Yet, 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 births. 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 Twin Birth Rates
A 2012 study took a more specific look at the data and established 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 30 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 2014, the twin birth rate was 33.9/1,000.
In those terms, it is evident that the twin birth rate increased from less than 2% of babies born in 1980 to over 3% of babies born in 2009. After 2009, the rate of increase did not continue to rise at the same pace. It remained stable and even slightly decreased from 2009-2012 to 33.1. Then, in 2014, it jumped slightly to a new high of 33.9. Keep in mind, however, that this number is calculated based on the number of overall births (singleton + multiple) in a given year. The actual number of twins was only slightly higher, as the overall number of births was actually lower. Here are the numbers:
In 2007, there were 4,316,233 live births. Of those, 138,961 were twins. The twin birth rate was 32.2.In 2018, there were 3,791,712 live births. Of those, 123,536 were twins. The twin birth rate was 32.6.
There were fewer twins born in 2018 than in 2007, but there were also fewer births overall.
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% 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%. The states with the highest twin birth rate in 2018 were:
Connecticut (39.7/1,000)Michigan (36.9/1,000)Illinois (36.1/1,000)New Jersey (36.0/1,000)Alabama (35.6.1000)
The states with the lowest twin birth rate in 2018 were:
New Mexico (25.4/1,000)Wyoming (28.6/1,000)Washington (29.4/1,000)Nevada (30.1/1,000)Arkansas (30.2/1,000)
Reasons for More Twin Births
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. 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 30. In fact, twin birth rates have risen with advancing age, peaking at 35 to 39 years and then declining. However, twin birth rates have been highest among women who are age 40 and older. The study also illustrates the difference in twin birth rates according to age. In 2009:
Women aged 40 or over: 7% of all births were a twin deliveryWomen 35-49: 5% of births were a twin deliveryWomen under age 25: 2% of births were a twin delivery.
This increase correlates to a shift in the age distribution of women giving birth during the 30 years of the study. Where only 20% of women giving birth in 1980 were over age 30 the same population accounted for 35% of births after 2000. Not surprisingly, the increasing older age of mothers influences twin birth rates because of the higher spontaneous (i.e., without the use of fertility therapies) twinning rates of women in their 30s. That correlation continues to hold true in recent years. In 2014, the majority of multiples were born to mothers over the age of 30.
Fertility Treatments and Twin Births
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 30 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.
