Fetal Heterotaxy (Situs Ambiguus, Isomerism)
Some of the baby's organs are not in their usual left-right places
What is heterotaxy?
Heterotaxy means some organs are not in their usual right-left positions. In some babies, both sides of the body may develop with more right-sided features or more left-sided features. Doctors call this right or left isomerism.
It can affect the heart, stomach, liver, spleen, intestines, lungs, and large veins.
Heterotaxy is not one single heart defect. Each baby can have a different group of findings.
Two common patterns
Left isomerism / polysplenia pattern
There may be several small spleens. A large vein from the lower body may not follow its usual path to the heart. Some babies have heart defects or a slow heartbeat (heart block).
Right isomerism / asplenia pattern
The spleen may be missing or may not work well. Some babies have serious heart defects. The veins that bring blood from the lungs to the heart may connect in unusual ways.
Some babies have a mixed pattern that does not fit either group.
What tests are usually offered?
- A detailed ultrasound of the baby.
- A fetal echocardiogram, which is a detailed heart ultrasound.
- Genetic counseling and possible genetic testing.
- Sometimes fetal MRI or other imaging.
What does it mean for the baby?
The outlook can be very different from one baby to another.
- Some babies have mild problems.
- Some have serious heart defects and may need surgery after birth.
- If the spleen is missing or does not work well, the baby may need extra protection from infection.
- Some babies have problems with the intestines or heart rhythm.
Pregnancy and delivery
You may be followed by maternal-fetal medicine and pediatric cardiology. Delivery may be planned at a hospital with newborn and heart specialists if the baby has an important heart problem.
One important point
Heterotaxy describes the body's left-right arrangement. It does not predict one single outcome. The care team will explain the baby's exact heart, veins, spleen, stomach, and other findings.
Important: This handout is for education only. It does not replace care from your obstetrician, maternal-fetal medicine specialist, genetic counselor, or pediatric cardiologist.