In TCM, yin is used to describe things/aspects that are still, cool, dim, descending, moist and nourishing, while yang refers to anything that has the properties of warm, bright, ascending, active and dynamic.
In the context of menstrual cycle, yin plays an important role during post-menstrual phase (follicular phase), it relates to hormonal triggers that stimulate follicles to develop, and the factors that support follicle’s growth and maturation. Yang performs a very important part after ovulation (luteal phase), when dynamic transformations happen (ovulation itself, the journey of the egg down the tube, fertilisation).
Everything in universe has yin and yang aspects. They are inseparable, interchangeable, mutually creating and transforming. There are two transforming phases during each menstrual cycle – period and ovulation. The start of cycle is right the time when yang is at its zenith and yin is at the beginning of growth. Ovulation heralds that yin, at its zenith, transforms into yang. Although short, transformations do happen at those stages. Therefore, qi and blood activities should be regulated to make sure transformations happen undisturbed.
Above-mentioned is the basic theory used in practice when we see patients come for fertility problems. We always say that we need to regulate period first. This is the reason. Only when yin and yang at its optimum level can pregnancy happen. This theory also applies to most women’s problems.