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  • Tyagal, Patan, Lalitpur
  • We undertook a case-control study to evaluate whether some occupational conditions during pregnancy increase the risk of delivering a small-for-gestational-age (SGA) infant and whether taking measures to eliminate these conditions decreases that risk.Methods. The 4536 cases and the 4441 controls were drawn from 43898 women who gave birth between January 1997, Canada, and March 1999. After delivery, the women were interviewed over the telephone.Results. An irregular or shift-work-schedule alone increased the risk of an SGA infant. A cumulative index of the following occupational conditions also increases the risk: standing, lifting heavy loads, noise and high psychological demands with low social support. The risk of having an SGA infant increased with the number and severity of the conditions (Ptrend =.004 and odds ratios=1.00 to 1.08 to 1.28, 1.43 and 2.29 respectively for 0, 2, 3, and 4 conditions). The risks were close to those of unexposed women if the conditions were removed before 24 weeks.

    Conclusions. Preventive measures can lower the chance of an SGA baby.

    The risk of fetal growth retardation is increased infant mortality and morbidity.1,2,5-8 Medical conditions.1-3,5-7,5-7, and occupational factors.5,8,9

    Several epidemiological studies10-31 have observed a significant effect of some occupational conditions on fetal growth, including long hours of work,13-20 shift work,20,21 prolonged standing,12,15,16,19,25,26,28 lifting loads,20 and high psychosocial stress.30,31 However, some studies showed no effect.10,11,17,18,23 In several studies, limitations related to the measurement of exposure may have led to an underestimation of the true effect. These limitations include having a reference group that includes moderately exposed workers,10-12,14,15,17,18,20-24,27 measuring occupational conditions based on job title,25 and failing to take into account changes in occupational conditions that occur during pregnancy.10,11,13,15,17,19-22,25-27,30,31 The latter limitation is important because previous studies have suggested that workers most heavily exposed during early pregnancy are more likely to experience a reduction in exposure throughout the pregnancy or to take earlier antenatal leave.12,14,16,23,28,29

    After an assessment of the worker’s work conditions by a physician, the Commission de la sante et de la securite au travail (the governmental agency responsible for safety and health at work) determines the relevance of this measure. The willingness of the employer or union to provide preventive measures is not a condition. Nor does the woman’s ability to take them. The employer must reassign the pregnant worker in a safe job. Otherwise, she can withdraw from work and receive 90% of her salary up to 4 weeks before the baby’s due date. After that time, the law of parental coverage allows her to return to her regular job and benefit from it. The context is favorable for evaluating the impact of eliminating hazardous occupational conditions if there are discrepancies in the application of the law in the province.

    We measured the association between some occupational conditions (schedule, posture, physical effort, psychosocial factors), both individually and cumulatively, and the risk of delivering a small-for-gestational-age (SGA) infant. The risk of delivering a small-for-gestational age (SGA) infant was also evaluated by assessing whether these occupational conditions could be eliminated through preventive measures (changes in working conditions, withdrawals), before or after 24 weeks.

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    METHODS

    Study Design, and Population

    This was a case control study. This was a case-control study. The source population consisted mainly of Quebec women who had given birth to a singleton in six regions. The regional public health agencies are able to obtain copies of all Quebec hospital birth certificates within a few hours after the delivery. The Commission d’acces a l’information du Quebec granted us permission to access personal data on birth certificates.

    • Mother’s name, telephone number
    • Types of birth (single and multiple)
    • Information about the birth weight, gender, date, and time of birth for infants
    • Length and duration of pregnancy
    • Information about mother’s birth date, civil status, education
    • Multiple live births or stillbirths prior to the mother

    Soon after birth, the attending physician records the infant’s gender, birthweight, and length on the birth certificate. The length of a pregnancy can be estimated by comparing the actual delivery date with the expected date. This is done by comparing the doctor’s clinical and ultrasonic evaluations and the last menses to determine the latter. We used information from birth certificates to classify births as either SGA cases (n=3409 [7.8%]), or noncases. SGA cases are infants who were born below the 10th percentile of gestational age, based on gender-specific Canadian standards.33 A random selection of 20% of noncases (n=8130) was used as a control group.

    Data Collection

    Interviewers reached out to the women via telephone as soon as possible following receipt of their birth certificate. However, they were not allowed to contact them earlier than two weeks after the birth. For both controls and cases, the median time between childbirth and interview took 30 days. Interviewers stated that they were from the public health department and Laval University. The Commission d’acces a l’information du Quebec granted access to the woman’s phone number and name. All information collected would be kept confidential and anonymous. They explained the study to the woman, asked for her participation and checked her eligibility. The case mothers were unable to contact 270 (7.9%), and 75 (2.2%) refused to participate. In contrast, the numbers for the control mothers were 442 (5.4%) and 126 (1.5%). There were 10 626 women (3564 cases and 7562 control) who agreed to take part. The interview was completed by 5977 women (4371 controls and 1606 cases).

    The computer-assisted telephone interview took 20-30 minutes and could be done immediately or later if the woman is available. The questionnaire detailed the following work conditions: hours worked each day, evening work [6:00 PM to 10:59 PM], night work [11:00pm to 5:59 AM], schedule regularity], standing posture; lifting [weight and frequency], pushing or pulling objects); break times, piecework or assembly line work; psychosocial variables and occupational, environmental conditions (e.g. noise, whole-body vibrations, exposure to environmental tobacco smoking). After reviewing the questionnaires by Mamelle et. al.34, McDonald’s et. al.35 and after consulting ergonomists about the validity of the exposure data obtained from questionnaires 36-42, we developed the questions.

    Karasek’s model of psychological demands, decision letitude and support at work was used to evaluate psychosocial factors at workplace. These factors were measured using a French translation43 of Karasek’s questionnaire.44,45 The median value for psychological demand and decision latitude was used. Four levels of job strain were found when cross-stratifying psychological requirements and decision latitude was used. Social support level was also a factor in the 3 highest levels job strain.

    As a first step, we documented the working conditions at the start of pregnancy. We asked mothers when they changed their work conditions during pregnancy. This allowed us to document the changes in work schedules, posture, and effort. We asked mothers to indicate when and why they stopped working (e.g. legally justifiable preventive withdrawal, medical problems, close to the expected delivery span>

    This section documented the mother’s medical history (before and after pregnancy), the characteristics of her newborn (gender, gender, birth-date, expected delivery date according to the doctor, congenital anomalies), and mother’s lifestyle (physical activities, smoking, caffeine, alcohol and drug use) and sociodemographic characteristics.

    226 (3.8%) of the 5977 women interviewed indicated that their interview data (birthweight, expected delivery date, and date of birth) indicated a case/control status other than the one determined from birth certificate data. To verify the information, 168 (74.3%) of these women gave us access their hospital records. 161 (95.8% of the requests) were answered by Archivists. The information received led to an amendment of the case or control status for only 88 (55.8%) of these 161 subjects. This resulted in 1536 cases and 4441 checks that can be analyzed.

    Analysis

    SAS software versions 6.12-8 (SAS Institute Inc., Cary NC) was used for all analyses. Logistic regression was used to determine the relationship (1) between SGA births and occupational conditions, or (2) between SGA births and potential confounders. To calculate odds ratios (ORs), 95% confidence intervals (95% CSS span), and standard errors, we used beta coefficients and standard error.

    Bivariate analysis determined the association between each variable that was considered a priori to be a possible confounder (obstetrical histories, mother’s medical profile, mother’s lifestyle, sociodemographic characteristics, and the risk of having a SGA infant). In the regression models, covariates were identified as factors statistically associated with SGA births. These included those with ORs less than or equal 0.8 or greater to 1.2 on at most one stratum. Multiple logistic regression was used to calculate ORs that relate occupational conditions to SGA borns. This adjustment included all covariates. As long as the OR did not differ by more than 10% from the full model, covariates could be removed one at a time. The final model was adjusted for any occupational conditions at the start of pregnancy (see Table 3> footnotes).

    TABLE 3–

    Adjusted Odds Ratios (ORs) and 95% Confidence Intervals (CIs) for Having a Small-for-Gestational-Age Infant, by Occupational Condition at the beginning of Pregnancy and Early (< 24 wk), Late ( > 23 wk), or No Elimination of Condition by Preventive Measures During Pregnancy: Workers Giving Birth Between January 1997 and March 1999, Quebec, Canada.

    Preventive Measures to Eliminate Condition During Pregnancy

    Condition at the Beginning of Pregnancy. Early Late Not Eliminated

    Cases, No. Controls, No. OR (95%CI) Cases No. Controls, No. OR (95%CI) Cases No. Controls, No. OR (95%CI) Cases No. Controls, No. OR (95% CI).

    Hours worked/wka

    20-34b 470 1284 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . . .

    35-39 526 1676 0.9 (0.8, 1.1) 116 353 0.8 (0.7, 1.1) 56 165 0.9 (0.6, 1.2) 354 1158 1.0 (0.8, 1.2)

    >= 40 519 1430 1.0 (0.8, 1.1) 147 356 1.0 (0.8, 1.2) 88 204 1.1 (0.8, 1.5) 284 870 1.0 (0.8, 1.2)

    Work schedulea

    Day onlyb 900 2713 1. . . . . . . . . . . . . . . . . . . . . . . . . . .

    Evening, but no night hours 432-1194 0.9 (0.8.1.0) 124 381 0.0.7 (0.5.9) 85 199 1.0.7 (1.3) 223 614 1.0(0.8,1.2)

    Night hours 177 463 0.8 (0.7, 1.0) 104 292 0.7 (0.6, 1.0) 23 71 0.7 (0.4, 1.1) 50 100 1.2 (0.8, 1.7)

    Unknown 6 20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Schedule regularity

    Regularb 1249 3731 1. . . . . . . . . . . . . . . . . . . . . . . . . . .

    Irregular or shift work 266 659 1.2 (1.0, 1.4) 103 290 1.0 (0.7, 1.2) 57 113 1.5* (1.0, 2.1) 106 256 1.3* (1.0, 1.7)

    Standing, h/d a

    < 2b 331 1125 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . . .

    2-3 331 1005 1.1 (0.9, 1.3) 38 100 1.2 (0.8, 1.9) 19 65 1.0 (0.6, 1.7) 274 840 1.1 (0.9, 1.3)

    4-6 505 1354 1.1 (0.9, 1.4) 176 498 0.9 (0.7, 1.2) 100 220 1.4* (1.0, 1.9) 229 636 1.2 (0.9, 1.5)

    >= 7 348 906 1.0 (0.8, 1.2) 183 491 0.9 (0.7, 1.2) 66 177 0.9 (0.6, 1.3) 99 238 1.2 (0.9, 1.6)

    Demanding posture, h/da,c

    < 1b 866 2728 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . . .

    >= 1 649 1662 1.0 (0.9, 1.2) 293 764 0.9 (0.8, 1.1) 120 263 1.2 (0.9, 1.5) 236 635 1.1 (0.9, 1.3)

    Lifting, kga

    0b 724 2292 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . . .

    1-6 308 873 0.9 (0.8, 1.1) 95 265 0.8 (0.6, 1.1) 58 122 1.1 (0.8, 1.6) 155 486 1.0 (0.8, 1.2)

    >= 7 478 1197 1.0 (0.9, 1.2) 233 626 0.9 (0.7, 1.1) 80 173 1.2 (0.9, 1.6) 165 398 1.2 (0.9, 1.5)

    Unknown 5 28 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Pushing/pulling objects

    Nob 965 2919 1. . . . . . . . . . . . . . . . . . . . . . . . . . .

    Yes 550 1471 1.0 (0.9, 1.2) 225 651 0.9 (0.7, 1.1) 117 230 1.2 (0.9, 1.6) 208 590 1.0 (0.9, 1.3)

    Piecework/assembly linea,d

    Nob 1383 411 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . .

    Yes 132 278 0.9 (0.7, 1.2) 51 109 0.9 (0.6, 1.4) 35 60 1.2 (0.7, 1.9) 46 109 0.8 (0.6, 1.2)

    Noisea,d,e

    Nob 1239 3750 1. . . . . . . . . . . . . . . . . . . . . . . . . . .

    Yes 276 640 1.1 (0.9, 1.3) 102 235 1.0 (0.7, 1.3) 39 84 1.1 (0.7, 1.7) 135 321 1.2 (1.0, 1.5)

    Job strain by social support

    Low strainb 292 972 1. . . . . . . . . . . . . . . . . . . . . . . . . . .

    Moderate-passive strain

    Moderate or high support 281 750 (0.9,1.3) 66 173 1.5 (0.7,1.3) 56 102 1.5 (1.0,2.1) 159 475

    Low support: 131 335 1.0.8, 1.3. 41 104 0.9.0.6.1.4 22 47 1.2.0.7.2.0 68 184.1.1.0.8.1.5

    Moderate-active strain

    High support or moderate support 261 898.1.0 (0.8.1.2) 49 130.1.1 (0.8.1.6) 16 68.0.6 (0.3.1.1) 196 700.1.0 (0.8.1.2)

    Low Support 102 275.1.2 (0.9.1.6) 22 56.1.0 (0.6.1.7) 8 23.1.2 (0.5.2.7) 72.196 1.3 (0.9.1.7)

    High strain

    Moderate or high support 188 536 1.0.8.1.3. 58.177 0.0.8.1.2. 32.67 1.3.0.8.2.1. 98 292 1.1.0.8.1.4.

    Low Support 236 540 (1.2, 1.5), 81 182 (1.1, 1.5), 38 73 1.3, 2.0) 112 285 (1.2, 1.6) 38 73

    Support not applicable to high or moderate strains 23 69 90.9 (0.5, 1.5). . . . . . . . . . . . . . . . . . . . . . . . . . .

    Strain or support unknown 1042 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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    aAdjusted to all other occupational conditions at the start of pregnancy, smoking during the third trimester (yes or no), mother and father’s heights (cm), and the average age of other children at their home (no children =6 years, >6 years). N = 1515 cases, 4390 controls.

    Preference category.

    Squatting, benting, lifting your arms above the shoulders, or any other demanding position.

    Considered to be eliminated if there was a preventive withdrawal from the workplace.

    A person who shouts or speaks loudly to be heard 2m away (due to background noise).

    Low job strain = high level of decision making and low psychological need; moderate-passive job stress = low decision-making ability and high psychological demands; moderate-active job strain = high job demand and high decision-making latitude; High job strain = high job demand and low decision-making latitude.

    g Adjusted to work schedule, regularity, standing and pushing/pulling, noise, smoking during the third trimester (yes or no), mother’s height (cm), average age of other children (no children), =6 years, >6 years). N = 1524 cases, 4417 controls.

    Workers without supervisors or coworkers were not eligible for social support.

    *P < .05.

    We also evaluated the impact of occupational conditions being eliminated by preventive measures, such as early or late modifications to working conditions or withdrawal from work. The groups of workers who were exposed to a particular working condition at the start of pregnancy were split into three groups based on whether or not the condition was removed during pregnancy. These 3 groups had different SGA risks compared to those who were not exposed at the start of the pregnancy.

    A cumulative index of occupational conditions was calculated for which the OR adjusted to the pregnancy was less than 1.2. This index was adjusted for covariates to evaluate its association with SGA risk. To evaluate a dose-effect relationship, we used the kh2 test. 46 We also examined the effect of eliminating indexed conditions during pregnancy using the same method as for single occupational conditions.

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    RESULTS

    In bivariate analyses, many variables were associated with SGA-risk (Table 1 span-data-preserver-spaces=”true “>).). Congenital anomalies, maternal smoking, caffeine intake, and maternal alcohol consumption were the strongest associations (ORs >=2). SGA risk was not associated with housework, physical activity during the first trimester and over-the-counter drug abuse, or mother’s country of birth. Data not shown. All variables in Table 1> were included in the multivariate initial models because they could be confounders.

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