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Together forever? Twins who ape physically connected to each other face a difficult decision: to separate or remain joined for life.

For 16-year-old twins Abigail and Brittany Hensel, everything takes teamwork, whether it's playing basketball, typing IMs, or walking across the room. That's because these girls share a body, with each twin controlling one arm and one leg.

Abigail and Brittany are conjoined twins meaning they are physically connected. Each girl has a separate head, heart, stomach, and spinal column, but they join together at the torso and share their lower organs (see Inside Brittany and Abigail, p. 14). Neither girl can feel any sensation from her sister's side of the body, but they coordinate their movements to swim and play on school sports teams. On their 16th birthday, they even got their driver's licenses. Abigail controls the pedals and shifter, Brittany the blinkers and lights, and both girls steer. They had to pass the driver's test twice--once for each twin.

Abigail and Brittany say they're satisfied with life as a twosome, but the same is not true for all conjoined twins. In each case, families and doctors face a tough question--whether or not to attempt a risky surgery to separate them.

TWO FOR ONE

Why are some twins conjoined? In a mother's womb, a single zygote, or fertilized egg, usually grows into a single baby. If the egg splits in two, then two babies develop. These twins are commonly called identical twins. But since no two people are completely identical, some experts prefer the term monozygotic (from one zygote), or MZ twins. Most doctors believe that conjoined twins start off like other MZ twins, but the zygote doesn't split completely (see Nuts & Bolts, p. 15), so the embryos, or developing young, remain joined as they grow.

"The places where they can be joined are variable," says Dr. Hardy Hendren, former chief of surgery at Children's Hospital Boston. Some are attached at the head, while others are joined at the chest, abdomen, or pelvis.

Conjoined twins are rare--only one in every 40,000 to 60,000 births. But most die before or shortly after birth. Dr. James O'Neill, a pediatric surgeon at Vanderbilt University Medical Center in Tennessee, explains, "The majority of conjoined twins are joined in ways that are not compatible with life." For instance, they may share a brain or heart that is abnormally developed or too weak to support both twins.

TOGETHER, OR APART?

Whether or not doctors can attempt separation depends on which organs the twins share. "We try to quickly assess the major structures that lie in the region of the bridge between the two," says Dr. Hendren. In some cases, such as with a shared brain, doctors determine that separation is impossible. But other organs are different. For example, twins joined at the abdomen and pelvis often share a colon. "What we do in that circumstance is divvy up the colon, so that each baby gets haft a colon, in terms of length. And they will thrive with that. You can do perfectly well with half a colon," says Dr. Hendren. This is what Dr. Hendren did for 4-month-old Hussein and Hassan Mohamed of the United Arab Emirates. During the 25hour surgery, he also divided the boys' shared liver. This was possible because the liver regenerates, or grows replacement tissue. The boys are now active, healthy 7-year-olds.

Once doctors decide to go ahead with surgery, medical technologies such as ultrasound, CAT scans, and MRI give them an edge they didn't have in the past. These imaging techniques can create 3-D visualizations of the internal organs they'll attempt to separate. "That has made a huge difference, because you can take those images and turn them around, and turn them upside down, and look at them from various views," says Dr. O'Neill. "With better imaging, it means that the surgical teams are much more prepared and can avoid unforeseen problems."

Such technology turned out to make all the difference in the case of Maria de Jesus and Maria Teresa Alvarez. These conjoined twins were born in Guatemala on July 25, 2001. They spent the first 12 months of their lives attached at the head. Everyday tasks, such as bathing, were next to impossible; one sister had to be held upside down while the other was dipped right side up into the tub.

When deciding if surgery was possible for the girls, doctors used high-energy x-rays to create a radiograph. This image showed that the twins were conjoined at the skull but they have two separate brains. With that knowledge, the parents decided to go ahead with the surgery--which proved to be successful.

RISKY BUSINESS

Even when doctors believe separation is possible, surgery can be a difficult choice for families. The surgery

is extremely complicated, and there's always the risk that one or both twins won't survive or that they'll develop infections or other problems afterward. If doctors successfully separate them, the twins will have to follow up with reconstructive surgeries and rehabilitation. The Alvarez twins, for instance, have had surgeries to reposition bones. Maria Teresa also suffered a brain infection that caused physical and mental damage, but she's come a long way through therapy. The girls, now pre-schoolers, spend 10 to 15 hours a week on physical therapy to help strengthen their bodies. And when they get older, they'll need surgery to replace the missing skull portions on top of their heads that resulted from separation. Their caregiver, Jenny Hull, explains that between surgeries they live much like other little girls, playing with friends and swimming. "They're the happiest little girls you'll ever meet," says Hull.

Doctors sometimes feel that the risks of leaving conjoined twins together are greater than the risks of surgery. Because of the strain on shared organs, some sets wouldn't live long if they remained joined. The toughest situation for families comes when doctors know they can save only one baby. When conjoined twins named Amy and Angela Lakeberg were born in 1993, they shared one heart that couldn't support both babies. Doctors went into the surgery knowing that Amy would die. Angela also died 10 months later from respiratory problems.

Even when both conjoined twins are healthy, families may decide to risk separation because they worry that their children won't be able to live traditional lives if they remain joined. But Abigail and Brittany's parents decided that the risks of separation were too high. If the girls survived the surgery, they would be severely disabled because they weren't born with two full sets of limbs. In Joined for Life: Abby and Brittany Turn 16, a documentary that recently aired on television, Abigail said, "We never wish we were separated, because then we wouldn't get to do all the things that we can do," such as running and playing sports. Brittany agreed, adding, "We don't know any other way."

To view a segment of the documentary, Joined for Life: Abby and Brittany Turn 16, visit: www.figure8films.tv/ content/shows/jfl16.htm

1. What is the main idea of this article?

(A) The Alvarez twins were successfully separated.

(B) Families of conjoined twins and their doctors must decide if the benefits of separation outweigh the risks of surgery.

(C) Conjoined twins are rare--only one in every 40,000 to 60,000 births.

(D) If doctors separate conjoined twins, the twins will have to undergo rehabilitation.

2. What do doctors use to determine which organs conjoined twins share?

(A) Ultrasound (B) CAT scans (C) MRI (D) All of the above

3. What is another term used to describe identical twins?

(A) Monozygotic (B) Embryonic (C) Fraternal (D) Dizygotic

ANSWERS

1. b 2. d 3. a

PRE-READIN6 PROMPTS

Jump-start your lesson with these pre-reading questions:

* Although more male conjoined twins develop in the womb than female conjoined twins, the female twins are more likely than the male twins to be born alive. Approximately 70 percent of all living conjoined twins are girls. What are some factors that determine the chances of survival for conjoined twins?

* Approximately 40 to 60 percent of conjoined twins emerge stillborn, and about 35 percent survive for only one day. The overall survival rate of conjoined twins is between 5 percent and 25 percent. Conjoined twins are extremely rare. One of about how many births do you think produce conjoined twins?

CRITICAL THINKIN6:

* Hold a class debate on this topic: Do you think it's ethical to separate conjoined twins when it's likely that one will die during the surgical procedure?

CROSS-CURRICULAR CDNNECTIONS:

MATH: In 2002, there were 4,021,726 births in the U.S. Of the births, 125,134 produced twins. Approximately what percent of the births that year produced twins? (Answer: 3.1 percent)

In 2002, 6,898 births in the U.S. produced triplets and 434 births produced quadruplets. Approximately what percent of the births that year produced triplets and quadruplets. (Answer: 0.17 percent; 0.01 percent)

RESOURCES

* For an article that explains the surgical process that separated Maria Teresa and Maria de Jesus Alvarez, read "Separating Sisters," by Nicole Dyer, Science World, November 29, 2002.

* Conjoined twins who are attached only at the upper part of the skull are called craniopagus twins. To learn other terms that describe conjoined twins based on the body parts at which the twins are joined, visit: www.medhunters.com/articles/sharedLives.html

* For a historic time line on known cases of conjoined twins, visit: www.twinstuff.com/conjoined.timeline.htm

* For a lesson plan on twins and information on how to purchase a copy of the Discovery Channel program Mystery of Twins, visit this Web site: http://school.discovery.com/lessonplans/programs/themysteryoftwins/

CHECK FOR UNDERSTANDING

DIRECTIONS: Defend or dispute the following. (Hint: Defend means to explain why a statement is correct. Dispute means to explain why a statement is incorrect.)

1. A single zygote can produce identical twins, including conjoined twins.

2. Conjoined twins have a high survival rate. In most cases, they can be easily separated.

ANSWERS

1. Defend: When a single zygote splits in two, two identical embryos will form. These embryos develop into identical, or monozygotic, twins. Most doctors believe that conjoined twins start off like other monozygotic twins, but the zygote doesn't split completely, so the embryos remain joined as they grow.

2. Dispute: Most conjoined twins die before or shortly after birth. That's because the majority of conjoined twins are joined in ways that are not compatible with life. For instance, they may share a brain or heart that is abnormally developed or too weak to support both twins. Whether or not doctors can attempt separation depends on which organs the twins share. Doctors must assess the major structures that lie in the region of the bridge between the twins. In some cases, such as with a shared brain, doctors determine that separation is impossible. Even if a separation is possible, the surgery is extremely complicated. There's always the risk that one or both twins won't survive or that they will develop infections or other problems afterward.
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Title Annotation:LIFE REPRODUCTION
Author:Adams, Jacqueline
Publication:Science World
Date:Apr 2, 2007
Words:1823
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