IVF success more likely in summer months, study suggests

Couples undergoing IVF may have up to double the pregnancy success rate in summer compared to colder months, new research suggests.

A study of 1,100 women found pregnancy rates were significantly higher in warmer weather, with spring treatments 75 per cent more successful than those in winter.

Researchers from the Reproductive Hospital of Guangxi in China analysed data from patients who had their first IVF treatment between June 2021 and October 2023, using fresh rather than frozen embryos.

Pregnancy rates peaked when outdoor temperatures reached 26 to 30 degrees Celsius, leading researchers to suggest that treatments in warmer conditions may improve outcomes.

Women having IVF in spring were 75 per cent more likely to become pregnant, while summer treatment was linked to a 53 per cent increase, both compared with winter.

For those using the “long protocol” – a standard UK method involving medication to suppress natural hormones before stimulating the ovaries – pregnancy rates were twice as high during summer months.

The researchers said higher levels of vitamin D, produced naturally when the skin is exposed to sunlight, may help explain the seasonal pattern. The so-called “sunshine vitamin” is thought to support hormonal balance and may boost fertility, although the exact mechanism is unclear.

Low vitamin D levels have been linked to several long-term health conditions, including heart disease, multiple sclerosis and osteoporosis. The NHS recommends that people in the UK take vitamin D supplements during winter, when sunlight is limited.

“Season and temperature may influence clinical pregnancy rates,” the research team said, adding that IVF treatment in summer could help improve success – although further studies are needed to confirm the findings.

While similar trends have been reported in previous research, the reasons for the seasonal differences remain uncertain. The researchers noted that the effects of temperature and season on conception are “not fully understood”.

They called for further research to explore vitamin D’s specific role in pregnancy, but emphasised that individual factors remain key to IVF success.