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To investigate the association between leisure-time physical activity (PA) and fecundability.
Prospective cohort study.
Internet-based observational study of Danish women who were planning a pregnancy (2007–2009).
A total of 3,628 women aged 18–40 years at baseline.
Main Outcome Measure(s)
Time to pregnancy (TTP). Fecundability ratios (FRs) and 95% confidence intervals (CIs) were derived from discrete-time Cox models, with adjustment for potential confounders, such as body mass index (BMI).
We observed an inverse monotonic association between vigorous PA and fecundability (≥5 h/wk vs. none: FR 0.68, 95% CI 0.54–0.85) and a weak positive association between moderate PA and fecundability (≥5 vs. <1 h/wk: FR 1.18, 95% CI 0.98–1.43) after mutual adjustment for both PA types. Inverse associations between high vigorous PA and fecundability were observed within subgroups of age, parity status, and cycle regularity, but not among overweight or obese women (BMI ≥25 kg/m2).
There was evidence for a dose-response relationship between increasing vigorous PA and delayed TTP in all subgroups of women with the exception of overweight and obese women. Moderate PA was associated with a small increase in fecundability regardless of BMI. These findings indicate that PA of any type might improve fertility among overweight and obese women, a subgroup at higher risk of infertility. Lean women who substitute vigorous PA with moderate PA may also improve their fertility.
In 2006, Denmark adopted a national goal of increasing the prevalence of physical activity (PA) in the adult population to ≥75% by the year 2021, with “physically active” defined as moderate intensity activity for ≥30 minutes every day (
). Given that previous research shows little effect of moderate-intensity PA on menstrual characteristics, some scientists postulate that the effect of PA on fertility may be positive up to a certain level of activity and then have a deleterious effect above that threshold level of activity (
Epidemiologic studies of PA and infertility have been inconclusive, with some showing reduced risk among vigorous exercisers and others showing increased risk at the highest levels of frequency or intensity. Most of these studies have been based on fertility clinic populations (
). In one study based on a fertility clinic population, women who had engaged in PA for ≥4 hours per week for <10 years had a 40% reduced likelihood of live birth, a threefold increased risk of in vitro fertilization (IVF) cycle cancellation (i.e., oocytes not retrieved), and a twofold increased risk of implan-tation failure and pregnancy loss compared with women not regularly engaged in PA (
). The third report, based on a population-based prospective cohort study of Norwegian women, found an increased risk of infertility (all types) among women reporting the highest levels of PA intensity and frequency (
We examined the influence of leisure-time PA on time to pregnancy among Danish women enrolled in a prospective cohort study. We further stratified the results by body mass index (BMI) to assess whether the effect of PA differed according to overweight or obesity, which have been shown to be strong predictors of fecundability in our cohort and other studies (
). Briefly, recruitment began in June 2007 with placement of an advertisement on a health-related website (www.netdoktor.dk) and a coordinated media strategy involving radio, print media, online news sites, and television. Enrollment and primary data collection were conducted via a self-administered questionnaire on the study website (www.snart-gravid.dk). Before enrollment, participants read a consent form and completed an online screening questionnaire to confirm eligibility. Eligible women were aged 18–40 years, residents of Denmark, in a stable relationship with a male partner, and not receiving any type of fertility treatment. Participants provided a valid e-mail address and their Civil Personal Registration number—a unique 10-digit personal identification number assigned to every resident by the Central Office of Civil Registration (
The baseline questionnaire collected information on demographics, reproductive and medical history, and lifestyle and behavioral factors. During the first 6 months of the study, participants were randomized to receive either a short- or long-form baseline questionnaire, with similar completion frequencies for both versions (
). After the first 6 months, newly enrolled participants received the long-form baseline questionnaire. Follow-up questionnaires evaluated changes in various exposures, frequency of intercourse, and clinically recognized conception. Participants were contacted every 2 months by e-mail for 12 months or until clinically-recognized conception. Women who conceived were asked to complete one questionnaire during early pregnancy to assess changes in exposures, after which active follow-up ceased. Cohort retention after 12 months of follow-up was ∼82% (
). The Snart Gravid study was approved by all appropriate Institutional Review Board committees, and consent was obtained from each of the participants via the internet.
Assessment of Physical Activity
On the baseline questionnaire, women reported the average number of hours per week that they engaged in PA during the past year. They were asked to report moderate and vigorous types of activity separately. Categories of response were none, <1, 1, 2, 3–4, 5–6, 7–9, and ≥10 hours per week. Examples were provided for vigorous (“running, fast cycling, aerobics, gymnastics, or swimming”) and moderate (“brisk walking, leisurely cycling, golfing, or gardening”) types of activity. Based on the Compendium of Physical Activities (
), we estimated total metabolic equivalents (METs) of PA per week by summing the metabolic equivalents from vigorous exercise (h/wk multiplied by 7.0) and moderate exercise (h/wk multiplied by 3.5).
Assessment of Covariates
Data on age, weight, height, parity, smoking history, current alcohol consumption, last method of contraception, and frequency of intercourse were self-reported on the baseline questionnaire and were updated every 2 months by follow-up questionnaire. At baseline, women reported whether their cycles were currently regular and, if so, their usual cycle length when not using hormonal contraception (“number of days from the first day of one menstrual period to the first day of the next menstrual period”). We calculated BMI as [weight in kilograms]/[height in meters]2. Self-reported height and weight among women who delivered infants conceived during our study showed excellent agreement with measures provided by the Danish Medical Birth Registry (
On each follow-up questionnaire, women reported the date of their last menstrual period, whether they were currently pregnant, and whether they had experienced any other pregnancies since the date of their last questionnaire, including miscarriage, induced abortion, or ectopic pregnancy. Total cycles at risk (rounded to the nearest whole number) were calculated as follows: (days of attempt time at study entry/usual cycle length) + ([(last menstrual period date [LMP] from most recent follow-up questionnaire − date of baseline questionnaire completion)/usual cycle length] + 1), with observed cycles at risk defined as those contributed after study entry. For women with irregular cycles, we estimated usual cycle length based on the baseline LMP date, expected date of their next menstrual period, and LMP dates recorded over follow-up. Because we anticipated that the results would be less reliable among women with irregular cycles, we evaluated results separately among women with and without regular cycles.
After 30 months of recruitment, 5,460 women registered at the study website. Of these, we excluded 1,063 (19%) women who had been trying to conceive for >6 cycles at study entry: 274 women (5%) with insufficient or implausible information about their LMP date or date of first pregnancy attempt, and 495 women (9%) who did not complete a follow-up survey. After these exclusions, 3,628 women remained in this study. The 601 women (16.6%) who were subsequently lost to follow-up at some point during the year after enrollment (mean follow-up of 5.3 months) had lower parity (28.6% vs. 34.4%), higher BMI (mean 24.7 vs. 24.0 kg/m2), and heavier smoking histories (mean 2.8 vs. 2.0 pack-years) than the 3,027 women who were followed to a study end point. The two groups were similar regarding total PA (mean 24.1 vs. 24.9 MET-h/wk) and other baseline characteristics (e.g., mean age: 28.2 vs. 28.5 years; mean alcoholic drinks per week: 3.0 vs. 2.9; >4 years of higher education: 20.1% vs. 25.0%; gravidity: 43.6% vs. 45.8%; and use of oral contraceptives as last method of contraception: 61.4% vs. 61.6%).
We analyzed vigorous PA in categories of none (reference), <1, 1, 2, 3–4, and ≥5 hours per week, and moderate PA in categories of <1 (reference), 1, 2, 3–4, and ≥5 hours per week. Continuous variables for moderate and vigorous PA were coded as the midpoint of each category (assigning 11 h/wk to the top category). We categorized total MET-h/wk in 10-unit increments, with 20–29 as the reference category (because it was associated with the highest fecundability in our cohort) and ≥60 as the maximum exposure category. We allowed for the possibility of a nonlinear relation or threshold effect of each PA variable on fecundability by fitting a restricted cubic spline model (
The fecundability ratio (FR) represents the cycle-specific probability of conception among exposed women divided by that among unexposed women. We used a discrete-time analogue of the Cox proportional hazards model to estimate FRs and 95% confidence intervals (CIs) for moderate, vigorous, and total METs of physical activity in association with time to pregnancy, in cycles (
). We evaluated time to any pregnancy regardless of pregnancy outcome. Women were censored if they did not conceive after 12 cycles, the typical amount of time after which couples seek medical assistance for infertility (
). Women contributed cycles at risk until they reached a study end point: pregnancy, use of fertility treatments, loss to follow-up, or the end of observation (12 cycles), whichever occurred first. The Cox model allowed for “delayed entry” into the risk set, which occurs when women enter the study after having tried to conceive for one or more cycles. Therefore, risk sets were based only on cycles at risk observed after study entry (
). Based on these criteria, we controlled for female age (<25, 25–29, 30–34, ≥35 y), partner's age (<25, 25–29, 30–34, ≥35 y), BMI (<20, 20–24, 25–29, ≥30 kg/m2), alcohol consumption (drinks per day), pack-years of smoking (never smoked, <5, 5–9, ≥10 pack-y), frequency of intercourse (<1, 1, 2–3, ≥4 times/wk), and last method of contraception (barrier methods, oral contraceptives, other hormonal contraceptives, natural family planning). Alcohol consumption and frequency of intercourse were modeled as time-varying variables. Further control for “doing something to time intercourse” made very little difference in the effect estimates for vigorous or moderate PA (<1% in FRs). To assess the independent effects of vigorous and moderate PA, we further controlled for each type of PA simultaneously in the final multivariable model. The proportion of missing data at baseline ranged from as low as 0.19% (age at menarche) to as high as 4% (pack-years of smoking); proportions were 0.25% and 0.27% for vigorous and moderate PA, respectively. We used multiple imputation methods to impute missing covariate values (
). All potential confounders were included in the imputation procedure.
In secondary analyses, we evaluated the extent to which the associations changed when pregnancy losses were excluded from the outcome definition. In these analyses, women who reported an abortion or ectopic pregnancy were censored at their estimated time to pregnancy (
). We stratified by age, parity status, BMI, cycle regularity, and number of cycle attempts before study entry. We assessed departure from the proportional hazards assumption by plotting the log-log survivor functions for each exposure variable in categoric form, where parallel log-log survivor curves indicated proportional hazards.
Baseline characteristics of the study population according to hours of vigorous PA per week are presented in Table 1. Vigorous PA was positively associated with education and higher frequency of intercourse, and was inversely associated with BMI, waist circumference, caffeine intake, current smoking, parity, and the report of “doing something to time intercourse.” Women in the highest category of total MET-h/wk of PA tended to have longer and irregular cycles (data not shown).
Table 1Baseline characteristics of 3,628 Danish women participating in a prospective cohort study of pregnancy planners, according to level of vigorous physical activity.
In multivariable models that mutually controlled for both types of PA, we observed a monotonic inverse association between vigorous PA and fecundability, and a weak positive association between moderate PA and fecundability (Table 2). When we considered the contribution of both types of PA to total activity levels, we found that higher levels of PA were associated with reduced fecundability (≥60 vs. 20–29 MET-h/wk: FR 0.74, 95% CI 0.56–0.97).
Table 2Physical activity at baseline and time to pregnancy.
The effect of vigorous PA was relatively uniform across levels of age, parity, and attempt time at study entry (Table 3). Although we also observed an inverse association between vigorous PA and fecundability among women with BMI <25 kg/m2 (vigorous PA ≥5 vs. 0 h/wk: FR 0.58, 95% CI 0.45–0.75), there was no evidence of an inverse association among overweight and obese women (BMI ≥25 kg/m2) (vigorous PA ≥5 vs. 0 h/wk: FR 1.22, 95% CI 0.74–2.02). In fact, for most categories of vigorous PA above “none” among overweight and obese women, there were weak positive associations between vigorous PA and fecundability (FRs ranged from 1.12 to 1.22, with the exception of FR 0.76 for 3–4 h/wk). The inverse association between vigorous PA and fecundability among lean women (BMI <25 kg/m2) was still apparent after the exclusion of underweight women, defined as BMI <18.5 kg/m2 (data not shown). Within subgroups of selected covariates, increasing levels of moderate PA were either weakly positively associated with fecundability or not associated with fecundability (Supplemental Table 1, available online at www.fertstert.org). Women who engaged in 20–39 MET-h/wk of PA (from all activity sources) had the highest fecundability in our cohort, regardless of BMI (Supplemental Table 2, available online at www.fertstert.org).
Table 3Vigorous physical activity and time to pregnancy, stratified by selected factors.
Vigorous physical activity, hours per week
Age at baseline, y
Body mass index (kg/m2)
Cycle attempts before study entry
Moderate physical activity, h/wk
Note: FR adjusted for cycle number, age, partner's age, body mass index, alcohol consumption, pack-years of smoking, intercourse frequency, last method of contraception, and moderate physical activity (when applicable); 95% CI in parentheses. FR = fecundability ratio; CI = confidence interval.
Results for vigorous PA within levels of moderate PA (<5 vs. ≥5 h/wk) are shown at the bottom of Table 3, using a single reference category of women engaged in <5 hours of moderate PA and no vigorous PA. Fecundability was lowest for the women engaged in ≥5 h/wk of both moderate and vigorous PA (Table 3). Within each of the two levels of moderate PA, increasing levels of vigorous PA were associated with decreasing fecundability. Notably, women who engaged in ≥5 hours of moderate PA but reported no vigorous PA had increased fecundability compared with the least active women (<5 hours of moderate PA and no vigorous PA).
The overall results were virtually unchanged when we further controlled for cycle length and cycle irregularity, which are potential mediators of the PA–time to pregnancy association (vigorous PA ≥5 vs. 0 h/wk: FR 0.68, 95% CI 0.54–0.85; total PA ≥60 vs. 20–29 MET-h/wk: FR 0.75, 95% CI: 0.57–0.99). Among women with regular cycles, point estimates were less precise, but they were consistent with the results based on all women (vigorous PA ≥5 vs. 0 h/wk: FR 0.66, 95% CI 0.51–0.86; total PA ≥60 vs. 20–29 MET-h/wk: FR 0.74, 95% CI 0.53–1.02). Similar associations were observed when pregnancy losses were excluded from the outcome definition (data not shown).
Figure 1 shows the overall association between vigorous PA (h/wk) and fecundability using restricted cubic splines. Results are also presented according to categories of BMI at baseline (Supplemental Figs. 1 and 2, available online at www.fertstert.org). The observed patterns were consistent with the categorical results presented above: Fecundability decreased with increasing hours of vigorous PA per week, in a dose-response fashion, overall and among lean women. In contrast, low levels of vigorous PA were associated with increased fecundability among overweight and obese women up to ∼2 h/wk, after which vigorous PA had little effect on fecundability.
In this prospective cohort study of Danish women aged 18–40 years, vigorous PA was associated with reduced fecundability in all subgroups of women examined, with the exception of overweight and obese women (BMI ≥25 kg/m2), among whom PA of any type either modestly increased or had little effect on fecundability. In contrast, moderate PA was associated with a modest increase in fecundability overall and did not appear to have any deleterious effect on fertility among lean or overweight/obese women.
The finding of reduced fecundability among the highest-intensity exercisers agrees with some (
). Furthermore, the finding that any PA, regardless of type, may be associated with a modest increase in fecundability among overweight/obese women is supported by an intervention study showing that moderate PA coupled with weight loss can enhance fertility in obese women (
The conflicting results across studies regarding the effect of high intensity PA on female infertility may be attributable to the type of infertility studied. Two previous studies focused only on ovulatory infertility (
). The mechanisms by which high-intensity PA has a deleterious effect on female fertility might involve factors other than ovulation, such as impaired implantation. In support of this hypothesis, the study by Morris et al. of the success of IVF treatment reported a higher rate of implantation failures among women with high levels of PA (
). Morris et al. also found that associations were stronger among the women who engaged in cardiovascular activities (e.g., running, aerobics, or bicycling) as their primary exercise compare with those who engaged in walking (
), which agrees with our results for vigorous versus moderate types of PA.
Not all women entered our study when they were first attempting to conceive, introducing a possibility of both differential and nondifferential misclassification of PA. However, the observation of reduced fecundability among the high-intensity exercisers with ≤2 cycle attempts before study entry suggests that bias due to left truncation did not have a large influence on our results. More than 96% of the women in our cohort with a viable pregnancy reported using home pregnancy tests to confirm their pregnancy, suggesting that bias due to differential recognition of early pregnancy loss (which may be as high as 25% [
), the study's restriction to pregnancy planners entailed the omission of a nonnegligible fraction of total pregnancies. If pregnancy intention was related both to PA and fertility potential, our results would not apply to women with unplanned pregnancies.
Another limitation is that we did not validate our measures of physical activity. However, vigorous PA was correlated with other lifestyle and behavioral variables (e.g., education, BMI, and parity) in the expected direction. In addition, we did not ask about specific types of PA, but rather ascertained the number of hours of activity per week for all types of vigorous or moderate PA combined. Given that specific sports with varying intensities may have different effects on fecundability—e.g., an Iranian study showed that endurance and weight-category sports may confer a higher risk of amenorrhea or oligomenorrhea than other sports (
)—our results may have been influenced by nondifferential misclassification, which most likely would have biased the effect of high PA toward the null. Use of a baseline measure of physical activity, as opposed to one that was updated throughout follow-up, may have resulted in misclassification. For example, women who took longer to conceive could have modified their exercise patterns, thereby introducing differential exposure misclassification. Nevertheless, after stratifying the data by attempt time at entry into the study, we did not find strong evidence of bias in our findings. We were also unable to examine the different causes of subfertility in this study. The time to pregnancy measure represents a combination of different causes contributing to couples' subfertility, and therefore the associations reported in this study are likely to reflect the overall effect of PA on fertility.
Cohort retention in this study was similar to that reported in other large volunteer cohort studies (
). PA levels were similar for the small proportion of women lost to follow-up and women followed to a study end point, implying that bias due to selective losses was unlikely. Another consideration is that this study enrolled a self-selected sample of pregnancy planners recruited via the internet; but there is little reason to believe that such women would differ from the general population of women planning a pregnancy in ways that would lead to biased effect estimates. Two Scandinavian birth cohort studies, in which population registry data were used to compare differences between study participants and all women giving birth in the general population, showed that nonparticipation at study outset had a small impact on effect estimates (
In summary, although the present study found evidence of a dose-response relation between increasing vigorous PA and delayed time to pregnancy, results were equivocal among overweight and obese women. Moderate PA was associated with a small increase in fecundability regardless of BMI. These findings indicate that PA of any type might improve fertility among overweight and obese women, a subgroup at higher risk of infertility. Lean women who substitute vigorous PA with moderate PA may also improve their fertility. Future research investigating individual types of PA in relation to fertility, and whether overweight or obese women might benefit from increased PA when planning a pregnancy, is warranted.
The authors thank the study staff and all of the women who participated in the Snart Gravid study. The authors also thank Ms. Tina Christensen for her support with data collection and media contact, Dr. Donna Baird for her feedback on questionnaire development, and Mr. Thomas Jensen for his assistance with website design. We also thank Ms. Kristen Hahn, Ms. Rose Radin, and Ms. Kristen Banholzer for their general assistance with the manuscript.
Supplemental Table 1Moderate physical activity and time to pregnancy, stratified by selected factors.
Moderate physical activity, hours per week
Age at baseline, y
Body mass index (kg/m2)
Cycle attempts before study entry
Note: FR adjusted for cycle number, age, partner's age, BMI, alcohol consumption, pack-years of smoking, intercourse frequency, last method of contraception, and vigorous PA; 95% CI in parentheses. FR = fecundability ratio; CI = confidence interval.
Supplemental Table 2Total physical activity and time to pregnancy, stratified by selected factors.
Total MET-hours of physical activity per week
Age at baseline, y
Body mass index (kg/m2)
Cycle attempts before study entry
Note: FR adjusted for cycle number, age, partner's age, body mass index, alcohol consumption, pack-years of smoking, intercourse frequency, and last method of contraception; 95% CI in parentheses. FR = fecundability ratio; CI = confidence interval.
L.A.W. has nothing to disclose. K.J.R. has nothing to disclose. E.M.M. has nothing to disclose. H.T.S. has nothing to disclose. A.H.R. has nothing to disclose. E.E.H. has nothing to disclose.
Supported by the National Institute of Child Health and Human Development ( R21-050264 ) and the Danish Medical Research Council ( 271-07-0338 ). The content of this article is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.