Pregnancy and child

Sperm quality may be lower during summer

“Trying for a baby? Now’s the best time: Men have healthier sperm in winter and spring”, reports the Mail Online. While the Mail’s report is broadly accurate, the news only really applies to men from couples having trouble conceiving.

The Mail’s story is based on a study of the quality of semen samples from more than 6,000 Israeli men referred to an infertility clinic.

Researchers found that sperm with normal quality – and so theoretically the greatest chance of leading to a successful conception – were seen in samples collected during the winter months. The researchers conclude that the pattern is consistent with diminished sperm quality during the summer and a gradual improvement during autumn and winter.

In samples with abnormally low concentrations of sperm, the pattern was slightly different, with sperm quality peaking in spring and autumn.

This study raises the possibility that men may experience seasonal variations in fertility. However, the research is based on analysing samples from a fertility clinic, so the results may be unrepresentative of all men. It is also worth bearing in mind the contrast between the British climate and the Israeli climate and whether the same effect would be seen here.

The best way for men to help improve their fertility is to avoid alcohol and smoking and try to maintain a healthy weight.

Where did the story come from?

The study was carried out by researchers from Ben-Gurion University of the Negev, Israel. The source of funding for this study was not reported, though the authors reported no conflicts of interest.

The study was published in the peer-reviewed American Journal of Obstetrics and Gynecology.

The results of this study were reported accurately by the Mail Online, with the exception of the headline. The study has not directly examined whether couples are more likely to conceive in the spring and winter compared to other times of the year, so the claim that ‘now is the best time to try for a baby’ are unfounded based on the evidence presented in the study.

Obviously, sperm quality is an important factor in determining whether a successful conception will take place as a result of intercourse, but other factors, such as the women’s fertility and
general health should also be taken into account.

What kind of research was this?

This was a cross-sectional study examining the relationship between the time of year and a number of measures of sperm quality. It aimed to determine whether there is a seasonal sperm variation pattern.

The research can tell us little more than an observed cyclic pattern in sperm quality exists (at least in Israeli men).

The study has not examined whether this directly affects fertility, or whether couples are more likely to conceive at certain times of the year than at other times.

What did the research involve?

The researchers analysed the computerised results from 6,455 semen samples from 6,447 couples referred to an infertility clinic.

Men referred to the clinic were advised to abstain from sexual activity for one to three days before providing a semen sample, and the exact abstinence period was recorded on arrival at the clinic. Semen samples were delivered to the laboratory between 30 and 60 minutes after ejaculation.
Researchers then measured:

  • semen volume
  • sperm concentration
  • percentage of ‘total motility’ (what proportion of the sperm were moving), and the percentage of sperm with fast and slow motility (a higher proportion of ‘fast’ sperm make conception more likely)
  • percentage of sperm with normal morphology (shape) – a high proportion of abnormal sperm can be a cause of difficulty conceiving 
  • acrosome index – the acrosome is a cap-like structure which develops over the front of the head of the sperm. It contains the enzymes that break down the outer membrane of the ovum (egg), allowing fertilization to occur. The acrosome index is the percentage of sperm with a normal acrosome (clearly visible with a smooth oval shape comprising 40% to 70% of the sperm head). As with morphology and motility, the acrosome index contributes towards the possibility of a successful conception

Sperm samples were classified according to sperm concentration:

  • normal concentration (normozoospermic) – which was defined as having at least 20 x 106 sperm/ml
  • low concentration (oligozoospermic) defined as having between 4 x 106 and 19.99 x 106 sperm/ml

Variations in sperm variables were analysed according to the season on the day the semen sample was given, and again by the season 70 days before the day of the sample (when the sperm would be forming). So if a sample was given in May, it was ‘made’ during March. They then looked at if there was a correlation between when it was made and its quality – the same analysis as above but shifted by over two months.

The researchers adjusted for potential confounding factors such as age and the duration of abstinence prior to providing a sample.

What were the basic results?

For samples with normal sperm concentration:

  • semen volume was constant during the year
  • sperm concentration peaked during the spring months (March to May)
  • the percentage of motile sperm peaked during the summer months (June to August), and was lowest during winter (December to February), but this peak was mainly due to an increase in slow moving sperm. The greatest percentage of fast moving sperm was seen during the winter months, and was lowest during the summer months
  • the greatest percentage of sperm with normal morphology was seen during the winter months, and was lowest during the summer months

When the researchers analysed data according to the season when the sperm was made (70 days before the sample was collected), the results for all semen variables were similar.

For samples with low sperm concentration:

  • semen volume and sperm concentration did not vary significantly throughout the year
  • the percentage of motile sperm was highest during summer and autumn (September to November) and lowest during winter. The percentage of slow moving sperm also peaked during autumn, but the percentage of fast moving sperm did not vary over the year
  • the greatest percentage of sperm with normal morphology was seen during spring, dropped during the summer months and recovered during autumn
  • the acrosome index varied in a similar manner: it was greatest during spring, dropped during the summer months, and recovered again during autumn

When the researchers analysed data according to the season the sperm was made (70 days before the sample was collected), the results were less statistically significant.

How did the researchers interpret the results?

The researchers conclude that their analyses show a “relationship between the rotation of the seasons and annual semen variations.” The rhythm is slightly different in semen with normal and reduced sperm concentration.
They suggest that:

  • semen with normal sperm concentration will perform best during winter
  • semen with low sperm concentration will perform best in spring and autumn


This interesting study raises the possibility that humans, like other animals, may experience cyclic variations in fertility due to variations in semen quality with the seasons. The researchers found that for men with normal semen concentration, the summer months were associated with diminished sperm quality, and a gradual improvement in sperm quality is observed during the autumn and winter months.

It has already been observed that sperm quality is dependent on temperature, so the observation that sperm quality peaks in winter months when it is colder may be consistent with this.

However, it should be noted that all samples analysed were from couples attending a fertility clinic, and therefore may not be representative of the general population. Also, the Israeli climate differs markedly from the UK. For this reason, any seasonal differences – if they were found in this country – may be somewhat different.

The research has also not directly examined whether these observations were consistent with increased or decreased chance of conceiving at certain times of the year. However, birth registry data from Israel shows that births peak during autumn, consistent with conception during winter.

For these reasons, further research would be required to see if the seasonal increase in sperm quality actually lead to successful conception in the majority of cases.

NHS Attribution