its report on the Effects of Pregnancy on Work Performance, these guidelines continue to influence clinical decisions and workplace policies. Provisional clinical guidelines derived from the NIOSH lifting criteria that account for recent evidence for maternal and fetal health are presented and aim to improve the standard of care for pregnant workers.
Clinicians play an important role in decisions concerning work activity restrictions during pregnancy. This role is underscored in the United States where federal regulations for occupational lifting are lacking and limited opportunities for paid antenatal leave are available. Findings from a recent survey show that recommendations to pregnant workers in physically demanding jobs vary widely, possibly because of limited clinician training in occupational health and the absence of current authoritative guidelines.
To address the need for updated empirically based recommendations that can be uniformly applied, this paper presents provisional clinical guidelines for occupational lifting in pregnancy based on an extensive review of the literature linking occupational lifting to maternal and fetal health and application of the National Institute for Occupational Safety and Health (NIOSH) lifting equation.
Literature review criteria
An extensive review of the literature linking occupational lifting to maternal and fetal health was conducted to inform the application of the NIOSH lifting equation to pregnant workers. Using the PubMed MESH Thesaurus terms, pregnancy, lifting, occupational diseases, preeclampsia, hypertension, pregnancy complications, pelvis, sacroiliac joint, pubic symphysis, range of motion articular, joint instability, relaxin, estradiol, biomechanics, stress mechanical, occupational exposure, low back pain, human engineering, and gait, we conducted an electronic search of PubMed, Web of Science, Cochrane Library, the OSH References Collection, CINAHL, Directory of Published Proceedings, EMBASE, Google Scholar, PsycNET, ScienceDirect, CISDOC, OCLC First Search, EBSCOHost, and OSH References Collection. This electronic search was initially completed in July 2009 (inclusive of all preceding dates) and then updated in August 2012 (since July 2009). The search terms were used to search the Defense Technical Information Center and Google for possible unpublished research. The initial electronic database search was supplemented by manual searches of published reference lists, review articles, and conference abstracts.
Occupational lifting and fetal-maternal health outcomes
Several etiological mechanisms are thought to influence maternal and/or fetal health for pregnant women working in jobs with high exertion demands, such as heavy manual lifting. These mechanisms include venous insufficiency, excessive intraabdominal pressure, ligament laxity, and increased demands on the musculoskeletal system because of fetal load. Venous insufficiency is thought to play a role in the relationship between occupational physical activity and fetal health (eg, growth retardation) and preeclampsia. Mechanical compression, altered venous tone, and poor venous return from the lower extremities may be exacerbated by constrained postural demands (eg, prolonged standing, stooping), inducing conditions of fetal hypoxia. Increased intraabdominal pressure has been hypothesized to explain significant associations between forward flexion of the upper body (or stooping) and preterm delivery and spontaneous abortion.
The bulk of the epidemiological evidence shows a small increased risk of lower birthweight for gestational age in relation to heavy physical work. The evidence is strongest in research involving women in developing countries, which may increase the likelihood of maternal nutrition influences but also may signal more strenuous working conditions coupled with more limited opportunities to avoid or reduce exposures.
Evidence on the association between lifting and miscarriage also shows a generally consistent pattern of a slightly elevated significant risk, with odds ratios (ORs) most often in the range of 1.5–2.0 ; however, a Finnish case control study of physiotherapists found a notably higher OR of 3.5, 95% confidence interval (CI), 1.1–9.0 between heavy lifting (often related to patient transfers) and spontaneous abortion. Investigations on the relationship between occupational lifting and preterm birth is more limited than for other fetal health outcomes, and the findings more consistently suggest no association. For a more detailed summary of the epidemiological evidence related to fetal health outcomes, we recommend the systematic reviews conducted by Bonzini et al (2007) and the 2009 Guideline Development Group of the Royal College of Physicians.
Fewer studies have investigated the association between maternal health outcomes and heavy physical work load, despite evidence showing a higher use of antenatal sick leave and hospital visits among those employed in heavy work. One previous study showed a positive association between heavy lifting (10-20 kg or 22-44 lb) in early pregnancy (occurring more than 20 times per week) and preeclampsia. Additionally, a 2-fold increased risk of preeclampsia was found for pregnant women with high physical activity at work (composite score).
An explanatory model by Paul et al (1994) suggests that pregnancy-related musculoskeletal problems arise, at least in part, from reduced load-bearing capacity associated with joint laxity. Although the mechanisms underlying laxity are unknown, the condition presents early in pregnancy and persists beyond 6 weeks postpartum. The associated reduction in ligament rigidity is believed to weaken joint stability, increasing demand on stabilizing muscles.
Many researchers have identified laxity as a contributing factor in pregnancy-related pelvic girdle pain, low back pain, and knee pain, although direct evidence is lacking. Although the hormonal basis for laxity has been questioned, laxity itself is a well-established phenomenon that deserves further attention, especially in relation to short- and long-term maternal health consequences of occupational lifting and other physical job demands. The pregnancy-related musculoskeletal risk model by Paul et al (1994) also calls attention to increased load on the musculoskeletal system because of increased abdominal mass and the change in the center of mass.
Occupational lifting and fetal-maternal health outcomes
Several etiological mechanisms are thought to influence maternal and/or fetal health for pregnant women working in jobs with high exertion demands, such as heavy manual lifting. These mechanisms include venous insufficiency, excessive intraabdominal pressure, ligament laxity, and increased demands on the musculoskeletal system because of fetal load. Venous insufficiency is thought to play a role in the relationship between occupational physical activity and fetal health (eg, growth retardation) and preeclampsia. Mechanical compression, altered venous tone, and poor venous return from the lower extremities may be exacerbated by constrained postural demands (eg, prolonged standing, stooping), inducing conditions of fetal hypoxia. Increased intraabdominal pressure has been hypothesized to explain significant associations between forward flexion of the upper body (or stooping) and preterm delivery and spontaneous abortion.
The bulk of the epidemiological evidence shows a small increased risk of lower birthweight for gestational age in relation to heavy physical work. The evidence is strongest in research involving women in developing countries, which may increase the likelihood of maternal nutrition influences but also may signal more strenuous working conditions coupled with more limited opportunities to avoid or reduce exposures.
Evidence on the association between lifting and miscarriage also shows a generally consistent pattern of a slightly elevated significant risk, with odds ratios (ORs) most often in the range of 1.5–2.0 ; however, a Finnish case control study of physiotherapists found a notably higher OR of 3.5, 95% confidence interval (CI), 1.1–9.0 between heavy lifting (often related to patient transfers) and spontaneous abortion. Investigations on the relationship between occupational lifting and preterm birth is more limited than for other fetal health outcomes, and the findings more consistently suggest no association. For a more detailed summary of the epidemiological evidence related to fetal health outcomes, we recommend the systematic reviews conducted by Bonzini et al (2007) and the 2009 Guideline Development Group of the Royal College of Physicians.
Fewer studies have investigated the association between maternal health outcomes and heavy physical work load, despite evidence showing a higher use of antenatal sick leave and hospital visits among those employed in heavy work. One previous study showed a positive association between heavy lifting (10-20 kg or 22-44 lb) in early pregnancy (occurring more than 20 times per week) and preeclampsia. Additionally, a 2-fold increased risk of preeclampsia was found for pregnant women with high physical activity at work (composite score).
An explanatory model by Paul et al (1994) suggests that pregnancy-related musculoskeletal problems arise, at least in part, from reduced load-bearing capacity associated with joint laxity. Although the mechanisms underlying laxity are unknown, the condition presents early in pregnancy and persists beyond 6 weeks postpartum. The associated reduction in ligament rigidity is believed to weaken joint stability, increasing demand on stabilizing muscles.
Many researchers have identified laxity as a contributing factor in pregnancy-related pelvic girdle pain, low back pain, and knee pain, although direct evidence is lacking. Although the hormonal basis for laxity has been questioned, laxity itself is a well-established phenomenon that deserves further attention, especially in relation to short- and long-term maternal health consequences of occupational lifting and other physical job demands. The pregnancy-related musculoskeletal risk model by Paul et al (1994) also calls attention to increased load on the musculoskeletal system because of increased abdominal mass and the change in the center of mass.
Low back and pelvic girdle pain
Low back pain (LBP) and pelvic girdle pain (PGP) are common during pregnancy, with LBP occurring in up to two thirds of pregnancies and PGP occurring in nearly 20%. Data from numerous studies show that LBP prevalence is most elevated in months 6 and 7. Because women often underreport LBP/PGP to their prenatal provider, the topic may not garner sufficient clinical attention. Two studies indicate that the prevalence of severe LBP and/or PGP symptoms ranges from 15% to 20% ; however, studies investigating antecedents for pregnancy-related low back and/or pelvic girdle pain have rarely considered occupational exposures. Importantly, severe cases of pregnancy-related LBP/PGP have been reported to trigger or exacerbate comorbid conditions, affecting patient well-being and functional status. Two studies report increased sleep disturbance and impaired daily living. Other research shows elevated depression among those with pregnancy-related LBP/PGP.
Activity limitations resulting from LBP/PGP during pregnancy and the postpartum period have been shown to interfere with weight loss and resumption of leisure-time physical activity levels needed for health maintenance. Additionally, patients with both LBP and PGP have been found to be at greatest risk of persistent pain postpartum and to experience greater disability. Although back pain spontaneously resolves postpartum for most, those with persistent pain were more likely to have had back pain prior to pregnancy, present with early onset of symptoms, and exhibit higher pain severity during pregnancy.
Existing guidance on occupational lifting
For the past 29 years, clinical management for physical job activities, including lifting, has relied on the American Medical Association’s (AMA) Council on Scientific Affairs published guidance on the effects of pregnancy on work performance. These guidelines define permissible weight limits “that healthy employees with normal uncomplicated pregnancies should be able to perform…without undue difficulty or risk to the pregnancy” ( Table 1 ). Evidence suggests that these guidelines continue to inform physician practice and workplace policy.
| Week of gestation | Intermittent lifting a | Repetitive a | ||
|---|---|---|---|---|
| Metric | US customary | Metric | US customary | |
| 20 | – | – | >23 kg | >51 lb |
| 24 | – | – | 11-23 kg | 24-51 lb |
| 30 | >23 kg | >51 lb | – | – |
| 40 | <14 kg | <31 lb | <11 kg | <24 lb |
a Intermittent and repetitive were not defined in the AMA guidance.
The AMA’s guidelines apply to repetitive lifting beginning in the 24th week or intermittent lifting beginning in the 30th week of pregnancy, permitting up to 51 pounds. The AMA’s recommended weight allowance drops in the final week of pregnancy to less than 24 pounds for repetitive and less than 31 pounds for intermittent lifting. An unpublished statement from the AMA’s 1999 Annual Meeting encourages physicians to “consider the potential benefits and risks of occupational activities and exposures on an individual basis, and work with patients and employers to define a healthy work environment for pregnant women and encourages employers to “minimize heavy lifting.”
Certain aspects of the AMA guidelines are nonspecific (eg, repetitive and intermittent lifting were not defined), and they do not inform the clinician how to take into consideration lifting task conditions, such as object location at the time of the lift (eg, near or far from the front of the body), which may leave pregnant workers at risk of overexertion injury.
The following statement on pregnancy and work by the American College of Obstetricians and Gynecologists (ACOG) from 1979 highlights other nonspecific guidance provided to clinicians about patient employment conditions: “The normal woman with an uncomplicated pregnancy and a normal fetus in a job that presents no greater potential hazards than those encountered in normal daily life in the community may continue to work without interruption until the onset of labor and may resume work several weeks after an uncomplicated pregnancy.”
Although acknowledging evidence associating physical job demands (standing, lifting) with preterm or small-for-gestational-age outcomes, specific recommendations on employment conditions are also absent from the recent American Academy of Pediatrics/ACOG Guidelines for Perinatal Care (6th edition), which state, “Women with medical or obstetric complications of pregnancy need to make adjustments based on the nature of their activities, occupations, and specific complications.” More recent guidance by the American College of Occupational and Environmental Medicine for reproductive and developmental hazard management does not address lifting or other physically strenuous work activities.
All military services have policies exempting pregnant women from some work activities, yet few specifically address lifting in pregnancy. The policy of the US Army exempts soldiers from wearing “load-bearing equipment” after pregnancy has been confirmed, and the US Air Force policy precludes wearing “heavy gear” after 20 weeks’ gestation. All military services designate the obstetrical health care provider as the authority for recommending restricted duty for pregnant personnel, although a newer Army policy also mandates an “occupational health interview” for pregnant service women. Occupational health consultation is optional in the US Navy and the US Marine Corps, but, when sought, these military services offer the only lifting-specific guidance.
Citing both the 1984 AMA guidelines and the 1991 Revised NIOSH lifting equation, the Navy and Marine Corps technical manual states that lifting may generally continue up to the level a woman was accustomed prior to pregnancy. The guidance adds that “additional restrictions in the third trimester of pregnancy include limiting or prohibiting … lifting weights that are bulky or awkward or that approach the woman’s maximal (prepregnancy) lifting capacity. As pregnancy progresses, it is wise to reduce the physical workload and ensure rest periods of adequate frequency and duration. In late pregnancy, a pregnant woman should not do any task that may require a Valsalva (bearing-down) maneuver.”
NIOSH lifting equation
Following a detailed scientific review by a panel of experts, the empirically derived NIOSH Work Practices Guide for Manual Lifting was published in 1981 to reduce overexertion injury in the general working population in association with 2-handed lifting of compact loads. After consideration of new evidence, the original NIOSH lifting guidelines were expanded and replaced in 1991 by the revised NIOSH lifting equation.
The lifting equation is an ergonomic job assessment tool used to evaluate the specific conditions of a lifting task to compute a recommended weight limit (RWL). The RWL represents the weight of the load that nearly all healthy workers could lift, up to 8 hours per day, without an increased risk of developing lifting-related LBP. By healthy workers, NIOSH means workers who are free of adverse health conditions (or other conditions such as pregnancy) that may increase their risk of musculoskeletal injury. According to the authors of the lifting equation, the RWL provides weight limits that would be acceptable to 90% of healthy women.
The lifting equation defines a maximum RWL of 51 pounds, which is considered safe for an ideal lift (ie, infrequent 2-handed lifting of compact loads close to the body without twisting, stooping, or reaching up or forward). Because lifting conditions deviate from this ideal, the RWL value is reduced in accordance with specific task conditions such as lifting frequency and location of the object at the start of the lift (eg, lifting from the floor, overhead, or far in front of the body).
The task conditions of a lift are associated with corresponding metabolic and biomechanical loads or demands so, for example, as the distance between a worker and a load lifted in front of the body increases, the RWL for that lifting task would be reduced from the ideal lift starting value of 51 pounds (the condition when an object is held close to the body) to a maximum value of 20 pounds (the condition when an object is held very far from the body). Those interested in knowing more about the lifting equation and the task parameters used to compute the RWL are encouraged to access the Applications Manual from the NIOSH web site ( http://www.cdc.gov/niosh/docs/94-110/ ) and to read the article by Waters et al.
Provisional clinical guidelines for occupational lifting
Motivated by the need for practical, evidence-based weight limits to aid clinical decision making, we applied the NIOSH lifting equation to define RWLs for a broad range of lifting patterns for pregnant workers. Criteria related to the distance objects are held in front of the body while lifting and the height of the object lifted relative to the floor, task conditions that influence the metabolic and biomechanical load, were used to define 9 “lifting zones.” Visual representations of these lifting zones are shown in Figure 1 .
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