Do you know the best time in the month to try for a baby? Traditionally, it is all about the fertile window, the five or six magical days in which pregnancy can occur. Ovulation, which typically happens around the 14th day of a cycle, releases a mature egg into the fallopian tube – but the egg only lasts for 24 hours. Sperm, which survive for five days, should ideally be supplied as often as possible during this fertile window. You can monitor this window by watching the calendar, checking the cervical mucus to see if it looks like runny egg whites, and investing in an ovulation predictor kit.

But such monitoring can take the sexiness out of sex. And a new study this week in Fertility and Sterility makes even more demands on couples trying to conceive: its findings suggest that it may be better to have sex every day. Yes, every single day. The study finds that a lot of sex may prepare a woman’s immune system for pregnancy. Sexually active women in the study had higher levels of cytokines, molecules released by type 2 helper T cells from the immune system that help reduce the “foreignness” of sperm or embryos, and the likelihood that the body will attack them. The lead author, Tierney Lorenz from the Kinsey Institute for Research on Sex, Gender, and Reproduction, says that the immune system is critical for a healthy pregnancy – from making sure the sperm isn’t attacked as an invader to helping the fertilised egg implant into the uterus. Lorenz thinks that more frequent sex sends a message to the immune system that it’s time to reproduce.

The solution

However, the study of 30 women did not actually follow them up to see if they became pregnant, and Lorenz says that the relationship between sex, changes in the immune system and the likelihood of having a baby is far too complex to be made clear from this one piece of research. As to how often couples should have sex, Lorenz says that women get so much advice that you should just “do what works right in your relationship”. There is no evidence from this study that daily sex will increase fertility, though Lorenz believes that the more your immune system gets the message that it’s time to reproduce, the more this could eventually increase your chances of pregnancy.

In the meantime, NICE in its latest guidance last year, advises couples who want to get pregnant to have sex two or three times a week throughout the month. This makes it more likely you will hit a fertile window without looking for it, and reminds the immune system to play nicely with incoming sperm.