Suzanne Severy Comeau and her sister, Jennifer, as 12-year-old twins entwined in the same bed after the death of their grandfather, awoke simultaneously with the same nightmare.
In college, the phone would ring and one would know it was the other on the line. Even today, the twins end up buying the same birthday card for each other 75 percent of the time.
"There was a special bond between us, and we felt unique," said Comeau, now 37 and a teacher, who grew up in the 1970s when multiple births were oddities.
But today, with more women postponing marriage and resorting to fertility treatments, twins are a fact of modern life. Between 1980 and 2002, the incidence of twins jumped 65 percent, according to statistics from the Centers for Disease Control and Prevention.
When Comeau goes back to her kindergarten in Carlisle, Mass., this fall, 15 of the 70 children -- or 20 percent of the entire class -- will be either a twin or a triplet.
"We've been aware of the increase in twins for a while because it's a small community, and we know who are families are," said Patrice Hurley, principal at the public school in Carlisle, an affluent town of 5,500. "It's really an exciting situation."
The National Center for Health Statistics reports that 136,328 — or about 3 percent of the babies born in the United States in 2002 — were multiples. Of those, 128,665 were twins, 7,110 triplets, 468 quintuplets and 85 higher-order births.
In the three decades since Comeau and her twin were born, the average age of mothers for all births rose from 24.6 years to 27.2 years. The largest increase is for women in their 30s and 40s, with a jump in birthrate from 2 percent to 9.1 percent.
The trend in delayed childbirth is "universally observed" nationwide and among all groups in the population, according to the CDC's Web site.
Comeau, who has taught for nine years, has also observed that her students' parents are "a little older than the norm."
"It's not a surprise to have so many twins in the class," joked Comeau. "People say there's something interesting in the water in Carlisle."
Teachers like Comeau work hard to get to know twins individually and not to mix them up, but mistakes happen.
"We are only human, after all," she said. "But we try to let them be who they are and celebrate all the things that make them different."
Twins can occur in two ways. In the case of identical -- or monozygous -- twins, a single fertilized egg splits, creating two embryos of the same sex with the same DNA.
With fraternal — or dizygous — twins, the woman releases two eggs, which are fertilized by two different sperm. The twins are genetically like ordinary siblings, only born at the same time, and can be different sexes.
Identical twins are a random occurence, although anecdotally families say they can skip generations. Comeau's paternal grandmother had stillborn twin boys. The rate among identical twins has remained relatively unchanged.
But fraternal twins are on the rise. Some of the increase can be attributed to a woman's age, but more often fertility treatment is the cause, says Dr. F. Sessions Cole, director of newborn medicine at St. Louis Children's Hospital in St. Louis.
Hormone treatments stimulate the ovaries to produce more than one egg; and in in vitro fertilization, doctors harvest, fertilize and reimplant multiple eggs to increase a woman's chance of conception. The more eggs, the better the odds.