31st March 2004

Researchers have found that pregnancy from IVF is more likely between May and September, probably due to biological programming that we have inherited from our primate origins.

The British Fertility Society hear today how investigators from IVF clinics in Liverpool and Chester discovered that not only were significantly less drugs required to stimulate ovulation in women during the months with most daylight, but the implantation and successful pregnancy rates were also markedly increased in months with longer daylight hours.

Although historical records show that natural birth rates show seasonal trends, conflicting evidence existed on the seasonal variability of IVF. Now, UK researchers have carried out analysis on nearly 3000 standardised procedures that took place over a three-year period.

Trends seen between commonly defined seasons were not significant but when they analysed results from the months with most daylight hours (May-September) against those with least (November-February), they found striking differences.

The amount of gonadotrophins (hormones used to stimulate the ovary to produce eggs) required for ovulation was greater during the darker days. The eggs harvested at different times of the year did not vary in quality; fertilisation rates were unchanged. However, implantation of the embryo into the uterus and completion of successful pregnancy was significantly higher during the months with most daylight – 20% compared to 16%.

Local effects of the hormone melatonin may therefore be responsible. Melatonin levels naturally cycle in response to light and dark, and all mammals have physical responses to this cycling – the most obvious being the natural sleep/wake patterns. Until recently it was thought that melatonin acted only through the pituitary gland in the brain. This natural system is purposefully switched off in women who are undergoing fertility treatment, so it seems that melatonin acts directly on the tissues of the female reproductive system to make it more fertile in lighter months.

Dr Simon Wood, who carried out much of the study, commented ‘Although we may not think that our bodies function differently from month to month, our work indicates that human fertility is still influenced by these primitive mechanisms common to all mammals.

Notes for Editors

1Seasonality and the role of sunlight in Assisted Conception cycles is presented at The Annual British Fertility Society Meeting 2004, in Cheltenham, UK

2SJ Wood*,#,##, A Quinn#,##, CR Kingsland# and Dr Lewis-Jones#,## from *Dept Obs & Gyn, Countess of Chester Hospital, Chester, #Hewitt Centre for Reproduction, Liverpool Womens Hospital, Liverpool and ##Dept of Obs & Gyn, University of Liverpool, Liverpool, UK