In the 1960s in the United States Landrum B. Shettles developed the Shettles method, which is a procedure for couples to use prior to and during an intercourse to increase their chances of conceiving a fetus of their desired sex. Shettles, a physician, who specialized in obstetrics and gynecology, found a difference in the size and shape of male sperm cells that he correlated with the different sex chromosomes they carry. Based on that finding, Shettles developed procedures for couples to follow based on whether they desire a female or a male fetus and published them in the 1970 book, Your Baby’s Sex: Now You Can Choose. The Shettles method is based on the idea that male-producing sperm prefer alkaline conditions, whereas female-producing sperm prefer acidic conditions. The method provides couples with a procedure intended to enhance the favored environment for the sperm that will supposedly produce the desired sex, including female douches to be used before intercourse and how to time sexual intercourse within the female menstrual cycle. The book Your Baby’s Sex: Now You Can Choose, made the Shettles method a widely popular method of natural sex selection.