Seasonal variation, temperature, day length, and IVF outcomes from fresh cycles

Summer versus winter: the impact of the seasons on oocyte quality in in vitro fertilization cycles

OBJECTIVE

The aim of this study was to determine the effects of seasons (winter vs. summer) on oocyte quality in infertile women undergoing ovulation induction for in vitro fertilization.

METHODS

This retrospective cross-sectional study assessed 155 cycles of in vitro fertilization-induced ovulation in women, with 71 and 84 cycles occurring in the summer and winter, respectively. Oocytes were evaluated for quality, with 788 and 713 assessed during summer and winter, and classified according to Nikiforov's categories: (a) category I, good quality; (b) category 2, medium quality; and (c) category 3, low quality.

RESULTS

Thickened zona pellucida (p<0.001), increased perivitelline space (p<0.001), oocyte shape abnormalities (p=0.01), and the presence of refractile bodies (p<0.0001) were more frequent in the summer cycles, whereas cytoplasmic granularity (p<0.001) was more frequent in the winter cycles. In winter, we observed a higher frequency of category 3 (p<0.001) and category 2 (p<0.001) oocytes and a lower frequency of category 1 (p<0.001) oocytes.

CONCLUSION

Oocyte dysmorphisms were found in 70-80% of cases and were more common in winter. The main features include a thickened zona pellucida, enlarged perivitelline space, irregular shape, and cytoplasmic granularity. This implies better-quality oocytes in the summer than in the winter. However, retrospective studies have limitations due to data collection biases and potential confounding variables such as diet and exercise. Future research is needed to confirm these findings and explore the underlying mechanisms.

Association between ambient temperature exposure and pregnancy outcomes in patients undergoing in vitro fertilization in Shanghai, China: a retrospective cohort study

STUDY QUESTION

Does ambient temperature exposure affect outcomes including clinical pregnancy and live birth in women undergoing IVF?

SUMMARY ANSWER

Both extreme cold and hot ambient temperatures were significantly associated with adverse pregnancy outcomes of IVF cycles.

WHAT IS KNOWN ALREADY

Heat exposure has been linked to adverse pregnancy outcomes worldwide. However, the effect of ambient temperature on infertile women undergoing IVF treatment is unclear.

STUDY DESIGN, SIZE, DURATION

A retrospective cohort study was conducted from a database of 3452 infertile women who underwent their first fresh or frozen embryo transfer in the Shanghai First Maternity and Infant Hospital from April 2016 to December 2020.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Daily mean ambient temperature exposure for each patient was obtained based on their residential address. Temperature-stratified multiple logistic regression analysis was performed to investigate associations between temperature exposure and pregnancy outcomes after controlling for confounders. Vulnerable sub-groups were identified using forest plots.

MAIN RESULTS AND THE ROLE OF CHANCE

The clinical pregnancy rate and live birth rate were 45.7% and 37.1%, respectively. Regarding clinical pregnancy, a higher temperature during cold weather was significantly associated with a higher pregnancy rate in the period about 11 weeks before ovarian stimulation (adjusted odds ratio (aOR) = 1.102, 95% CI: 1.012-1.201). Regarding live birth, an increased temperature during cold weather was significantly related to a higher live birth rate in the period after confirmation of clinical pregnancy or biochemical pregnancy, with the aORs of 6.299 (95% CI: 3.949-10.047) or 10.486 (95% CI: 5.609-19.620), respectively. However, a higher temperature during hot weather was negatively associated with the live birth rate in the periods after confirmation of clinical pregnancy or biochemical pregnancy, with the aORs at 0.186 (95% CI: 0.121-0.285) or 0.302 (95% CI: 0.224-0.406), respectively. Moreover, the decline in live birth rates during cold and hot weather was accompanied by increased rates of early miscarriage (P < 0.05). Stratified analyses identified susceptibility characteristics among the participants.

LIMITATIONS, REASONS FOR CAUTION

Climate monitoring data were used to represent individual temperature exposure levels according to the patient's residential address in the study. We were not able to obtain information of personal outdoor activity and use of indoor air conditioners in this retrospective study, which may affect actual temperature exposure.

WIDER IMPLICATIONS OF THE FINDINGS

This study highlights that the ambient temperature exposure should be taken into account during IVF treatment and afterwards. There is a need to be alert to extremes in cold and hot ambient temperatures, especially during the period of follicle development and pregnancy. With this knowledge, clinicians can scientifically determine the timing of IVF treatment and reinforce patients' awareness of self-protection to minimize adverse pregnancy outcomes associated with extreme temperatures.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by a grant from the Clinical Research Plan of Shanghai Hospital Development Center [SHDC2020CR4080], a grant from the Science and Technology Commission of Shanghai Municipality [19411960500], and two grants from the National Natural Science Foundation of China [81871213, 81671468]. B.W.M. is supported by a NHMRC Investigator grant (GNT1176437). B.W.M. reports consultancy for ObsEva, and research grants from Merck KGaA, Ferring and Guerbet. The other authors have no conflict of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Embryo transfers performed during daylight savings time led to reduced live birth rates in older patients

PURPOSE

To study the impact of undergoing an embryo transfer during the week of daylight savings time transition on live birth rates.

METHODS

We performed a retrospective observational cohort study of patients undergoing embryo transfer at an academic infertility practice during the week of spring or fall daylight savings time transition (cases), or the 2 weeks preceding and following the daylight savings transition (controls) between 2015 and 2021. The primary exposure was completion of an embryo transfer during the week of daylight savings time transition. The primary outcome was a comparison of live birth rate per embryo transfer among individuals undergoing an embryo transfer during the week of daylight savings time (DST) transition and those who did not.

RESULTS

A total of 309 embryo transfers occurred during the week of daylight savings transition and 1242 embryo transfers occurred in the control group outside of the daylight savings transition week. The live birth rate after embryo transfer during DST transition weeks was 39.2% (121/309) compared to 40.8% (507/1242) (p = 0.59). When restricting the analysis to individuals (age > 37 years), the live birth rate after embryo transfer during DST transition week was 23.5% (24/102) compared to 34.8% (149/429) (p = 0.03). This difference persisted in the mixed-effects regression model demonstrating that after adjusting for relevant covariates, embryo transfer during DST transition weeks resulted in a 45% decrease in the odds of achieving a live birth.

CONCLUSION

Daylight savings time transition may be associated with less favorable outcomes after embryo transfer among an older infertile patient population. Future work is needed to prospectively examine the influence of circadian rhythm disruption on reproductive outcomes.

Season at the time of oocyte collection and frozen embryo transfer outcomes

STUDY QUESTION

Does the meteorological season at the time of oocyte retrieval affect live birth rates in subsequent frozen embryo transfers?

SUMMARY ANSWER

Frozen embryo transfers resulting from oocytes retrieved in summer have 30% increased odds of live birth compared to frozen embryo transfers resulting from oocytes retrieved in autumn, regardless of the season at the time of embryo transfer.

WHAT IS KNOWN ALREADY

Season at the time of frozen embryo transfer does not appear to be associated with live birth rate. One study in the northern hemisphere found increased odds of live birth with frozen embryo transfer resulting from oocytes collected in summer when compared to those collected in winter.

STUDY DESIGN, SIZE, DURATION

Retrospective cohort study including all frozen embryo transfers performed by a single clinic over eight years, from January 2013 to December 2021. There were 3659 frozen embryo transfers with embryos generated from 2155 IVF cycles in 1835 patients. Outcome data were missing for two embryo transfers, which were excluded from analysis. Outcomes were analysed by the season, temperatures, and measured duration of sunshine at the time of oocyte collection and at the time of frozen embryo transfer.

PARTICIPANTS/MATERIALS, SETTING, METHODS

There were no significant differences between patients with oocyte collection or embryo transfers in different seasons. Meteorological conditions on the day of oocyte collection and the day of frozen embryo transfer, and in the preceding 14- and 28-day periods, were collected including mean, minimum, and maximum temperatures, and recorded duration of sunshine hours. Clinical and embryological outcomes were analysed for their association with seasons, temperatures, and duration of sunshine with correction for repeated cycles per participant, age at the time of oocyte retrieval, and quadratic age.

MAIN RESULTS AND THE ROLE OF CHANCE

Compared to frozen embryo transfers with oocyte retrieval dates in autumn, transfers with oocyte retrieval dates in summer had 30% increased odds of live birth (odds ratio (OR): 1.30, 95% CI: 1.04-1.62) which remained consistent after adjustment for season at the time of embryo transfer. A high duration of sunshine hours (in the top tertile) on the day of oocyte retrieval was associated with a 28% increase in odds of live birth compared to duration of sunshine hours in the lowest tertile (OR 1.28, 95% CI: 1.06-1.53). Temperature on the day of oocyte retrieval did not independently affect the odds of live birth. The odds of live birth were decreased by 18% when the minimum temperature on the day of embryo transfer was high, compared with low (OR: 0.82, 95% CI: 0.69-0.99), which was consistent after correction for the conditions at the time of oocyte retrieval.

LIMITATIONS, REASONS FOR CAUTION

This was a retrospective cohort study, however, all patients during the study period were included and data was missing for only two patients. Given the retrospective nature, causation is not proven and there are other factors that may affect live birth rates and for which we did not have data and were unable to adjust, including pollutants and behavioural factors. We were also not able to stratify results based on specific patient populations (such as poor- or hyper-responders) nor report the cumulative live birth rate per commenced cycle.

WIDER IMPLICATIONS OF THE FINDINGS

These findings may be particularly relevant for patients planning oocyte or embryo cryopreservation. Given the increasing utilization of cryopreservation, identification of factors that influence outcomes in subsequent frozen embryo transfers has implications for future therapeutic and management options. Further studies to clarify the physiology underlying the influence of sunshine hours or season on subsequent frozen embryo transfer outcomes are required, including identification of specific populations that may benefit from these factors.

STUDY FUNDING/COMPETING INTERESTS

No funding was provided for this study. S.L. has received educational travel assistance from Besins, Merck and Organon outside the submitted work. R.H. is National Medical Director of City Fertility and Medical Director of Fertility Specialists of Western Australia, has received honoraria from MSD, Merck Serono, Origio and Ferring outside the submitted work, and has equity interests in CHA SMG. C.R., M.W., and E.N. declare that they have no conflicts of interest.

TRIAL REGISTRATION NUMBER

N/A.

Are ICSI results affected by months and seasons? A retrospective cohort study of fresh and frozen cycles

In present study, we aimed to investigate the effect of seasonal variation on ICSI results both in fresh and frozen embryo transfer cycles. Between January 2007 and December 2019, a total of 4039 ICSI cycles (3227 fresh and 812 frozen ET) which resulted in embryo transfer were included in the study. We found no significant difference in the pregnancy outcome among different seasons and months. The best results were obtained for spring (41.0%) and the least for winter (37.1%) in fresh cycles and the best for spring (42.5%) and the least for winter (29.7%) in FET cycles. In monthly distribution, the best results were obtained for September (57.0%) and the least for November (24.1%) in fresh cycles and the best for October (49.3%) and the least for December (20.2%) in FET cycles. Our study did not show any significant influence of the months or seasons on clinical pregnancy rates in either fresh and frozen embryo transfers. However, the difference among months in frozen cycles was notable although it did not reach statistical significance. It can be suggested that the strict control of reproductive hormones especially in fresh cycles as well as the standardized laboratory temperature minimize the influence of seasonal effect on ICSI outcome.

Season and temperature do not affect cumulative live birth rate and time to live birth in in vitro fertilization

Objective

To explore whether season and temperature on oocyte retrieval day affect the cumulative live birth rate and time to live birth.

Methods

This was a retrospective cohort study. A total of 14420 oocyte retrieval cycles from October 2015 to September 2019. According to the date of oocyte retrieval, the patients were divided into four groups (Spring(n=3634);Summer(n=4414); Autumn(n=3706); Winter(n=2666)). The primary outcome measures were cumulative live birth rate and time to live birth. The secondary outcome measures included the number of oocytes retrieved, number of 2PN, number of available embryos and number of high-quality embryos.

Results

The number of oocytes retrieved was similar among the groups. Other outcomes, including the number of 2PN (P=0.02), number of available embryos (p=0.04), and number of high-quality embryos (p<0.01) were different among the groups. The quality of embryos in summer was relatively poor. There were no differences between the four groups in terms of cumulative live birth rate (P=0.17) or time to live birth (P=0.08). After adjusting for confounding factors by binary logistic regression, temperature (P=0.80), season (P=0.47) and duration of sunshine(P=0.46) had no effect on cumulative live births. Only maternal age (P<0.01) and basal FSH (P<0.01) had an effect on cumulative live births. Cox regression analysis suggested no effect of season(P=0.18) and temperature(P=0.89) on time to live birth. Maternal age did have an effect on time to live birth (P<0.01).

Conclusion

Although season has an effect on the embryo, there was no evidence that season or temperature affect the cumulative live birth rate or time to live birth. It is not necessary to select a specific season when preparing for IVF.

Seasonal fluctuation of in vitro fertilization encounters in the United States

PURPOSE

To examine patient and hospital characteristics related to seasonal fluctuation in in vitro fertilization (IVF).

METHODS

This retrospective cohort study examined 33,077 oocyte retrievals identified in the National Ambulatory Surgery Sample. Exposure assignment was monthly IVF encounters: low-volume months (<25%ile), mid-volume months (≥25/<75%ile), and high-volume months (>75%ile). Main outcomes were patient and hospital characteristics related to the exposure groups, assessed with a multinomial regression model.

RESULTS

The median IVF encounters were 977 per month, ranging from 657 to 1074 (absolute-difference 417). January, July, and December were the lowest-quartile volume months, ranging from 657 to 710 encounters per month (low-volume months). May, August, and November were the top-quartile volume months, ranging from 1049 to 1074 encounters per month (high-volume months). In a multivariable analysis, patients undergoing IVF in the low-volume months were younger and less likely to have infertility or comorbidities. Patients undergoing IVF in the high-volume months were more likely to have lower household income and receive IVF at urban teaching facilities. Northeastern residents were less likely to have IVF in the low-volume months but more likely to have IVF in the high-volume months. Sensitivity analyses showed that the lowest-to-highest variability in monthly IVF encounters was higher in Northeast region compared to other regions (320 vs 50-128); infertility patients compared to those without (317 vs 190); privately insured patients compared to self-pay (227 vs 156); and older patients compared to younger (234 vs 192).

CONCLUSION

This study suggests substantial seasonal fluctuation in IVF oocyte retrieval in the USA based on patient and hospital factors.

Influence of climatic conditions in the mesothermal climate area on pregnancies following elective fresh single blastocyst transfer

PURPOSE

To investigate the effect of climatic parameters in the mesothermal climate area on clinical pregnancy and live birth following fresh single blastocyst transfer.

METHODS

This study investigated clinical pregnancies and live births that resulted from 555 ovarian stimulation cycles followed by fresh single blastocyst transfer. The samples were stratified according to climatic conditions (low T, temperature < 12.9 °C; middle T, 12.9 °C ≤ temperature < 22.6 °C; high T, temperature ≥ 22.6 °C; low H, relative humidity < 62.1%; middle H, 62.1% ≤ relative humidity < 66.5%; high H, relative humidity ≥ 66.5%; short S, sunlight duration < 5.2 h; middle S, 5.2 h ≤ sunlight duration < 6.7 h; long S, sunlight duration ≥ 6.7 h). Clinical pregnancy and live birth rates among three groups from each climatic parameter were compared. Multivariable analyses were performed to investigate the effects of climatic conditions on blastocyst development, endometrial thickness, clinical pregnancy, and live birth.

RESULTS

A statistically significant difference was found in pregnancy rates among low T (48.8%), middle T (37.3%), and high T (36.6%) groups. Multivariable analyses revealed that temperature was associated with clinical pregnancy and live birth rates with adjustment for patient age, BMI, type of ovarian stimulation, endometrial thickness, and expansion grade of the transferred blastocyst. The association between climatic parameters and blastocyst development and endometrial thickness was not confirmed.

CONCLUSIONS

This study suggests that lower temperatures in the mesothermal climate area could favorably affect the rates of clinical pregnancy and live birth achieved by fresh single blastocyst transfer.

Summer is not associated with higher live birth rates in fresh IVF/ICSI cycles: a population-based nationwide registry study

STUDY QUESTION

Is summer associated with a higher live birth rate after fresh IVF/ICSI?

SUMMARY ANSWER

There was no support for a higher live birth rate after fresh IVF/ICSI when treatment was performed during the summer season.

WHAT IS KNOWN ALREADY

Seasonal variations in human natural conception and birth rates are well described. It has been hypothesized that serum vitamin D, levels of which are associated with sun exposure, may have a role in human natural conception rates. However, the association between seasons and IVF outcomes has not yet been clarified and conflicting reports have been published. Furthermore, it has been suggested that women with normal vitamin D levels have a better pregnancy outcome after ART compared to those with vitamin D insufficiency.

STUDY DESIGN SIZE DURATION

A nationwide, register-based cohort study including all first-time fresh IVF/ICSI treatments (n = 52 788) leading to oocyte retrieval in Sweden between 2009 and 2018 was carried out.

PARTICIPANTS/MATERIALS SETTING METHODS

All first-time fresh IVF/ICSI cycles leading to oocyte retrieval were identified in the National Quality Registry of Assisted Reproduction. Data collected included patient characteristics as well as information about the treatment cycle and pregnancy outcome. The patients were divided into season subgroups, (summer, autumn, winter and spring) based on the date of oocyte retrieval. The primary outcome was live birth rate, which was defined as the number of live births per oocyte retrieval and embryo transfer (ET). Other outcomes included clinical pregnancy per ET and miscarriage per clinical pregnancy. Logistic regression with multiple imputation was performed to evaluate whether there was an association between season and IVF/ICSI outcomes, with summer as reference. Adjustments were made for woman's age, year of treatment, BMI, total FSH/hMG dose, type of treatment, fertilization type, embryonic stage at ET and number of embryos transferred.

MAIN RESULTS AND THE ROLE OF CHANCE

Live birth rate per oocyte retrieval ranged between 24% and 26% among seasons. A significantly higher live birth rate was seen for spring compared with summer, 26% versus 24%, respectively (adjusted odds ratio (OR) 1.08, 95% CI 1.02-1.16, P = 0.02). No significant association was seen when winter and autumn were compared with summer. Live birth rate per ET ranged between 29% and 31% among seasons. A significantly higher live birth rate was seen for spring and autumn compared with summer, at 31% and 31%, respectively versus 29% (adjusted OR 1.08, 95% CI 1.01-1.16, P = 0.04 and adjusted OR 1.09, 95% CI 1.01-1.16, P = 0.02), respectively. No significant association was seen when winter was compared with summer. Clinical pregnancy rate varied between 36% and 38% and miscarriage rate between 16% and 18%, with no significant seasonal associations.

LIMITATIONS REASONS FOR CAUTION

Possible limitations are the retrospective design of the study and unmeasured confounders. Another limitation is that a generalized estimating equation (GEE) model was not used. The use of a GEE model would have made it possible to include all started fresh IVF/ICSI cycles since it allows for correction for any dependence between cycles within women.

WIDER IMPLICATIONS OF THE FINDINGS

The results of this large registry study give no support for the hypothesis that IVF/ICSI treatments performed during summer season, with the highest degree of sunlight and vitamin D synthesis, is associated with higher pregnancy and live birth rates. In fact, our results showed significantly lower live birth rates during summer compared with spring and autumn. However, the magnitude of this difference was small and unlikely of clinical value. We suggest that season should not be taken into consideration when planning and performing fresh IVF/ICSI treatments.

STUDY FUNDING/COMPETING INTERESTS

Financial support was received through the Swedish state under the agreement between the Swedish government and the county councils, the ALF-agreement (ALFGBG-70 940) and grants from the Hjalmar Svensson's Research Foundation (HJSV2021019 and HJSV2021037). None of the authors declare any conflict of interest.

TRIAL REGISTRATION NUMBER

N/A.

Effects of seasonal variations and meteorological factors on IVF pregnancy outcomes: a cohort study from Henan Province, China

OBJECTIVE

To investigate whether seasonal variations and meteorological factors influence pregnancy outcomes in women undergoing in vitro fertilization-embryo transfer (IVF-ET) treatment.

DESIGN

Retrospective cohort study.

SETTING

University-affiliated reproductive medical center.

SUBJECTS

Women aged < 35 years undergoing IVF from June 1, 2015, to June 1, 2019.

INTERVENTIONS

Cycles were divided into four groups according to the date of the beginning of ovulation induction: spring (659 cycles), summer (578 cycles), autumn (519 cycles), and winter (534 cycles).

RESULTS

The high-quality embryo rate was higher in autumn and winter than in cycles in which ovulation induction occurred in spring and summer (58.70% vs. 58.78% vs. 62.67% vs. 63.42%; P < 0.001). The results of linear regression analysis showed that the high-quality embryo rate was significantly correlated with the daily average temperature of ovulation induction (P = 0.037). The clinical pregnancy rates of cycles starting ovulation induction in spring, summer, and autumn were significantly higher than those starting in winter (70.71% vs. 73.18% vs. 70.13% vs. 65.17%; P = 0.031), while the biochemical pregnancy rate, early abortion rate, and live birth rate were not significantly different (P > 0.050). Multivariate logistic regression analysis showed significant seasonal variation in clinical pregnancy (OR = 1.643, 95% CI = 1.203-2.243; P = 0.002), and that a higher daily average temperature at the time of ovulation induction increased the clinical pregnancy rate (OR = 1.012, 95% CI = 1.001-1.022; P = 0.031).

CONCLUSIONS

In women younger than 35 years who undergo IVF treatment, the season and ambient temperature on the date of the beginning of ovulation induction may have an impact on embryo development and clinical pregnancy.

The association between season, day length, and temperature on clinical outcomes after cryopreserved embryo transfer

OBJECTIVE

To investigate whether there is an association between season, temperature, and day length at oocyte retrieval and/or embryo transfer (ET) and clinical outcomes in frozen ET cycles.

DESIGN

Retrospective cohort study.

SETTING

Large academically affiliated research hospital.

PATIENT(S)

A total of 3,004 frozen ET cycles from 1,937 different women with oocyte retrieval and transfer between 2012 and 2017.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Implantation, clinical pregnancy, spontaneous abortion, and live birth.

RESULT(S)

Frozen ETs with oocyte retrieval dates in summer had 45% greater odds of clinical pregnancy (odds ratio [OR], 1.45; 95% confidence interval [CI], 1.15-1.82) and 42% greater odds of live birth (OR, 1.42; 95% CI, 1.13-1.79) compared with those with oocyte retrieval dates in winter. A 41% greater odds of clinical pregnancy (OR, 1.41; 95% CI, 1.16-1.71) and 34% greater odds of live birth (OR, 1.34; 95% CI, 1.10-1.62) were observed among transfers with an average temperature at oocyte retrieval in the highest tertile (17.2-33.3 °C) compared with those in the lowest tertile (-17.2-6.7 °C). There were no consistent associations between clinical outcomes and day length at oocyte retrieval or between season, day length, or temperature at transfer of thawed embryos.

CONCLUSION(S)

Warmer temperatures at oocyte retrieval are associated with higher odds of clinical pregnancy and live birth among frozen ET cycles. The consistent associations seen with oocyte retrieval dates and the lack of associations observed with ET dates suggest that any seasonality effects on in vitro fertilization success are related to ovarian function and not uterine receptivity.