Views of the four-dimensional monad for men and women. The four-dimensional color ultrasound monad is a common examination technology in modern medicine, which can understand the condition of the fetus in the mother\’s body. For expectant parents, gender issues are a matter of great concern. So from what aspects should we look at the four-dimensional monads of men and women? 1. Conversion rate index The conversion rate index refers to other than placental tissue and amniotic fluid.