The number of twins in the U.S. is multiplying. According to a 2012 report by the National Center for Health Statistics, their birthrate rose 76 percent from 1980 to 2009. This could be due to the fact that more women over the age of 30 are having children and more are using fertility drugs and assisted reproductive technology, both of which increase the likelihood of multiple gestation. As many as one in three women who use fertility treatments may become pregnant with twins.
The result? As many as one in 30 babies born in the United States is now a twin. A recent report published in the New England Journal of Medicine says that by 2011, the incidence of triplet or higher-order births decreased by 29 percent from its peak in 1998. Researchers attributed this trend -- in part -- to changes in medical guidelines aimed at reducing multiple births resulting from IVF. (A recent study found that other fertility treatments, such as ovulation-inducing medications or insemination, have overtaken IVF as the main source of multiples resulting from fertility treatment.)
With so many twins among us these days, it's high time we celebrate some of the most interesting facts about them.
1. Identical twins do not have identical fingerprints.
A photo of actual fingerprints from identical twins in 1952.
You might think that because identical twins supposedly share almost the same DNA, they must also have identical fingerprints. Well, that's not true. Fingerprints are not solely generated based on DNA. When identical twins are conceived, they start out with the same fingerprints, but during weeks six through 13 of pregnancy, as the babies start to move, they each touch the amniotic sac, and unique ridges and lines are formed on each twin's hand that result in different fingerprints.
2. Massachusetts has the most twin births of any state in America.
At nearly 4.5 for every 100 live births, Massachusetts has the highest rate of twin births. Connecticut and New Jersey follow with 4.2 twins per every 100 births. Researchers hypothesize that more multiple births occur in "affluent towns outside of Boston" because of a higher concentration of wealthier women who have pursued careers. These women are more likely to attempt to have children at a later age and seek reproductive assistance. The state with the lowest rate of twin births is New Mexico.
3. Mirror image identical twins have reverse asymmetric features.
About 25 percent of identical twins develop directly facing each other, meaning they become exact reflections of one another. According to About.com, "they may be right- and left-handed, have birthmarks on opposite sides of their body, or have hair whorls that swirl in opposite directions." This occurs when the twins split from one fertilized egg more than a week after conception.
4. Identical twins do not always have the same genetics.
While identical twins derive from one fertilized egg that contains a single set of genetic instructions, also known as a genome, it's still possible for identical twins to have serious differences in their genetic makeup. Geneticist Carl Bruder of the University of Alabama at Birmingham closely studied the genomes of 19 sets of adult identical twins and found that in some sets, one twin's DNA differed in the number of copies of each gene it had. Normally, every person carries two copies of every gene, one inherited from each parent, but Bruder explains that there are "regions in the genome that deviate from that two-copy rule, [and] these regions can carry anywhere from zero to 14 copies of a gene."
5. Mothers of twins may live longer.
A study published in the journal Proceedings of the Royal Society B focused on the population of women in 1800s Utah and found that women who gave birth to twins were very strong and healthy to begin with, so were likely to live long lives. As LiveScience writer Stephanie Pappas explains it, "Twins could be an evolutionary adaption in which healthy moms take the chance to pass on double their genes at once." However, because the data only includes women who conceived twins naturally -- IVF was obviously not used in the 1800s -- the findings are not definitive.
6. Tall women are more likely to have twins.
Gary Steinman, MD, PhD, an attending physician at Long Island Jewish (LIJ) Medical Center, discovered that taller women have more insulin-like growth factor (IGF), a protein that is released from the liver in response to a growth hormone that stimulates growth in the shaft of longer bones. Having higher levels of IGF results in increased sensitivity of the ovaries, thus increasing a woman's chance of ovulating. According to Steinman, the more IGF a woman has, the greater chance she has of becoming pregnant with twins, because IGF "govern[s] the rate of spontaneous twinning."
7. Also, women who eat a lot of dairy are more prone to conceiving twins.
Another study done by Steinman and published in the Journal of Reproductive Medicine found that women who eat more dairy products may increase their chances of conceiving twins. Steinman tested this by comparing twin rates from vegan mothers and non-vegan mothers. Those who consumed dairy were five times more likely to have twins. This is because cows, like humans, also produce IGF in response to growth hormone and release it into their blood. Then it gets released into their milk, which women consume.
8. It's possible that twins can have different dads.
In 2009, Mia Washington gave birth to twins who have different fathers -- which is said to be a one-in-a-million occurrence. Dr. Hilda Hutcherson, Clinical Professor of Obstetrics and Gynecology at Columbia University, explained to "Today" how it happened: Normally, a woman releases one egg each month. For Washington, there were two. At the same time, the mom had intercourse with two different men in the span of five days. Because sperm can remain alive in the reproductive tract for that long, each man's sperm fertilized one of eggs. Voila! Two babies, two dads.
9. Twins interact with each other in the womb.
In 2011, researchers at Umberto Castiello of the University of Padova in Italy studied 3D videos of twins in their mother's womb. At 14 weeks of gestation, twins were seen reaching for each other. By 18 weeks, they touched each other more often than they touched their own bodies. The researchers said that kinematic analyses of the recordings revealed that the twins made distinct gestures toward each other and were as gentle to the other twin's delicate eye area as they were when they touched their own.
10. Some conjoined twins can feel and taste what the other one does.
Susan Dominus wrote a piece for The New York Times about two conjoined twins, Krista and Tatiana Hogan, who are attached at the head through a "thalamic bridge," part of the brain that acts as a sort of "neural switchboard" and filters most sensory input. Scientists have hypothesized that this connection could result in one Hogan sister being able to taste and feel what the other twin is experiencing and to understand each other's thoughts. Dominus, who spent a considerable amount of time with the twins for her story, recorded these amazing observations:
"[Their parents noticed] when one girl's vision was angled away from the television, she was laughing at the images flashing in front of her sister's eyes. The sensory exchange, [researchers] believe, extends to the girls' taste buds: Krista likes ketchup, and Tatiana does not, something the family discovered when Tatiana tried to scrape the condiment off her own tongue, even when she was not eating it."
11. Forty percent of twins invent their own languages.
These twin babies are speaking their own special "language."
These languages are called autonomous languages. Researchers suspect that twin babies use each other as models in developing language when an adult model language is frequently absent. The "language" consists of inverted words and onomatopoeic expressions. These autonomous languages are formed when two very close babies are learning how to speak a real language alongside one another and naturally often play and communicate with each other. While this is more common among twins, since they are more likely to be around each other and developing at the same rate, this phenomenon can also sporadically occur between two babies who are not twins. The made up "languages" often disappear soon after childhood, once the children have learned a real language.