A recent study led by Shevach Friedler published in the medical journal Fertility and Sterility showed that laughter can promote pregnancy:

To test the idea, the research team had a medical clown visit their fertility clinic periodically over one year. Of the 219 women in the study, …half underwent embryo transfer on a day the clown was at the clinic.

During recovery from the procedure, each woman had a 15-minute visit from the clown, who performed a specific routine created by Friedler – who has studied movement and mime – and a colleague.

The researchers found that, compared to women who came to the clinic on a “non-clown” day, those who’d had a laugh were more than twice as likely to become pregnant, when other factors such as age, type of infertility and the number of embryos transferred, were taken into account.