Why More Multiple Births? Historically in the United States, the natural rate of multiple births tended to increase as mothers aged. Women in their mid to late 30s were more likely to have twins or triplets than those who were younger. The advent of assisted reproductive technology (ART) led to a dramatic increase in multiple births overall starting in the early 1990s; it is now women aged 40 and over who are most likely to experience this phenomenon. After peaking in the late 1990s, rates of triplet births declined, as the hazards of multiple births are more apparent.