Mothers of Multiples

Trends of Multiples

The availability of fertility drugs has made multiple births more common. Twin births have steadily increased from 1990 to 2004, climbing an average of 3 percent a year for a total increase of 42 percent since 1990, and 70 percent since 1980. However, the National Center For Health Statistics found that numbers began leveling off in 2004 at 32.2 twins per 1,000 births, remaining the same in 2005. In the United States in 2006, there were 127,085 twin births (3.0 percent of all births).

Birth rates of higher-order multiples also increased over the last two decades, soaring 400 percent between 1980 and 1998 but trending downward since, declining in 2005 for the seventh consecutive year to 61.8 per 100,000 live births. In the United States in 2006, there were 6,118 triple births (0.14 percent), 355 quadruplet births (.008 percent) and 67 quintuplet and higher-order births (.002 percent). It is important to note that these statistics do not differentiate between dizygotics and monozygotics, despite their vastly different origins.

Children from multiple births are likely to be born prematurely and underweight and to have worse outcomes than singletons. However, improvements in medical technology mean that multiples are more likely to survive, to the point where only very highorder births attract media attention. By contrast, the Dionne sisters, born in Canada in 1934, were highly publicized as the first quintuplets to survive infancy.

The prevalence of dizygotic twin and higher-order multiple births are generally due to four main factors. The first two—hereditary and ethnicity—are connected to genetics on the maternal side of the family. Hyperovulation, the dropping of more than one egg during ovulation, is a gene passed through the generations. The more eggs that are dropped, the greater the possibility for more than one egg to be fertilized at the same time. The next two are not so much factors as they are trends: age and medical interventions. The Centers for Disease Control and Prevention has found that more and more women are having children later in life.

When women reach 35 years of age and over, they may begin experiencing symptoms of perimenopause, including hyperovulation. Egg production is also heightened by fertility, enhancing therapies and assisted reproductive technologies such as in vitro fertilization (IVF). An estimated one-third of the increase in dizygotic births since the late 1970s and early 1980s has been attributed to the shift in the maternal age distribution; the remainder is likely the result of fertility and reproductive interventions. To date, there is no established research that connects physiological factors or medical interventions to the creation of monozygotic multiples; they are simply considered a random act. As a result, their numbers have remained constant, about 4 per 1,000 births.

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