Joined for life.

What's it like to have a sibling constantly at your side? For six-year-old Brittany and Abigail Hensel, its perfectly normal. They are...

Imagine this: You wake up each morning to find your sister lying beside you. To get dressed and tie your shoes, you use one hand and she uses another. You do everything else together, too, even sitting on the same chair at lunch and riding on the same bicycle. That's what life is like for six-year-old Brittany and Abigail Hensel.

Like most identical twins, the Hensel girls look very much alike. But unlike most twins, Britty and Abby share parts of the same body. They're conjoined twins--identical twins who are physically connected to each other.

Conjoined twins like Britty and Abby are rare. Only about 40 sets are born in the United States each year. Few survive as long as Britty and Abby. That's because conjoined twins often share vital organs, like a heart or brain. These shared organs are often malformed and may not be strong enough to support both twins.

But Britty and Abby each has her own head, heart, and stomach, which function normally. They also have separate spinal columns, though these are joined at the pelvis. The girls share three or four lungs (doctors don't know for sure), which provide plenty of oxygen for both twins. Most of their completely shared organs lie below the waist (see diagram, right).

LIVING TOGETHER

Britty and Abby lead relatively normal lives. They attend a regular school, and each does her own school work. They prefer to do some projects together, though. For example, to cut out paper dolls, one twin holds the paper, while the other uses the scissors.

But sometimes the girls don't want to do the same thing. For example, sometimes they want to play with different toys. What do they do then? "We flip a coin," says Abby.

Doctors aren't sure how the girls, with separate brains, coordinate their movements. Britty's brain controls the left side of the body, while Abby has command of the right. How did they learn to walk, swim, and ride a bike? Dr. Benjamin Carson of Johns Hopkins Children's Center speculates that Britty and Abby have managed so well because their nervous systems may be somewhat connected too.

MAKING A TWOSOME

How do conjoined twins develop. Like regular identical twins, they form when a newly fertilized egg, or zygote, splits inside a woman's womb (see diagram, p. 11). When the zygote splits completely in half, each "half" develops into a separate identical baby. But sometimes--as in Britty and Abby's case--the zygote doesn't fully separate.

Scientists are not sure why this happens. The zygotes that separate to form regular identical twins usually split within the first few days of pregnancy. Scientists think those that start to separate late--say, after 12 days--are more likely to stay partly joined.

TO SEPARATE OR NOT?

Sometimes conjoined twins can be separated after birth. Deciding whether to perform such a tricky operation is difficult for parents and doctors. According to Dr. Geoffrey Machin, an expert on conjoined twins, there are no general rules. "Certain kinds of conjoined twins have such little overlap [in organs] that they can be separated with no risks. Other ones are not separable, period," Dr. Machin says.

When Britty and Abby were born in 1990, doctors disagreed on whether the twins could or should be separated. The operation would divide the twins in half, leaving each girl with only one arm and one leg. Neither twin would be able to support artificial limbs, so they'd both probably be disabled for life.

In other cases, where conjoined twins share a heart, brain, or other vital organs, doctors generally decide to leave them together because these organs are very difficult to divide.

Abby and Britty's parents, Mike and Patty Hensel, never considered separating the girls. Doctors told them that the chances of both twins surviving the operation were very small. "How could you pick between the two?" asks Mike.

The twins support their parents' decision. "I'm not going to be separated," says gritty. Abby agrees.

TAKING A CHANCE

But not all parents of conjoined twins make the same choice.

When Megan and Shannon Fanning of Naperville, Illinois, were born two years ago, they were joined from chest to belly. The twins shared a single liver and had intertwined and partially connected intestines. The liver can be divided safely because it regenerates--it forms new tissue to replace a part that is lost. Even so, doctors thought the chances were poor that both twins would survive separation.

The twins' parents decided to take their chances. The day after the Fanning girls were born, a medical team rushed them to the Children's Memorial Hospital in Chicago for an eight-hour surgery. Doctors divided the girls' liver in half, then untangled and separated their intestines. The surgery was a success! Today Megan and Shannon are healthy, happy two-year-olds (see photo, left). The only sign that they were once joined at the stomach: They have no belly buttons!

Like the Fannings, some parents of conjoined twins think separation helps their children lead more normal lives. Doctors may also decide to separate conjoined twins if one twin is weaker or might die. Separation might save the healthier twin's life.

But some attempts at separation fail. Three years ago, Angela and Amy Lakeberg, another pair of conjoined twins, underwent separation surgery (see SW 11/5/93, p. 7). The girls were connected at the chest, sharing a liver and an abnormal heart. Their parents knew one twin would have to die to save the other. Amy died during surgery, but Angela pulled through. Ten months later, however, Angela died of respiratory problems. Her lungs had never fully recovered from the surgery.

What do you think? Should conjoined twins be separated? Read what some SW readers say (right), then debate and decide for yourself.

RELATED ARTICLE: What's Inside Britty and Abby

2 spinal cords joined at the pelvis 3 or 4 lungs 2 hearts with a common

circulatory system 1 liver 2 stomachs 3 kidneys 1 large intestine 1 small intestine 1 pair of ovaries 1 uterus 1 bladder

How Do Twins Form?

Identical twins form when a single fertilized egg (a zygote) splits in half and each half develops into a separate baby. These monozygotic (from one zygote) twins share the same genes and are always the same sex. If the zygote doesn't split completely, the twins will stay attached, or conjoined, and share certain body parts.

Fraternal twins are dizygotic. They form when two separate eggs are fertilized. These twins are no more alike than regular siblings.

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RELATED ARTICLE: Twins, triplets and more!

Conjoined twin births happen very rarely. Only abaut 40 cases are recorded in the United States each year. But other kinds of twin births are on the rise (see graph, right). In 1993 alone, an average of nearly 300 twins, triplets, and up were born each day! What has caused this multiple-birth baby boom?

According to Dr. Louis Keith, President of the Center for the Study of Multiple Births in Chicago, Illinois, more women are using fertility drugs to help them have babies. Some women have difficulty getting pregnant because their ovaries (female reproductive organs) don't release eggs on a regular monthly cycle. Fertility drugs stimulate egg release. But sometimes the drugs cause the ovaries to release many eggs at a time, instead of the usual one. That means more than one egg can be fertilized by sperm (male reproductive cells). The result: multiple babies.

Other women, says Dr. Keith, are choosing to start families later in life, when they're in their 30s. At this age, a woman's body is more likely to release more than one egg per month naturally.

Dr. Geoffrey Machin, a twin expert, adds that increasingly good nutrition may also play a role. "It may well be that you have to be well-nourished to be able to carry twins," says Dr. Machin.

Experts also cite improved prenatal care--care for expecting mothers. More rest, better nutrition, and more careful monitoring of mothers expecting multiple births mean more of these babies are born close to full-term (after nine months of pregnancy). That means the babies are more fully developed and likely to survive.

Are twins or triplets on the rise in your school or town? To find out, try checking school or hospital records.