Fraternal Twins or Identical Twins?

Doctors have reported the first known case of fraternal twins who shared a single placenta in the womb. The researchers have suggested that this unique form of twinning may have come about as a result of in vitro fertilization. The twins, a boy and a girl, are now 2 years old and ''normal and healthy,'' said Dr. Vivienne L. Souter, a geneticist who described them in Thursday's issue of The New England Journal of Medicine. Each child possesses a mixture of male and female blood, most likely because their blood systems ''communicated'' through the shared placenta, Dr. Souter said. But all their other tissues are entirely separate, with separate sets of genes. ''We have always been taught that this kind of thing couldn't happen,'' said Dr. Raymond W. Redline, a reproductive pathologist at Case Western Reserve University in Cleveland. ''When there is one placenta, we have assumed the twins are identical.'' The twins were conceived with in vitro fertilization, using donor eggs for implantation in a 48-year-old mother. After fertilization, the embryos were grown in a laboratory for five days, to the blastocyst stage containing 100 or more cells. Practitioners have found that pregnancy is more likely when they transfer blastocysts to a woman's uterus rather than three-day-old, eight-celled embryos. At some point, either just before or just after the embryos were transferred to the mother's womb, it seems that the outer shells of the two blastocysts may have fused, Dr. Souter said. This caused the development of only one placenta, while the embryos remained separate. As the pregnancy progressed and doctors viewed the twins with ultrasound, they assumed that the mother was carrying identical twins. But amniocentesis showed one male fetus and one female. ''Obviously, it raises a question about whether culturing the embryos for a long time in vitro could predispose for this,'' Dr. Souter said. ''But it could be completely unrelated to I.V.F. We don't know for sure.'' In an earlier case, reported in 1998 in The New England Journal, two fraternal twins conceived via I.V.F. seemed to have fused entirely, creating what researchers called a ''chimeric hermaphrodite.'' This was a child with both male and female sex organs -- a testicle and an ovary. By age 4 1/2, the child was developing as a normal, healthy boy, according to The New England Journal's study.