A simple urine test may determine whether a birthmark is likely to develop into a threat to health.
Most birthmarks are no risk to health
The breakthrough also raises hopes of developing new drug therapy for those birthmarks that are hard to treat.
US research found urine samples from people with aggressive birthmarks had higher levels of compounds associated with development of new blood vessels.
The research, compiled at Boston's Children's Hospital, is published in the journal Pediatrics.
Most birthmarks vanish early in life or stay in proportion. But some grow more aggressively and threaten health.
There are two main types of disfiguring birthmark caused by abnormal vessel development.
The most common, hemangiomas - or strawberry birthmarks - are caused by the abnormal multiplication of the cells lining the blood vessels.
They grow rapidly in the first year of life, then usually shrink and disappear.
However, a small number grow quite large, and can cause obstruction, ulceration and other problems.
They can be treated by drugs which block the formation of new blood vessels
The second type - known as vascular malformations - can be a more serious problem.
They are linked to problems with the development of either blood or lymphatic vessels that occur before birth.
They usually grow in proportion to the child, but sometimes progress during adolescence or pregnancy, or after surgery or trauma, and in rare instances become fatal.
No drugs are currently available to treat this type of birthmark, and surgery and injection techniques produce mixed results, and can be dangerous.
The Boston team found urine samples from patients with advanced, aggressive birthmarks of either type were more likely to show elevated levels of two compounds associated with the development of new blood vessels.
One is a family of enzymes called matrix metalloproteinases (MMPs), the second a growth factor called bFGF.
In two patients the researchers were able to show that MMPs levels plummeted after treatment.
The Boston team say their work is the first to suggest that new blood vessel formation plays a role in the development of vascular malformations, as well as hemangiomas.
This raises the prospect that drugs which are effective against hemangiomas may also work against vascular malformations.
Researcher Dr Steven Fishman said: "Prior to this study, we had thought it was not possible to treat vascular malformations with drugs, since congenital anomalies generally do not respond to drugs.
"This study gives us hope that with further research we'll be able to develop drug treatments."
In the meantime, urine testing for MMPs may help physicians know when a vascular anomaly is about to become aggressive and needs intervention.
Dr Fishman said: "It can be very hard to tell whether an anomaly will progress.
"It can sit there and do nothing, or go on to destroy the nose or other nearby tissues.
"What we've shown is that the presence of MMPs in urine correlates with how aggressive the lesions are."
Professor John Harper, director of the vascular anomalies service at Great Ormond Street Hospital in London, told the BBC News website the research was "very interesting".
"Potentially it could be important in understanding and managing these birthmarks," he said.
"But this is the first report of its type, and we need more work to corroborate the findings, and to assess how useful they are likely to be in the future."