Everybody is or should be ” interested in twins,” said Horatio Hackett Newman. From Chang and Eng, the original Siamese twins, to Mary-Kate and Ashley Olsen, twins fascinate scientists and the public alike.
Dr. Kurt Benirschke, emeritus professor of medicine at the University of California, San Diego (UCSD), presented a lecture called “The Human Twinning Process” Jan. 30 at The Bishop’s School as the fourth in the school’s “Leaders in Science” lecture series. Benirschke shared his expertise with students of the La Jolla private school, as well as interested members of the community.
In the past, the birth of twins was a relatively rare occurrence. Not so today.
“Twinning has increased markedly,” Benirschke said.
Advanced infertility treatments have contributed to this increase. To Benirschke, this is a worrisome trend. While most twins are born healthy, he noted, “There are some problems with twins.”
Twins come in two types: identical and fraternal. Fraternal twins are formed when a woman releases two eggs, which are fertilized by two sperm. A woman may release two eggs instead of one because of high hormone levels. Higher hormone levels may be an inherited trait, or may be due to age: older women are more likely to have fraternal twins.
Although they share a womb, genetically, fraternal twins are no more alike than any other pair of siblings. Fraternal twins can be boy-girl, boy-boy, or girl-girl pairs.
On the other hand, explained Benirschke, “Identical twinning is an accident of pregnancy.”
Initially, there is only one embryo. For reasons poorly understood, early in pregnancy the embryo splits into two separate individuals. Each will develop into a twin.
Because they come from a single egg and single sperm, the twins will be genetically identical. Identical twins are always the same sex.
Both fraternal and identical twinning is on the rise. For natural conception, the twinning rate is around 2 percent. However, with assisted reproductive technology, such as in-vitro fertilization, more than one-third of pregnancies result in multiple births. In 2002, 69 pregnancies in the United States involved five or more babies.
This is a cause for concern, said Benirschke, because having more than one baby can be a risky business. Twins, triplets and higher-order multiple births have an increased risk of early delivery. This may cause low birth weight and serious disabilities, such as cerebral palsy.
Benirschke focused on one particular medical condition that affects identical twins: Twin-to-Twin Transfusion Syndrome (TTTS). In TTTS, the babies’ blood vessels overlap in the placenta. Blood can travel between the twins.
Sometimes, the blood flow between the twins is unequal. One twin pumps blood into the other. The twin that is losing blood becomes anemic, while the recipient swells with fluid.
According to the March of Dimes, identical twins have a 15 percent chance of developing TTTS. Fortunately, treatment is available. Benirschke explained that doctors can use lasers to seal off the connecting blood vessels. Then, each twin is able to develop normally on its own.
Because multiple births carry such risks, some countries have taken the step to regulate assisted reproductive technology. Benir-schke noted that in Belgium and Holland, doctors may only implant one embryo into a woman.
In the United States, while some guidelines exist, there are no laws prohibiting transfer of multiple embryos.
“We’re messing around with nature a little,” Benirschke said.
Is it worth the risk?”
The next lecture in the series is “Live and Let Die,” presented by John Reed, M.D., Ph.D., of The Burnham Institute on Tuesday, Feb. 27, 7 p.m. The “Leaders in Science” lecture series is free and open to the public. For information, visit http://www.bishops.com/news_information/science_lecture_series.htm.