Historical relationship between temperature and conception rates
Estimating the historical relationship between temperature and conception rates, we find that exposure to a hot day (daily mean temperature > 25 °C) slightly reduces conception rates in the week of the exposure (− 0.18%, p = 0.068) and the following week (− 0.29%, p = 0.029) compared with a day with a mean temperature of 15–20 °C (Fig. 1a). Two weeks after exposure, weekly conception rates are more strongly decreased, by 0.85% (p = 0.000), whereas the coefficients on weeks 3 and 4 are both − 0.80% (p = 0.000). Five weeks after exposure, the impact is lower (− 0.44%, p = 0.000). Between weeks 6 and 10, the coefficients are practically zero, whereas from week 11, they begin to increase, and until week 22, they are consistently positive, indicating an increased conception rate over this period. However, most of the 95% confidence intervals include zero. These results suggest that exposure to hot temperatures changes the timing of some conceptions that do not disappear completely but are delayed by several weeks.
We investigate the possible displacement further by calculating the sum of the coefficients (total effect) over weeks (lags) 0–5 and 6–25. These calculations show the extent to which the initial change in conception rates over weeks 0–5 is compensated by a rebound in the later weeks. The total effect of exposure to hot temperature is − 0.034 log points over weeks 0–5 and 0.017 log points over weeks 6–25 (Fig. 1b). This suggests that approximately half of the short-term decline is compensated by a rebound within 6 months following the exposure. Nevertheless, the total impact remains negative: exposure to a > 25 °C day reduces the overall conception rate over a 26-week period by 0.06% (p = 0.012). The impacts of exposure to a 20–25 °C day are similar but lower in magnitude. The cumulative effects over weeks 0–5 and weeks 6–25 are − 0.011 and 0.003 log points, respectively (see also Fig. 6 in the Appendix for the individual coefficients). Importantly, temperature exposure seems to have a monotonic, nonlinear effect. Colder temperatures below the omitted category have small positive effects over weeks 0–5 and small negative effects over weeks 6–25, with no apparent differences between temperature categories. In sum, temperatures between ≤ − 5 °C and 15–20 °C seem to have more or less similar impacts on the conception rate, but as temperature increases above 20 °C (and especially above 25 °C), conception rates decrease in the short term (up to 5 weeks after the exposure) and partially rebound after that.
In theory, three mechanisms can drive the decline in conception rates over the next few weeks after the heat exposure. First, heat might reduce sexual activity. Second, it could change conception chance. Third, it might influence the chance of a clinically unrecognized loss of an embryo. Unfortunately, the data we use do not allow us to determine the exact importance of these channels. However, that we see a small effect in the week of the exposure and larger effects later suggests that hot weather has no sizeable negative influence on sexual behavior. Indeed, previous studies report that heat does not decrease sexual activity (Hajdu and Hajdu 2019), but interest in sex is rather driven by holidays and cultural/religious celebrations (Wood et al. 2017).Footnote 6 The second channel might be an important one. As mentioned before, experiments with mammals suggest that the conception chance is diminished by heat exposure (Wettemann et al. 1979; Jannes et al. 1998; Yaeram et al. 2006; Paul et al. 2008). Human studies report that heat suppresses spermatogenesis (Macleod and Hotchkiss 1941; Robinson et al. 1968; Brown-Woodman et al. 1984; Carlsen et al. 2003; Wang et al. 2007; Ahmad et al. 2012; Garolla et al. 2013; Zhang et al. 2015). Although the results of these papers are not directly comparable with our study, they are similar in that they usually report a prolonged but reversible (U-shaped) impact on various sperm parameters. This suggests that exposure to heat decreases the conception rate by reducing human reproductive health (e.g., sperm quality). Finally, because a sizeable portion of human pregnancies ends in a clinically unrecognized pregnancy loss (Wilcox et al. 1988; Zinaman et al. 1996) and therefore is not included in any administrative dataset, we cannot rule out that exposure to hot weather before the conception also diminishes the survival probability of the fetus (before clinical recognition).
We test the sensitivity of the results by a wide range of additional model specifications: controlling for lagged weekly conception rates (up to 25 weeks), excluding precipitation controls, including year-by-season fixed effects, excluding county-by-week quadratic time-trends (Fig. 7, Appendix), including more temperature lags (Fig. 8, Appendix), applying alternative clustering of the standard errors (Fig. S2 of the Supplementary Materials, Hajdu and Hajdu 2020a), and using the total number of women as the denominator in the calculation of the conception rate (Fig. S3 of the Supplementary Materials, Hajdu and Hajdu 2020a). We also use narrower (3 °C wide) temperature categories (Fig. 9, Appendix), estimate the relationship between temperature and conception rates on an aggregated (country level) dataset (Fig. S4 of the Supplementary Materials, Hajdu and Hajdu 2020a), and estimate a polynomial distributed lag specification where the temperature (and precipitation) coefficients are defined as a 6th order polynomial (Fig. S5 of the Supplementary Materials, Hajdu and Hajdu 2020a).Footnote 7 We also use daily minimum or maximum temperature instead of daily mean temperature (Fig. S6 of the Supplementary Materials, Hajdu and Hajdu 2020a). The results using 3 °C wide temperature categories suggest that the effect of temperature is increasing past 25 °C. In addition, some of the alternative specifications result in slightly smaller and less precise estimates. Nevertheless, none of these changes alters the main conclusions.
No apparent differences are observed between the earlier and later years in our sample, counties below and above the median per capita income, or counties below and above the median yearly average temperature (Fig. S7–S9 of the Supplementary Materials, Hajdu and Hajdu 2020a). These results are not surprising as our sample covers a relatively short period, and the differences across the counties in terms of per capita income or yearly average temperature are usually not very large.
In addition, as placebo tests, the temperature and precipitation variables are replaced with weather data that were measured exactly 1, 2, or 3 years later. Because conception rates could not have been affected by temperature in the distant future, zero or close to zero coefficients should be observed in the placebo regressions. These estimations further support the credibility of the baseline results (Fig. 10, Appendix). In general, as expected, the estimated individual coefficients are usually insignificant. The total impacts show fairly random patterns.
We also estimated Eq. (3) separately for conceptions ending in live births, induced abortions, and spontaneous fetal losses. These results are depicted in Fig. 2. The impacts over lags 0–5 are similar, although for the less frequent pregnancy outcomes, the statistical uncertainty is much higher. However, we can observe important differences over lags 6–25. For conception rates calculated from pregnancies ending in live births, the rebound is substantial and mirrors the initial impacts.Footnote 8 But for conception rates calculated from pregnancies ending in induced abortions and spontaneous fetal losses, a similar rebound is basically non-existing. These results are likely to reflect that individuals who desire to have a baby are very likely to eventually have one, even if it is delayed because of exposure to hot weather. However, if heat exposure prevents an unintended pregnancy, then it is less likely that the “missing” conception will be replaced by another pregnancy a couple of months later. Because most pregnancies ending in live births are planned/intended in Central and Eastern Europe (Bearak et al. 2018), it is not surprising that we can observe a large rebound. In contrast, induced abortions are much more likely to be the result of unplanned pregnancies. Therefore, a sizeable rebound after the initial decline due to exposure to heat is not expected when analyzing conception rates calculated from pregnancies ending in induced abortions. Regarding spontaneous fetal losses, numerous factors may contribute to the observed pattern of the estimated impacts (lack of rebound). First, unplanned pregnancies have higher odds of miscarriage (Maconochie et al. 2007). Second, some intended conceptions from the summer months (when they are more likely to be exposed to heat) are likely to occur a couple of months later as a result of the heat exposure. This shift, however, influences in utero temperature exposures of the fetuses. Their first trimester exposure to hot days will decrease, whereas their exposure during the second and especially third trimesters will increase. Because animal studies suggest that heat exposure during early pregnancy increases embryo loss (Ulberg and Burfening 1967; Edwards et al. 2003; Romo-Barron et al. 2019), this decreased first trimester exposure to hot days could lower the risk of miscarriage. Therefore, conceptions ending in a spontaneous fetal loss will increase to a lesser extent over weeks 6–25 after the exposure than a simple delay in the time of conception would predict.
Projected impacts of climate change
To quantify the impacts of climate change, the estimated temperature–conception rate relationship is combined with the projected changes in temperature distribution between the periods of 1986–2005 and 2040–2059 by calendar week. First, we show the projections for the overall conception rate. Next, we replicate these projections by conception type (pregnancy outcome). We present interquartile ranges and the ranges containing 95% of the projections.
Seasonal differences in conception rates are likely to be larger by the mid-twenty-first century because of climate change (Fig. 3a and b). We project a substantial decline between the 23rd and 42nd calendar weeks as a result of the increase in the number of hot days. The impacts are especially large for calendar weeks 30 to 38: the median projections in RCP 8.5 reflect a decline of between 5.5 and 7.5%. At the same time, conception rates are projected to increase in the first calendar weeks and especially in the last 10 weeks of the year. Regarding the annual impact of climate change, practically all projections suggest a decline in annual conception rates (Fig. 3c). The interquartile ranges of the projections spread from − 0.92% to − 0.47% for RCP 4.5 and from − 1.18% to − 0.61% for RCP 8.5.
Using alternative model specifications to estimate the historical temperature–conception rate relationship, in most cases, does not considerably alter the projected impacts of climate change (Fig. 11, Appendix). However, using narrower (3 °C wide) temperature categories results in a slightly stronger projected impact. This specification allows to account for the fact that the effect of temperature is increasing past 25 °C (see Fig. 9, Appendix), and the average temperature within the > 25 °C category will increase in the future (Fig. S10 of the Supplementary Materials, Hajdu and Hajdu 2020a). Nevertheless, the qualitative results are the same in all these estimations: seasonal differences in conception rates will increase because of climate change, and the annual rates will decrease by a few percent during the next decades.Footnote 9
Next, we calculate the impacts of climate change by conception type. We use historical estimates on the temperature–conception rate relationship from models where conception rates were calculated from pregnancies ending in (i) live births, (ii) induced abortions, or (iii) spontaneous fetal losses, and combine them with the projected temperature changes as was done before. Seasonal differences in conception rates will be larger for all kinds of conceptions (Fig. 4a, b, d, e, g, h). Conception rates in the summer and early autumn months are projected to decrease, whereas conception rates during winter and late autumn are projected to increase. A notable difference is that the winter/autumn increase is more significant for live births than for spontaneous fetal losses or induced abortions. As a result, the differences in terms of annual impacts are substantial (Fig. 4c, f, i). The overall conception rate based on live births seems to be unaffected by a climate change–induced shift in temperature distribution, whereas the annual conception rates based on induced abortions and spontaneous fetal losses are projected to decline by a few percent. That is, the annual decline in overall conception rate (Fig. 3c) is primarily driven by a change in the number of induced abortions and spontaneous fetal losses rather than by a change in the number of live births.
We note that although the annual conception rate based on live births (in other words, the number of births) will not change significantly as a result of climate change, the changing seasonal distribution of conceptions could have important consequences on the affected newborns. The warming climate will induce a shift in the timing of conception for a small fraction of live births. In general, conceptions will disappear from the summer months and will re-appear mostly in the winter and late autumn months (Fig. S11 of the Supplementary Materials, Hajdu and Hajdu 2020a).Footnote 10 Because of this shift, as highlighted before, the exposure of fetuses to hot days during the second and third trimesters of pregnancy will substantially increase, whereas the first trimester exposure will drop (Fig. S12 of the Supplementary Materials, Hajdu and Hajdu 2020a). A crude estimation suggests that the affected newborns will be exposed, on average, to around 16 additional hot days (> 25 °C) and 32 additional moderately hot days (20–25 °C) during the second and third trimesters due to the change in the conception date (Table 2, Appendix). Considering the whole pregnancy, these figures are 9 and 16 days, respectively. At the same time, the exposure to cold days will substantially decrease. Because there is a negative relationship between in utero exposure to hot weather (especially in the second and third trimesters) and health at birth (Deschênes et al. 2009; Sun et al. 2019; Hajdu and Hajdu 2020b; Barreca and Schaller 2020; Chen et al. 2020), the slight change in timing of conception could have a non-negligible impact. Further consequences are also possible, as temperature exposure during pregnancy influences adult outcomes too (Wilde et al. 2017; Isen et al. 2017; Fishman et al. 2019; Hu and Li 2019).