Relationship between seasons and pregnancy rates during intrauterine insemination. A historical cohort

ABSTRACT BACKGROUND: The underlying cause of seasonal infertility in humans is unclear, but is likely to be multifactorial. OBJECTIVE: The aim of our study was to compare the pregnancy rates among infertile women who underwent induced ovulation and intrauterine insemination (IUI) with the season in which the fertility treatment was performed. DESIGN AND SETTING: This retrospective cohort study was conducted on 466 patients who were treated in the reproductive endocrinology and infertility outpatient clinic of a tertiary-level women’s healthcare and maternity hospital. METHODS: Retrospective demographic, hormonal and ultrasonographic data were obtained from the patients’ medical records. Clomiphene citrate or gonadotropin medications were used for induced ovulation. The patients were divided into four groups according to the season (spring, winter, autumn and summer) in which fertility treatment was received. Clinical pregnancy rates were calculated and compared between these four groups. RESULTS: There were no significant differences between the seasonal groups in terms of age, infertility type, ovarian reserve tests, duration of infertility, medications used or length of stimulation. A total of 337 patients (72.3%) were treated with clomiphene citrate and 129 (27.7%) with gonadotropin; no significant difference between these two groups was observed. The clinical pregnancy rates for the spring, winter, autumn and summer groups were 15.6% (n = 24), 8.6% (n = 9), 11.5% (n = 13) and 7.4% (n = 7), respectively (P = 0.174). CONCLUSIONS: Although the spring group had the highest pregnancy rate, the rates of successful IUI did not differ significantly between the seasonal groups.


INTRODUCTION
Many environmental factors influence human fertility outcomes. Although extensive research has shown that mammalian fertility is influenced by seasonal changes, few studies have specifically evaluated seasonal effects on the human reproductive system. 1 Seasonal infertility may be due to physiological changes that are season-dependent. It was previously shown that season-dependent high environmental temperatures negatively affect sexual function and nutrient intake. [2][3][4] Conversely, a five-year study conducted in France, in which the effect of the photoperiod on seasonal infertility was examined, demonstrated that seasonal infertility was independent of environmental temperatures. 5 Melatonin affects several daily and seasonal rhythms, such as endocrine signaling during both circadian time and daytime. Melatonin concentrations follow different circadian rhythm in different living things. Nighttime exposure to light inhibits melatonin synthesis and secretion in both animal models 6 and humans. 7 Serum levels of melatonin are affected by the complex interaction of daily rhythm, exogenous factors and endogenous factors. Melatonin receptors have been identified in human reproductive tissues. 8 Melatonin plays a major role in reproductive activity and blastocyst implantation. It is transferred maternally to the fetus via the placenta or milk, during pregnancy and lactation, respectively, thus indicating that the maternal photoperiod is transferred to the fetus. 9 In addition, it has been shown that melatonin causes seasonal changes relating to fertilization, embryo quality, sperm concentration and chromatin condensation rates. 10  orcid.org/0000-0002-8361-7908 melatonin cause amenorrhea, which leads to decreased secretion of gonadotropin and prolactin in response to the photoperiod. 11 Several retrospective studies have evaluated the impact of seasonal variation on in vitro fertilization outcomes. Some of these studies considered climatic conditions, especially temperature and the number of hours of daylight. 12 However, few studies have evaluated seasonal effects on pregnancy rates among human patients undergoing intrauterine insemination (IUI). 13,14 Seasonality of infertility treatment may alter reproductive performance; and therefore, the timing of infertility treatment may result in improved pregnancy rates.

OBJECTIVE
This study compared pregnancy rates during different seasons in which fertility treatments were performed, in order to investigate whether seasonal variations were associated with pregnancy outcomes among infertile women who underwent induced ovulation and IUI.

Study design, settings and ethics
A retrospective cohort study based on medical records was con- All consecutive infertile couples who met the inclusion criteria during the study period of two years were recruited for this study.
All the study participants met the following criteria: < 41 years of age; normal hysterosalpingography (HSG) and/or laparoscopy findings; regular menstrual cycles with normal baseline levels of serum Patients undergoing treatments (both induced ovulation and IUI) that were performed in one season were included in the study.
When a treatment (induced ovulation or IUI) overlapped with the previous or next season, the patient was excluded from the study.
This study was conducted in Ankara (latitude: 32.87° N, longitude: 39.87° E; altitude: 891 m). Ankara has a continental climate, which means that it has cold, snowy winters and hot, dry summers.
Rainfall is mostly seen during the spring and autumn months.
During the 24-month study period, the average seasonal air temperatures were 22.3 °C in the summer, 12.9 °C in the autumn, 1.4 °C in the winter, and 11.0 °C in the spring. 16 Demographic, hormonal and ultrasonographic data relating to the patients were copied from the patients' files and the hospital's electronic database. The patients were divided into spring, winter, autumn and summer groups.

Fertility treatments
Patients with unexplained or anovulatory infertility were first treated with clomiphene citrate. Women who received clomiphene citrate treatment but were unable to conceive were next treated with gonadotropin. Patients who declined clomiphene citrate treatment, were over the age of 35 years or had experi-  The patients underwent a serum pregnancy test on day 14 following IUI. Clinical pregnancy was defined as the presence of a gestational sac with an accompanying fetal heartbeat detected by means of ultrasound, at least five weeks after IUI.

Statistical analysis
All analyses were performed using the Statistical Package for the Social Sciences 15.0 (SPSS, Chicago, IL, USA) for Windows. Normal distribution of data was assessed using the Kolmogorov-Smirnov test. Continuous variables were presented as mean ± standard deviation (SD). Intragroup differences were investigated using one-way analysis of variance (ANOVA).
Categorical variables were expressed as the number (with percentage). Differences between data categories were evaluated using the chi-square or Fisher's exact test. Statistical significance was assumed based on a probability of 0.05.
The sample size calculation was performed using the DSS statistical software package for research sample size calculations. 17 The primary aim of this study was to compare the differences in clinical pregnancy rate between the seasons. It was calculated that a minimum of 80 participants in each group would be required to demonstrate a difference of at least 10% between the groups, with a power of 80% at the 5% significance level. This difference of 10% was taken both from a pilot study 14 and from our clinical experiments.

RESULTS
A total of 466 women were enrolled in the study. The study participants were divided into four groups according to the season in which induced ovulation and IUI treatment were received.
The spring, winter, autumn and summer groups contained 154, 105, 113 and 94 patients, respectively. No significant differences were observed between the groups in terms of age, primary infertility rate, baseline hormone levels, antral follicle count or duration of infertility (P > 0.05). The cycle characteristics, including the induced ovulation treatment protocol and duration of stimulation, were also similar among the groups ( Table 1) The overall clinical pregnancy rate was 11.4% in this cohort.
Although the clinical pregnancy rate was highest in the spring season, there was no significant difference between the seasons.

DISCUSSION
In this study, we aimed to investigate the seasonal variations of IUI success. To the best of our knowledge, this was the first study to analyze the seasonal variations of pregnancy rates following IUI in Turkey. Women were evaluated over a period of more than 24 months in this study. Regardless of whether the analysis com-  High temperatures above the thermo-neutral zone have been shown to reduce birth rates and delay the onset of puberty. 26 It has been suggested that heat stress is a probable factor in the development of seasonal infertility 27 and that this negatively affects embryo development. 28 Heat shock proteins, which appear in response to heat stress, 29 are found in the ovaries. 30 It has been shown that hyperthermia affects developing oocytes.
In a study by Palacios, the pregnancy rate after artificial insemination of dairy sheep was significantly affected by seasonal meteorological variables. 31  However, we did not find any significant difference in relation to seasonal changes. On the other hand, we did not measure temperature, weather or daily exposure to light.
The relationship between birth, fertility and seasonal change is unclear. Roenneberg and Aschoff 34 defined a circadian rhythm that changed over time for birth rates around the world. Pregnancy rates in relation to seasonal fecundability were also analyzed in their study. They hypothesized that the biological rhythm of conception is influenced by social or by environmental factors. Their conclusion was that although conception and birth rhythms vary from country to country, these rhythms have changed their characteristics recently, after having remained stable for more than a century.
The limiting feature of our study was that we did not know the exact meteorological data and temperatures. The sleep patterns and stress levels, which may have been affected by melatonin, were unknown. In addition, changes to people's lifestyles are increasing, as an inevitable consequence of modern life. Today, people live in houses with a constant temperature, are exposed to artificial light instead of sunlight and come into less and less contact with the external environment. All of these factors may lead to lower but more stable reproductive performance, through elimination of the confounding effects of environmental conditions.

CONCLUSION
Although we found a higher pregnancy rate among women undergoing infertility treatment during the spring season, we did not detect that seasonal variation had any statistically significant influence on the success of IUI. New studies with higher power may find a significant difference between the outcomes from infertility treatment and seasonal changes. In this regard, further large-scale studies are required, in order to better evaluate the effects of seasonal variability on pregnancy.