Are there more twins in the world? It has been widely assumed that there has been an increase in twins over the years, largely attributed to fertility treatments. It seems like there are twins everywhere you turn; perhaps you notice more and more families with double strollers out and about in the mall or more frequent announcements in the media from celebrity parents celebrating the birth of twins. Television shows like "Kate Plus Eight" (formerly "Jon and Kate Plus Eight") or "Texas Multi Mamas" shine a spotlight on multiple birth. Schools report record registration of twins and multiples and educators wrestle with the issue of classroom placement. The increasing incidence of twins has even generated legislation to ensure that twins' rights are protected in school.
The Increase in Twins
A 2012 study takes a more specific look at the data and establishes some trends about the increase in the twin birth rate in the United States between 1980 and 2009. A National Center for Health Statistics (NCHS) brief from January 2012 provides the following information:
• In 1980, 1 in 53 babies was a twin.
• In 2009, 1 in 30 babies was a twin.
This represents a 76% increase in the twin birth rate in the thirty years from 1980 - 2009. The study estimated that an additional 865,000 more twins were born during this thirty years than if the twin birth rate had not increased during those decades. To put this in other terms:
• In 1980, the twin birth rate was 18.9/1,000.
• In 2009, the twin birth rate was 33.3/1,000.
In those terms, it is evident that the twin birth rate increased from less than 2 percent of babies born in 1980 to over 3 percent of babies born in 2009.
Twin Birth Rates Across the States
All areas of the United States experienced a rise in the twin birth rate, but rates continue to differ among the states. This chart shows the twin birth rate in each state of the United States, comparing the rates in 1980 and in 2009 and the percent change over the years. Rates rose by at least 50 percent in 43 states and the District of Columbia, and five states (Connecticut, Hawaii, Massachusetts, New Jersey, and Rhode Island) saw the rate rise by more than 100 percent.
The states with the highest twin birth rate in 2009 were:
• Connecticut (45.9/1,000)
• Massachusetts (45.2/1,000)
• New Jersey (44.2/1,000)
• New Hampshire (40.3/1,000)
• New York (38.3/1,000)
The states with the lowest twin birth rate in 2009 were:
• New Mexico (22.3/1,000)
• South Dakota (24.6/1,000)
• Arizona (26.6/1,000)
• Nevada (27.4/1,000)
• Vermont (27.5/1,000)
Reasons for the Increase in Twins
So what explains the increase in the number of twins? Many assume that it is simply an increased utilization of reproductive technology. However the study identified another significant influence. Several factors have been identified as contributing to increased twinning. One study identified consumption of dairy products as a factor. (Cattle are given growth hormone to increase their production of milk and beef. When women ingest the milk from these animals, their own hormones react, stimulating ovulation.) Another associated rising obesity rates with increases in twinning, citing that overweight or tall women are more likely to have twins.
The 2012 study of twin birth rates identifies maternal age as a leading factor contributing to the increase in twins. The largest increase in twin birth rates was realized among women over the age of thirty. It says "Historically, twin birth rates have risen with advancing age, peaking at 35–39 years and declining thereafter (4). Since 1997, however, rates have been highest among women in their 40s." The study illustrates the difference in twin birth rates according to age. In 2009:
• Women aged 40 or over: 7 percent of all births were a twin delivery
• Women 35-49: 5 percent of births were a twin delivery
• Women under age 25: 2 percent of births were a twin delivery.
This increase correlates to a shift in the age distribution of women giving birth during the thirty years of the study. Where only 20 percent of women giving birth in 1980 were over age thirty, the same population accounted for 35 percent of births after 2000. "The increasingly older age of mothers over the decades would be expected to influence twin birth rates because of the higher spontaneous (i.e., without the use of fertility therapies) twinning rates of women in their 30s." The study estimates that one third of the increase in the twin birth rate can be attributed to this elevation in maternal age.
Fertility Treatments and the Twin Birth Rate
Fertility treatments are largely assumed to the cause behind the increase in twins, and this study supports that theory. The study cites infertility treatments as being responsible for about two-thirds of the increase in the twin birth rate from 1980 to 2009. Fertility therapies include the taking of fertility stimulating drugs or procedures to assist conception, such as in-vitro fertilization. The influence of fertility treatments is somewhat associated with the issue of advanced maternal age as women over the age of thirty are more likely to seek fertility assistance, the study acknowledges.
In the three decades during which twin birth rate data was studied, medical technology made fertility therapies more successful and more accessible. The use of reproductive assistance became more prevalent in the 1980's and 1990's. However, the processes have been refined in recent years to restrict the number of multiple births that result as an outcome of fertility therapy, recognizing that multiple birth is associated with elevated health risks and accompanied by greater health care costs.
Source: Martin JA, Hamilton BE, Osterman MJK. Three decades of twin births in the United States, 1980–2009. NCHS data brief, no 80. Hyattsville, MD: National Center for Health Statistics. 2012.