Well aware of how difficult conception or carrying a baby to term can be, some couples who hope to exercise a bit of control over an unpredictable experience are opting to do in vitro fertilization first.. ... In the United States, more than 2 percent of all infants born are conceived using assisted reproductive technology, of which I.V.F. is the most common. At least 12 million babies have been born globally using I.V.F. since 1978... ... At age 35, there is a 15 percent chance of conceiving naturally per month... At 40, that drops to 5 percent. ... I.V.F., however,...