Asymptomatic cases are especially problematic for anyone who's pregnant. Rubella virus can be passed from mother to fetus and cause congenital rubella syndrome, a condition which results in birth defects like heart problems, intellectual disabilities, hearing and eyesight loss, and liver or spleen damage. The mother is much more likely to have a miscarriage if she's infected early in her pregnancy, or to have a stillbirth. It's for this reason that women who are planning to get pregnant and haven't had the MMR (measles, mumps, rubella) vaccine yet get the shot—and why they then wait at least 30 days before trying to get pregnant. Even the rubella virus in the vaccine can cause harm to a developing fetus. That also means that if there's some kind of outbreak of rubella, a pregnant person can't get vaccinated to protect herself—she's just at the mercy of herd immunity, the phenomenon where high rates of vaccinated individuals protect those in the population who can't be vaccinated from disease.