According to Shettles 'theory, the relative alkalinity of a woman's reproductive tract during menstruation affected the speed of sperm carrying the X and Y chromosomes differently. X chromosomes, he said, flowed quicker when the alkalinity of the reproductive tract was relatively high.
薛特的理論是：在女性的排卵期，子宮頸分泌物明顯地呈鹼性反應。若夫妻在此時行房，最喜愛鹼性環境而游速較快的雄性精蟲（含 Y 染色體）便能跑在雌性精蟲（含 X 染色體）的前頭，一路領先到達輸卵管外側與卵子結合，如此一來得男率自然提昇。