Most people think just one sperm is needed to fertilise a woman’s egg and make a healthy pregnancy. This underpins a common view that all the other sperm — and all the other sex — are surplus to requirements, at least when it comes to conceiving a pregnancy. However, biologists now believe sexual intercourse is not just a sperm delivery process, but also a kind of biological communication. Regardless of whether fertilisation occurs, sperm and other components of the ejaculated fluid trigger subtle changes in the immune system of women. This has consequences for pregnancy should it happen later. More broadly, the importance of regular sexual activity also has implications for fertility planning, and for IVF and other forms of assisted reproduction, which generally do not take sexual practice or history into account. Sperm swim in a soup of molecular messages Evidence from animal research and clinical studies has led researchers to conclude seminal fluid — the fluid sperm are bathed in following ejaculation — plays an important role in fertility. Seminal fluid contains small molecules that act …