Cystic hygroma forms when small pockets of lymphatic tissue become separated from the rest of the lymphatic system during early fetal development. These isolated pockets can’t drain properly, so they fill with fluid and expand into cysts. About half of all cases are linked to a chromosomal abnormality, though the condition can also occur with a normal set of chromosomes or, rarely, develop later in life after trauma or infection.
How the Lymphatic System Goes Wrong
Early in pregnancy, the fetal lymphatic system develops a network of channels that eventually connect to the venous (blood) system. In a cystic hygroma, some of that lymphatic tissue gets sequestered, meaning it separates from the lymphatic sacs before those connections form. Because the tissue has no pathway to drain fluid into the bloodstream, lymph fluid accumulates and stretches the tissue into fluid-filled cysts.
This process typically begins in the first trimester. Cystic hygromas are most often detected during routine ultrasound screening between 10 and 14 weeks of gestation, when the fluid collection behind the baby’s neck becomes visible. What distinguishes a cystic hygroma from a simple thickening of the neck fold is that the fluid extends down the fetal back or contains internal dividing walls called septations. That distinction matters because cystic hygromas carry a significantly higher risk of chromosomal problems than a thickened neck fold alone.
Chromosomal Abnormalities
Genetic problems are the single most common cause. A 2025 systematic review found that among fetuses diagnosed with cystic hygroma, roughly 21% had Turner syndrome (a missing or partially missing X chromosome), 17% had Trisomy 21 (Down syndrome), 10% had Trisomy 18 (Edwards syndrome), and 3% had Trisomy 13 (Patau syndrome). Another 5% had other genetic diagnoses. Added together, chromosomal abnormalities account for more than half of all prenatally detected cases.
Turner syndrome deserves special attention here because it’s the most frequent single cause. In Turner syndrome, the developing lymphatic system is particularly prone to drainage failure, which is why cystic hygroma on an early ultrasound is one of the hallmark findings that prompts genetic testing.
Gene Mutations With Normal Chromosomes
Not every case shows up on a standard chromosome test. Some fetuses have a normal number of chromosomes but carry mutations in individual genes that disrupt lymphatic development. Noonan syndrome is the best-studied example. It’s caused by mutations in genes that control cell signaling, particularly one called PTPN11, along with others like KRAS, RAF1, BRAF, and MAP2K1.
In one diagnostic study published in the European Journal of Human Genetics, researchers tested 75 fetuses that had normal chromosomes but abnormal ultrasound findings. About 17% turned out to carry a mutation in a Noonan syndrome gene. This means that even when initial genetic screening comes back normal, there may still be a genetic explanation, and more targeted testing can sometimes find it. Noonan syndrome causes a range of features including heart defects, short stature, and distinctive facial characteristics, so identifying it prenatally helps families and doctors plan ahead.
Maternal Infections
Viral infections during pregnancy can occasionally produce fluid collections that resemble or accompany cystic hygroma. Parvovirus B19, the virus that causes “fifth disease” in children, is the most well-documented example. The virus itself doesn’t cause birth defects, but it can damage fetal blood vessel walls and infect red blood cell precursors, leading to severe anemia. That anemia, combined with leaky blood vessels, can trigger widespread fluid buildup in the fetus (hydrops fetalis).
In one study of hydropic fetuses with parvovirus infection, about 11% had cystic hygroma as part of their fluid pattern. This is a much smaller contributor than chromosomal causes, but it’s relevant because parvovirus infection is treatable. Doctors can perform fetal blood transfusions in severe cases, and many infections resolve on their own.
Cases With No Identified Cause
According to the Fetal Medicine Foundation, about 10% of fetal cystic hygromas occur in babies with a normal karyotype, no other obvious defects, and no identified genetic syndrome. In these cases, the hygromas often resolve on their own during pregnancy. The lymphatic system essentially corrects its drainage problem as it matures, and the fluid reabsorbs. These isolated cases carry the best prognosis by far. Fetal death occurs in roughly 90% of all cystic hygroma cases overall, but that figure is heavily driven by the cases with severe chromosomal abnormalities or hydrops. Hydrops, where fluid accumulates in multiple body compartments beyond just the neck, develops in 60 to 80% of cases and is the main driver of poor outcomes.
Postnatal and Adult Cases
Most cystic hygromas are detected before birth, but some are not discovered until after delivery, and a small number develop in adults. In babies, a cystic hygroma may not become noticeable until it grows large enough to see or feel, sometimes ranging in size from a grape to a grapefruit. These postnatal cases likely reflect the same embryological problem, just one that was too small to detect on prenatal imaging.
In adults, cystic hygromas can form after physical trauma or a respiratory infection damages the lymphatic system. These are rare and tend to occur in areas where lymphatic tissue is concentrated, particularly the neck and armpit. Unlike the prenatal form, adult cystic hygromas are not associated with chromosomal abnormalities and are typically treated as isolated structural problems.
How Common Cystic Hygroma Is
The overall incidence is estimated at 1 in 1,000 to 1 in 6,000 births, but the condition is found at a much higher rate among pregnancies that end in miscarriage, roughly 1 in 750 spontaneous abortions. That gap reflects the high rate of fetal loss associated with the chromosomal causes described above. Many affected pregnancies end before reaching viability, which means the condition is more common in early pregnancy than the birth rate alone suggests.