The average age of women in our cohort was 28.4 years (range 18–40 years), 67% were nulliparous, and 69% became pregnant within 12 cycles. Mean caffeine consumption reported in the baseline questionnaire was 137 mg/day (median= 86; range= 0 to 1425); over the follow-up period, average caffeine consumption declined to 130 mg/day (median= 80.6; range= 0 to 1425). In total, 59% of the women reported drinking regular coffee in the previous month, 51% reported drinking regular tea, and 75% reported drinking soda on the baseline questionnaire. Herbal or green tea consumption was reported by 39%, regular cola by 31%, diet cola by 45%, non-cola sodas (regular and diet) by <20% of the women, and decaffeinated coffee by 5%. Few women (7%) reported no consumption of caffeinated beverages, and there was little missing data (for example, 1%, 2%, and 3% women were missing baseline data on the number of coffee, tea, and colas, respectively). In general, consumption of all caffeinated beverages and sodas tended to decline progressively over the follow-up period, with the largest reductions occurring between baseline and the first follow-up.
Baseline characteristics of the study population varied by beverage type (). Women who consumed the most coffee tended to be older, have higher parity, and were more likely to smoke and drink alcohol than women who did not consume coffee. Women who drank large amounts of regular tea were somewhat older, drank more alcohol, and were slightly less likely to be current smokers than women who did not drink regular tea. BMI and physical activity were not strongly associated with coffee or tea consumption. In contrast, women who drank soda had higher BMIs and were less physically active than other women. Soda drinkers were also slightly younger and tended to have fewer years of education. Frequency and timing of intercourse were not strongly related to beverage consumption except among the small number of women (n=17) who drank 3 or more sodas per day. These women were less likely to have frequent intercourse, but more likely to time intercourse.
Selected characteristicsa of 3628 women according to consumption of caffeinated beverages at baseline: the Snart Gravid Study, 2007–2009.
Overall, we found little association between total caffeine intake (mg/day) and fecundability, using either caffeine exposure at baseline or updating exposure over follow-up; adjusted FRs for time-varying data ranged from 0.98 to 1.07 for categories of consumption above 100 mg/day compared with <100 mg/day (). We did not find a monotonic trend of coffee consumption on fecundability, and all FRs for coffee consumption were close to 1.0, whether using baseline or time-varying data.
Association of Baseline and Time-varying caffeine and beverage consumption with time to pregnancy (censored at 12 cycles)
Only 11% of women reported drinking one or more servings of regular tea per day at baseline. Women who drank regular tea had moderate increases in fecundability for 2+ servings per day (FR= 1.27 [95% CI= 0.98–1.64]), with no monotonic trend according to servings per day. There was little association between herbal or green tea consumption and fecundability (). The associations were similar using baseline data.
Women who drank 2 or more servings per day of any type of soda appeared to have lower fecundability (FRs ranging from from 0.48 to 0.79), but these estimates were imprecise. For all sodas combined, the adjusted FRs were 0.89 (95% CI= 0.80–0.98), 0.85 (0.71–1.02), 0.84 (0.57–1.25), and 0.48 (0.21–1.13) for <1, 1, 2 and 3+ servings per day respectively, compared with none. Sugar-sweetened sodas appeared to have a slightly stronger inverse association with fecundability than diet sodas, with FRs of 0.91 (0.83–1.00), 0.72 (0.52–1.01), and 0.58 (0.25–1.36), respectively for <1, 1, and 2+ sugar-sweetened sodas per day versus none and FRs of 0.97 (0.88–1.07), 1.07 (0.88–1.32), and 0.79 (0.51–1.23), respectively for <1, 1, and 2+ unsweetened sodas per day versus none (). For most soda variables, the FRs for baseline beverage consumption tended to be closer to the null than the time-varying results ().
The displays the association between number of servings of all types of soda per day and fecundability, using restricted cubic splines. As in the categorical analysis shown in , the FR is lower among women who reported drinking sodas compared with those who did not drink any sodas, but the estimates are imprecise.
Association between sweetened sodas and fecundability, fitted by restricted cubic splines. The curves are adjusted for age, partner age, pack-years of smoking, alcohol intake, body mass index, level of physical activity, and intercourse frequency.
Analyses of viable pregnancies, rather than all pregnancies, did not produce any major change in effect estimates for total caffeine or any individual beverage (data not shown). The results using months instead of cycles were very similar to those using estimated number of menstrual cycles as the underlying metric for time to pregnancy (data not shown).
In general, we found little evidence that the effect measures for each beverage varied by age, parity, smoking, alcohol intake, menstrual cycle regularity, and number of cycle attempts before study entry (eTables 1–3
). For example, results were similar to our overall results among women with regular cycles (FR= 1.04 [95% CI= 0.80–1.34], 1.18 [0.88–1.57] and 0.71 [0.47–1.08] for coffee [3+ servings/day], tea [2+servings/day], and sodas [2+ servings/day], respectively, compared with none). Similar results were also found in women who had been trying for 2 cycles or fewer at entry (1.12 [0.85–1.46], 1.22 [0.91–1.64] and 0.64 [0.41–1.00] for highest level of coffee, tea, and sodas, respectively, compared with none). Coffee appeared to have slight detrimental effects on fecundability among older women (age 30+ years) and parous women, at high levels of coffee intake (3+ servings per day). In contrast, consuming large amounts of coffee was associated with increased fecundability among younger women (age <30 years) and nulliparous women. Smokers who drank coffee also seemed to have slightly increased fecundability, while fecundability was reduced among non-smokers who consumed 3+ servings per day vs. none (FR=0.83 [95% CI= 0.55–1.23]). Coffee also appeared to be inversely associated with fecundability among the women who had been trying to conceive for 3–6 cycles at entry, while all FRs for coffee were greater than 1.0 among women who had been trying for 2 cycles or less. The associations between either tea or soda consumption and fecundability appeared to vary little according to categories of age, parity, smoking, cycle regularity, or the number of cycle attempts before study entry.