The morula becomes a blastocyst on day 6 after the ovum is fertilized. An inner cell mass develops at one end within this vesicle. The inner cell mass can form a whole fetus. Conjoined twins are produced when this inner cell mass, derived from a single zygote, incompletely splits late, after the 12th day of gestational life. Identical twins (monozygotic twins) occur when a single fertilized egg splits and develops into two individuals. Eight to 12 days after conception, the embryonic layers that will split to form monozygotic twins begin to develop into specific organs and structures. It's believed that when the embryo splits later than this — usually between 13 and 15 days after conception — separation stops before the process is complete, and the resulting twins are conjoined. An alternative theory suggests that two separate embryos may somehow fuse together in early development. What might cause either scenario to occur is unknown.
Conjoined twins can be diagnosed using standard ultrasound as early as the first trimester. More-detailed ultrasounds and echocardiograms can be used about halfway through pregnancy to better determine the extent of the twins' connection and functioning of their organs. False-positive results can occur before 10 weeks, however, when identical twins who share an amniotic sac (monoamniotic twins) may appear conjoined. If an ultrasound detects conjoined twins, a magnetic resonance imaging (MRI) scan may be done. It can provide greater detail about where the conjoined twins are connected and which organs they share.
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Doctors at Texas Children’s Hospital had their work cut out for them during a 26-hour operation to separate conjoined twins Knatalye Hope and Adeline Faith Mata. After spending their first 10 months of life conjoined at the chest and abdomen, Knatalye and Adeline underwent the grueling operation in which surgeons disjoined their chest wall, lungs, diaphragm, liver, intestines, colon, pelvis, and the lining of the heart, also known as the pericardial sac. "This surgery was not without its challenges with the girls sharing several organ systems," Dr. Darrell Cass, pediatric surgeon and co-director of Texas Children's Fetal Center, told KHOU. "Our team has been preparing for this surgery for months, and we've done everything from working with our radiology experts to build a 3-D model of their organs, to conducting simulations of the actual separation surgery."
Approximately 40 to 60 percent of conjoined twins arrive stillborn, and about 35 percent survive only one day. The overall survival rate of conjoined twins is somewhere between 5 percent and 25 percent. For some reason, female siblings seem to have a better shot at survival than their male counterparts. Although more male twins conjoin in the womb than female twins, females are three times as likely as males to be born alive. Approximately 70 percent of all conjoined twins are girls.