Yes, rubella is a teratogen. It is one of the most well-established teratogenic infections in humans. When a pregnant person contracts rubella, particularly during the first trimester, the virus can cross the placenta and directly damage developing fetal tissues. Infection in the first 11 weeks of pregnancy carries up to a 90% chance of the baby being born with birth defects, a condition known as congenital rubella syndrome (CRS).
How Rubella Crosses the Placenta
Rubella infection causes viremia, meaning the virus circulates in the bloodstream. During pregnancy, that circulating virus can infect the placenta itself. The virus damages the lining of blood vessels in the placenta, and as those damaged cells break away into the bloodstream, the virus gains access to fetal circulation. Rubella can also pass from maternal immune cells to specialized placental cells called extravillous trophoblasts, essentially bypassing the placenta’s protective barrier layer and reaching fetal blood vessels through an alternative route.
Despite rubella being considered the textbook example of a teratogenic virus, the exact mechanisms of how it causes birth defects are still not completely mapped out. What researchers do know is that once the virus reaches fetal tissue, it causes cell death and suppresses cell division in precursor cells that are critical during organ formation. Organs that are actively developing at the time of infection are the ones most affected, which is why timing matters so much.
Why the First Trimester Is Most Dangerous
The risk of CRS is tightly linked to when during pregnancy the infection occurs. The first 11 weeks carry the highest danger, with up to 90% of infected pregnancies resulting in a baby with CRS. This is the window when the heart, eyes, ears, and brain are undergoing their most rapid and complex development. Rubella doesn’t just cause one type of defect; it disrupts whatever organ system is forming at the moment the virus arrives.
After the first trimester, the risk drops significantly but doesn’t disappear entirely. Hearing loss, for example, can result from infection as late as the 20th week. Infection in the second and third trimesters is less likely to cause the severe structural defects seen with early exposure, but it can still lead to growth restriction and other complications.
The Classic Pattern of Birth Defects
CRS has a recognizable set of core features known as the Gregg triad, named after the Australian ophthalmologist who first connected rubella to birth defects in 1941. The three hallmark defects are:
- Sensorineural deafness: the most common single defect, caused by damage to the developing inner ear
- Cataracts: clouding of the lens in one or both eyes, sometimes accompanied by other eye abnormalities
- Heart defects: structural abnormalities of the heart, particularly patent ductus arteriosus and pulmonary artery stenosis
Beyond this classic triad, CRS can also cause microcephaly (an abnormally small head), intellectual disability, liver and spleen enlargement, low birth weight, and a characteristic blueberry-colored skin rash caused by abnormal blood cell production. Not every baby with CRS has all of these features. Some have a single defect, while others have many.
Problems That Appear Later in Life
One of the more troubling aspects of CRS is that some consequences don’t show up until years or even decades after birth. Children who appeared to escape the worst effects at birth can develop new problems over time. These delayed manifestations include diabetes, thyroid disorders, and growth hormone deficiency. Hearing loss that wasn’t present at birth can emerge later in childhood.
Eye problems can also progress. Glaucoma, corneal changes, and other forms of ocular damage may develop well after the initial cataracts are treated. Vascular problems are another late finding: the virus can cause thickening of artery walls, leading to high blood pressure and kidney-related complications. In rare cases, CRS survivors develop a devastating condition called progressive rubella panencephalitis, a slow inflammatory brain disease that can appear in the second or third decade of life.
CRS Is Still a Global Problem
In countries with widespread MMR (measles, mumps, rubella) vaccination programs, CRS has become rare. But globally, the picture is very different. An estimated 32,000 babies were born with CRS worldwide in 2019. Three-quarters of those cases, roughly 24,000, occurred in just 19 lower- and middle-income countries that had not yet introduced rubella-containing vaccines into their immunization programs by the end of 2023.
Some of the highest estimated CRS rates were in Nigeria (130 per 100,000 live births), Chad (105), Ethiopia (99), Guinea-Bissau (96), and Somalia (94). These numbers reflect what happens when a highly teratogenic virus circulates freely in populations without vaccine protection.
How Rubella Is Detected in Pregnancy
When a pregnant person is exposed to rubella or develops a rash illness, blood tests can determine whether infection has occurred. The key tests look for two types of antibodies. IgM antibodies appear shortly after a new infection and are detectable between 4 and 30 days after rash onset, though only about 50% of cases test positive on the day the rash first appears. Waiting five days after symptoms begin gives the most reliable result, with over 90% of cases testing positive at that point.
IgG antibodies show up by about day four and peak within one to two weeks. A single positive IgG result confirms immunity but doesn’t tell you when the infection happened. To determine timing, doctors can test IgG avidity, which measures how tightly the antibodies bind to the virus. Low-avidity IgG suggests infection within the past four months, while high-avidity IgG points to an older infection or prior vaccination. This distinction is especially important in early pregnancy, when determining whether infection occurred during the highest-risk window can guide decision-making.
Prevention Before and After Pregnancy
The MMR vaccine is the only reliable way to prevent rubella and CRS. It is contraindicated during pregnancy because it contains a live (though weakened) virus. Anyone planning a pregnancy should confirm their rubella immunity beforehand, ideally through a blood test showing positive IgG antibodies. If vaccination is needed, the current CDC guidance is to wait at least 28 days after receiving the MMR vaccine before attempting to conceive.
If a pregnant person is found to be susceptible to rubella, vaccination should happen as soon as possible after delivery. In the meantime, avoiding settings where rubella exposure is possible is the primary protective strategy. The CDC recommends that susceptible pregnant individuals stay away from potential exposure for six weeks (two incubation periods) after the last known case in their area.
Importantly, immunoglobulin injections are not recommended as routine post-exposure prevention. While they may reduce the severity of symptoms, they do not prevent the virus from entering the bloodstream or crossing the placenta. The CDC notes that immunoglobulin might modify symptoms and create an unwarranted sense of security without actually protecting the fetus.