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Running title: Physical activity and preterm birth

Preterm birth is a major reason for infant mortality and morbidity, representing a public health concern worldwide. Regular and voluntary physical activity is a healthy behavior to be incorporated by everyone, including pregnant women. On the other hand, some women are exposed to highly demanding occupational physical activities during pregnancy that might represent a threat to the fetus and to her own health. This paper is a literature review of studies about physical activity during pregnancy and the occurrence of preterm birth. Papers published between 1987 and 2007 (English/Spanish/Portuguese) were included. The differences between physical activity types with respect to its influences on preterm birth occurrence are described and discussed. Physiological characteristics of different physical activities, especially occupational and leisure time are considered. Contrary to other outcomes, preterm birth should not be studied by evaluating physical activity with instruments that combine all domains of physical activity (occupational, household, commuting and leisure time) in a single score or as a measure of energy expenditure. Although the effects measured by the studies are not strong and the evidence is impaired by many methodological flaws, it seems that recreational or leisure-time physical activities done on a regular basis are protective for this adverse pregnancy outcome. Studies on occupational physical activities, 60specially standing for long periods present contrasting results. Regardless of methodological aspects of the studies reviewed, there’s a chance that the real effect of occupational physical activity is being blurred by some underlying factors not easily measured in epidemiological investigations. Our conclusions do not reject the idea that working conditions might represent danger for the pregnancy outcome, but only raise the question that maybe the mechanisms by which employment-related physical activities are being considered up to now could be better and deeper studied. Future studies should pay extra attention to psychological and socioeconomic characteristics, without neglecting biological plausibility.

Leisure-time physical activity during pregnancy in the 2004 Pelotas Birth Cohort Study

OBJECTIVE: Physical activity during pregnancy is a poorly investigated subject on population level. The study aimed to describe duration, type and frequency of leisuretime physical activity during pregnancy, and to explore its associated factors.

METHODS: A population-based study was carried out during 2004 in Southern Brazil. A total of 4,471 mothers were interviewed soon after delivery. Physical activity was measured using a questionnaire, developed for the study. Results were obtained by Poisson regression.

RESULTS: In the sample, 14.8% of women reported to engage in some type of physical activity prior to pregnancy and 12.9% during pregnancy. In the first trimester, 10.4% of all mothers engaged in some type of physical activity; 8.5% in the second trimester and 6.5% in the third trimester. Only 194 mothers (4.3%) were active during the whole pregnancy. In the adjusted analysis, leisure-time physical activity was positively associated with schooling, physical activity advice during prenatal care, and family income (p<0.001), being employed during pregnancy (p=0.05), and number of pregnancies (p=0.02). Walking was the most frequent activity.

CONCLUSIONS: The prevalence of leisure-time physical activity is low among Brazilian pregnant women. Although physical activity is not perceived as being pregnancy-threatening, and current guidelines recommend it, this population’s behavior does not seem to be changing. Active lifestyle for both pregnant women and future mothers should be encouraged.

KEYWORDS: physical activity, pregnant women, population-based studies, physical fitness.

Effects of leisure-time physical activity on preterm birth

INTRODUCTION: Preterm birth (PB) is a well-known public health problem worldwide. Risk factors for such outcome have been described thoroughly in the literature. Among them, lifestyle aspects are of great interest as they are potentially modifiable.

OBJECTIVE: To investigate the relation between PB and leisure-time physical activity (LTPA) during pregnancy.

METHODS: A cross-sectional population-based study evaluated, retrospectively, LTPA during pregnancy in 4147 mothers participating in the “2004 Pelotas Birth Cohort Study�. All mothers from Pelotas (Southern Brazil) who delivered from January 1st to December 31st were interviewed soon after birth. Only singleton, live births were included in the present analysis. Gestational age was determined by last menstrual period, and when not available, by ultrasound or the Dubowitz method. LTPA was determined using an instrument developed by the researchers.

RESULTS: 14.6% of all births were considered preterm. Only 13.3% of the mothers reported engaging in LTPA during pregnancy. Crude analysis indicated that LTPA had a protective effect on PB. A regression model was fitted in order to establish factors associated to PB and to control for confounders. Protective effects of LTPA remained after adjustment for family income, mother’s schooling, age and morbidities during pregnancy. LTPA during the three trimesters (PR=0.55; 95% 0.32–0.96), LTPA in third trimester (PR=0.50; 95%CI 0.31–0.80) and minimum LTPA (_90 min./week) in the third trimester (PR=0.58; 95%CI 0.34–0.98) showed protective association with PB in the adjusted analysis.

CONCLUSIONS: Protective effects against preterm birth might be obtained by keeping an active lifestyle during pregnancy, especially being active during the entire pregnancy and the last trimester. Since no harm or risk increase was detected in exercising women, physical activity should be encouraged during antenatal care. Clinical research approaches are recommended to better understand the relation between LTPA and PB in order to determine optimal intensity, duration, frequency and type of exercise to be performed during pregnancy.

Keywords: preterm birth, physical activity, pregnancy, cohort studies