In rare cases, twins develop so closely that their body parts are joined together. Conjoined twins can be connected through a variety of body parts, including the head, chest, abdomen, pelvis and buttocks. Twins joined at the chest or abdomen are the most common type of conjoined twins, comprising 75% of all cases.
Common types of conjoined twins
The most common sites for twins to be joined are:
- Thoracopagus - Joined at the chest and facing each other
- Omphalopagus - Joined at the abdomen and facing each other
- Pygopagus - Joined at the buttocks and perineum, and facing away from each other
- Ischiopagus - Joined with a single bony pelvis and four normal lower extremities
- Craniopagus - Joined at the skull with or without brain connection
What causes conjoined twins?
Identical twins occur when a single fertilized egg (embryo) splits and develops into two individuals. The dominant theory on the origin of conjoined twins suggests that when the single embryo splits later, separation stops before the process is complete, leaving the babies joined. Alternatively, another theory holds that two separate embryos fuse in early development. What would cause either scenario remains unknown.
The exact rate of conjoined twins is not known, but estimates have varied from 1 in 25,000 to1 in 80,000 births. Maternal age and the number of prior pregnancies do not appear to be factors that influence the occurrence of this type of twins. However, use of assisted reproductive techniques (for example, in vitro fertilization) may result in an increased risk for conjoined twins.
How are conjoined twins diagnosed?
A routine ultrasound is the most common form of diagnosis for conjoined twins. The ultrasound might raise suspicion if the babies appear to be in the same pregnancy sac (meaning no dividing membrane). Then, if there is no dividing membrane, the babies' bodies might not appear to be separate or they might not change position relative to each other as time passes. Prior to 10 weeks of pregnancy, this condition may be diagnosed incorrectly by ultrasound. Multiple organ structures might be involved and/or developing abnormally.
MRI and echocardiogram are used to provide additional details. The more we know about the anatomy and how precisely the two babies are joined, the more accurately our team can assess whether surgically separating the twins is possible, and if so, how best to proceed.
Understandably, this is a very upsetting diagnosis. The fetal care specialists at the Colorado Fetal Care Center can help guide your family through pregnancy, delivery and care for your twins after birth. Successful surgical separation after birth is possible, but the outcomes depend on the type of connection as well as which organs and body structures are shared.
Managing pregnancy with conjoined twins
Conjoined twins can be delivered by cesarean section at our state-of-the-art facility, where our team specializes in managing this condition. Our fetal experts also have experience in procedures to surgically separate the twins after birth. There are no surgical procedures that can be performed while the babies are still in the uterus (referred to as fetal interventions) to separate conjoined twins.