Most likely related to the introduction of IVF in modern medicine the United States. If in the year 1080, the twins appeared with a frequency of 19:1000, by 2014 this figure has increased almost twice.
New medical technologies allow to get a child even partners with reproductive disorders. IVF and artificial insemination give thousands of new lives in America. However these methods lead to a significant increase in the probability of birth of twins or even triplets.
According to the data, about 36% of all newborn twins in 2011, appeared in the result of in vitro fertilization. Another important factor, according to scientists, the so-called "aging mom". Increasingly women are having children in later years. This is also reflected on the likelihood of conceiving twins.