Live birth rates in fresh IVF no higher after summer treatment than at other times

Published 12 September 2022

A large Swedish registry study has failed to find any link between ART carried out in the summer months – when vitamin D levels may be highest – and improved birth rates. Once again seasonal variability appears to have no impact on IVF success.

The seasons of the year should not be considered by fertility clinics – or their patients – when planning ART with fresh eggs and embryos, say Swedish researchers. Their recommendation comes from a large registry study which found no evidence that pregnancy and live birth rates are higher when treatment takes place in the summer, when sunlight is strong enough for the body to make vitamin D.(1)

The findings, based on more than 50,000 first-time IVF/ICSI cycles carried out over almost a decade, appear to debunk the theory that sun exposure boosts the chance of parenthood – or at least from treatment with assisted reproduction.

In fact, a slightly higher LBR did occur among women having ART during spring and autumn, which was found just statistically significant. However, they discounted this difference – for reasons not fully explained – as ‘small in magnitude’ and ‘probably negligible’ in terms of clinical impact and ‘unlikely of clinical value'. Hence, their message to discount the role of any season when choosing the timing of fertility treatment.

Seasonal variations in natural conception and birth rates are well documented, with the suggestion that light exposure and outside temperature could affect parameters for reproductive efficiency – for example, sperm quality. Vitamin D levels in blood from direct sunlight on skin are often attributed to an improvement in the chance of pregnancy.

However, the authors of this latest study write that any link between seasons and IVF/ICSI outcomes remains unclear, with reports conflicting. Several small studies have shown better pregnancy outcomes from ART performed in spring or summer, or at times of increased daylight.(2) However, a recent study based on fresh embryo transfer in more than 13,000 fresh IVF cycles found no such association.(3)

Discrepancies also exist between data on vitamin D and IVF: some studies have found that women with normal levels of the vitamin have higher success rates, although a recent systematic review and meta-analysis showed no such link.(4)

In a bid to explain if summer is indeed linked to better ART outcomes, the authors collected data on 52,788 first-time treatment egg retrievals between 2009 and 2018 from Sweden’s National Quality Registry of Assisted Reproduction. The registry holds information on all IVF and donor insemination treatments in Sweden, regardless of outcome. Details include treatment cycle and patient characteristics.

With adjustment made for confounders - including woman’s age, year of treatment, BMI, and number of embryos transferred (the vast majority were single transfers) - LBR was the primary outcome, per oocyte retrieval and ET. Other outcomes included miscarriage per clinical pregnancy, with miscarriage defined as loss of pregnancy up to gestational week 22.

Patients were divided into four subgroups – summer treatments (June, July, August), autumn (September, October, November), winter (December, January, February), and spring (March, April, May), in a total of 44,359 cycles.

LBR per retrieval ranged between 24% and 26% in all seasons, although a just significantly higher LBR was found for spring than summer (26% vs 24%; adjusted OR 1.08). No significant association was seen when winter and autumn were compared with summer.

The authors cite the large sample size and inclusion of only the first cycle for each patient as strengths of the study. One issue the authors highlight but do not discuss is that the number of IVF/ICSI treatments performed in the summer was lower (14% of the total) than in other seasons - because clinics in Sweden close for the vacation. Also not mentioned was the possible effect of vitamin D supplementation in Sweden, which may well have neutralised any effect of real summer sunlight.

1. Carlssson Humla E, Bergh C, Akouri R, et al. Summer is not associated with higher live birth rates in fresh IVF/ICSI cycles: a population-based nationwide registry study. Hum Reprod Open 2022; hoac036.
2. Vandekerckhove F, Van der Veken H, Tilleman K, et al. Seasons in the sun: the impact on IVF results one month later. Facts Views Vis Obgyn 2016; 8: 75-83. PMC5130306
3. Liu X, Bai H, Mol BW, et al. Seasonal variability does not impact in vitro fertilization success. Sci Rep 2019; 9: 17185.
4. Cozzolino M, Busnelli A, Pellegrini L, et al. How vitamin D level 328 influences in vitro fertilization outcomes: results of a systematic review and meta-analysis. Fertil Steril 2020; 114: 1014-1025.

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