Conjoined twins form when a single fertilized egg begins to split into identical twins but doesn’t fully separate, resulting in two individuals who remain physically connected and often share organs. The condition occurs in roughly 1 in every 50,000 to 100,000 births, and about 70% of conjoined twins are female. How much they share, and where they’re joined, depends on timing and positioning during the earliest days of embryonic development.
How Conjoined Twins Form in the Womb
All conjoined twins are identical twins, meaning they come from a single egg fertilized by a single sperm. In a normal identical twin pregnancy, the embryo splits completely within the first two weeks after conception, producing two separate babies with the same DNA. In conjoined twins, that split is incomplete.
Scientists have two main theories for how this happens. The first, called partial fission, proposes that the embryo begins to divide but stops partway through, leaving the twins attached. The second, called secondary fusion, suggests that two initially separate embryos develop so close together that they partially merge. Both theories can explain the patterns doctors actually observe, and the real answer may be that different types of conjunction arise through different processes.
The leading model today is the embryonic disk theory. It proposes that two body axes form within a single embryo, developing so close together that their growth signals overlap. Where those axes overlap, the twins build shared structures instead of separate ones. The initial distance between these two axes, and the angle at which they’re oriented, determines exactly where the twins will be connected and how many organs they’ll share. A small gap produces extensive sharing. A larger gap produces twins joined only by skin and soft tissue.
Where Conjoined Twins Connect
The point of attachment varies widely, and doctors classify conjoined twins by the body region where they’re joined. The most common type by far involves the chest and abdomen. These twins, who face each other with their torsos fused, account for about 75% of all conjoined twin cases. They typically share a single umbilical cord and often share major organs like the heart or liver.
Twins joined at the side develop when the two embryonic axes form parallel to each other rather than facing one another. This side-by-side orientation often produces mirror-image anatomy, where one twin’s organs are arranged as a reverse of the other’s. Twins joined at the head are much rarer but receive significant attention because separation surgery is especially complex when brain tissue or blood vessels are shared.
Twins joined along the back present differently from all other types. They tend to have two separate umbilical cords and mostly independent internal organs. This pattern fits the fusion theory: two embryos that were initially separate drifted close enough to fuse along their dorsal (back) surfaces, but because they developed independently for longer, they share fewer structures.
How Shared Organs Function
The circulatory system is the most critical shared structure in conjoined twins, and the degree of cardiovascular connection varies enormously from pair to pair. Among twins joined at the chest, about 75% share some part of their cardiovascular system. In some cases, the connection is minimal: two fully separate hearts sitting inside a shared sac of tissue. In others, the hearts are fused at the level of the upper chambers, the lower chambers, or both.
Even when hearts are partially connected, each twin can maintain a distinct heart rhythm. In one documented case, one twin had a condition that dramatically slowed her heartbeat while the other twin’s heart beat at a normal rate. The slow heartbeat did not drag the other twin’s rate down. This tells us that shared cardiac tissue doesn’t necessarily mean the hearts function as a single unit. Each twin’s nervous system can still control its own heart rhythm independently.
Shared livers are common in chest-and-abdomen twins, and the liver’s natural ability to regenerate makes it one of the more straightforward organs to divide during separation surgery. Shared intestinal tracts, kidneys, and bladders occur in twins joined at the pelvis. When twins share a brain, which happens only in those joined at the skull, they may share sensory experiences. Some pairs joined at the head have reported being able to see through each other’s eyes or sense what the other tastes.
Detection Before Birth
Conjoined twins can be identified on a routine ultrasound as early as 7 to 12 weeks of pregnancy. At that stage, a doctor can see that the embryos haven’t fully separated, but it’s too early to determine the extent of organ sharing. Around the halfway point of pregnancy (18 to 22 weeks), more detailed ultrasounds and fetal echocardiograms can map out the cardiovascular connections and assess how each twin’s organs are functioning.
If the ultrasound raises concerns about complex organ sharing, an MRI scan of the fetus provides finer detail about exactly which structures are shared and how deeply fused they are. All of this imaging drives the biggest decisions families and medical teams face: whether to continue the pregnancy, whether separation surgery is feasible, and how to plan the delivery itself, which is almost always by cesarean section.
What Determines Whether Separation Is Possible
The single biggest factor in whether conjoined twins can be separated is which organs they share and whether those organs can be divided or reconstructed. Twins who share a single heart generally cannot be separated without sacrificing one twin, which raises profound ethical questions. Twins who share a liver but have separate hearts are much stronger candidates for surgery because the liver can be split and will regrow.
Separation surgery is among the most complex procedures in medicine, often involving teams of 20 or more surgeons, anesthesiologists, and nurses working in stages over 10 to 24 hours. One of the biggest practical challenges is simply having enough skin to close the wounds after the twins are apart. Surgeons typically place tissue expanders, essentially inflatable silicone balloons, under each twin’s skin months before the separation to gradually stretch extra tissue. Complications with these expanders are common and sometimes require additional operations. In cases where expansion doesn’t produce enough coverage, surgeons use banked skin grafts from the twins’ own tissue as a backup.
Fluorescent dye mapping helps surgeons determine which blood vessels supply which twin’s tissue, so they can divide shared structures while preserving blood flow to both sides. Nutritional support before and after surgery is also critical, since conjoined twins often have higher metabolic demands and can lose significant blood volume during separation.
Success rates depend heavily on the type of conjunction. In one surgical series, six of seven pairs were successfully separated, but that group was pre-selected as strong candidates. For twins with extensive organ sharing, the outcomes are far less favorable. The overall success of any separation depends on the experience of the surgical team and the resources available at the hospital, which is why these procedures are concentrated at a small number of pediatric specialty centers worldwide.
Why It Happens More Often in Girls
Conjoined twins occur at a 3-to-1 female-to-male ratio. No one has identified a definitive genetic or environmental cause for conjoined twinning in general, let alone the sex disparity. No specific gene mutations, maternal exposures, or dietary factors have been linked to the condition. It appears to be a random event in embryonic development, occurring when the twinning process, which already happens unpredictably, goes one step further by failing to complete. The female predominance mirrors a pattern seen in identical twinning more broadly, where female pairs are slightly overrepresented, but the reason remains unexplained.