Couples who can not get pregnant because of unknown reasons, but who still have a good chance of spontaneous pregnancy within the next year, are eligible to participate in this trial. In this trial we investigate whether a superficial wound in the inside lining of the womb (this procedure is called 'endometrium scratching') could lead to increased spontaneous pregnancy chances in these couples. The idea is that the inside lining of the womb (the endometrium) will be better prepared for implantation of an embryo because of the superficial wound that has been created, and that could possibly result in increased pregnancy chances.