Return of the rhythm method

Every morning when she wakes up, Becca, a college student in Pennsylvania, puts a teardrop-shaped thermometer called the Daysy under her tongue. If it lights up green, she knows that day she and her boyfriend can have sex without a condom. If it’s red or yellow, they need to use protection.

With its blinking face and patented “LadyComp algorithm,” Daysy seems newfangled, but its core technology is one septuagenarians would recognize. It relies on the basal body temperature method of family planning, based on the fact that women’s bodies are a few degrees hotter just after ovulation. But while in past decades women who employed this method had to hand-chart their temperatures on graph paper, Daysy tracks the readings automatically and ports the data onto a companion iPhone app that lets the user see her “cycle forecast” and “temperature curve.” Because it’s personalized, it claims to predict a woman’s chance of pregnancy with an accuracy of 99.3 percent—roughly the same as that of birth-control pills.

Becca began using Daysy in June after stints on the Pill and on Nuvaring, the hormonal vaginal ring. (All of the women in this story asked me to use only their first names.) Both methods caused mood changes that she found unsettling, such as bouts of unexplained crying, and the Pill made her nauseous nearly every morning. She and her boyfriend had been together for a while, so although he was wary of “natural” contraception at first, the Pill’s nasty effects on Becca persuaded him to give it a try.