Certain infections, in pregnant women, put the fetus at risk for birth defects.
Cytomegalovirus is a common herpesvirus which usually causes no symptoms in immunocompetent adults. In pregnant women, however, it can infect the fetus. Congenital cytomegalovirus is the most common cause of birth defects. About 40,000 children a year are born with congenital cytomegalovirus, causing an estimated 400 deaths and 8000 permanently disabled children (due to hearing loss, vision loss, or intellectual disability.) Congenital cytomegalovirus is more common than fetal alcohol syndrome, Down syndrome, or spina bifida. 1-4% of pregnant women get the virus, and of those, 1% pass it to the fetus. My back-of-the-envelope estimates are that the risk of permanent disability given congenital cytomegalovirus is about 40x the baseline risk of birth defects. Cytomegalovirus is most often transferred by contact with young children’s bodily fluids (saliva, urine, etc).
To avoid cytomegalovirus infection, wash hands frequently, especially around contact with small children. There is no vaccine yet, but “passive immunization” with IgG for pregnant women with primary CMV infections cuts the risk of congenital CMV in the child by a factor of 50. If you have a mono-like illness while pregnant, you could get tested for CMV and possibly treat it with IgG.
Toxoplasmosis is a protozoan parasite, usually contracted from raw meat, unpasteurized milk, cat feces, or soil. It causes no symptoms in most immunocompetent adults (though it sometimes causes a flu-like illness), but in pregnant women, can be passed to the fetus. An estimated 400-4000 babies every year are born with toxoplasmosis. While it is transmitted by cats, contact with cats is not a risk factor for congenital toxoplasmosis, while contact with soil or consumption of raw meat are.  In a prospective study of pregnant women who had high antibody titers for the disease, 6.3% passed it on to the fetus; of babies born with toxoplasmosis, 15% had severe disease with cerebral and ocular involvement. Ocular toxoplasmosis infects the retina and causes a loss of visual acuity. Other possible symptoms include prematurity and low birth weight, hearing problems, seizures, and intellectual disability. Up to 80% of children born with congenital toxoplasmosis develop vision or cognitive impairments later in life.
To avoid toxoplasmosis, cook meat thoroughly and avoid contact with soil. There is prenatal treatment for toxoplasmosis but the evidence suggests it doesn’t work , whereas treatment given to newborns successfully prevents developmental abnormalities.
Infants can become infected with herpes at the time of birth if the mother has active lesions in the birth canal. It is rare (about 1000 births a year) but deadly (25% mortality.)
To avoid congenital herpes, get a C-section if you have active genital sores at the time of birth, and by practicing safe sex during pregnancy. (The risk of transmitting herpes is much lower if you have had it before your pregnancy; most cases of neonatal herpes result from a primary infection in the third trimester.)
Rubella, also known as German measles, is a virus which is now rare among vaccinated people. Congenital rubella syndrome is characterized by deafness, blindness, heart defects, and sometimes intellectual disability. The incidence of congenital rubella syndrome in the US is less than 1 a year.
To avoid rubella, get your vaccine before you get pregnant. If your parents followed standard recommendations, you should have gotten this as part of the MMR vaccine when you were 12-15 months and 4-6 years old.
BOPPANA, SURESH B., et al. “Symptomatic congenital cytomegalovirus infection: neonatal morbidity and mortality.” The Pediatric infectious disease journal 11.2 (1992): 93-98.
Nigro, Giovanni, et al. “Passive immunization during pregnancy for congenital cytomegalovirus infection.” New England Journal of Medicine 353.13 (2005): 1350-1362.
Cook, A. J. C., et al. “Sources of toxoplasma infection in pregnant women: European multicentre case-control studyCommentary: Congenital toxoplasmosis—further thought for food.” Bmj 321.7254 (2000): 142-147.
Desmonts, Georges, and Jacques Couvreur. “Congenital toxoplasmosis: a prospective study of 378 pregnancies.” New England Journal of Medicine290.20 (1974): 1110-1116.
Carter AO, Frank JW. Congenital toxoplasmosis: epidemiologic features and control. CMAJ. 1986;135:618–23.
Jones, J. E. F. F. R. E. Y., Adriana Lopez, and Marianna Wilson. “Congenital toxoplasmosis.” American family physician 67.10 (2003): 2131-2146.
Serranti, Daniele, D. A. N. I. L. O. Buonsenso, and P. I. E. R. O. Valentini. “Congenital toxoplasmosis treatment.” Eur Rev Med Pharmacol Sci 15.2 (2011): 193-8.
SYROCOT (Systematic Review on Congenital Toxoplasmosis) study group. “Effectiveness of prenatal treatment for congenital toxoplasmosis: a meta-analysis of individual patients’ data.” The Lancet 369.9556 (2007): 115-122.
McAuley, James, et al. “Early and longitudinal evaluations of treated infants and children and untreated historical patients with congenital toxoplasmosis: the Chicago Collaborative Treatment Trial.” Clinical Infectious Diseases 18.1 (1994): 38-72.
McLeod, Rima, et al. “Outcome of treatment for congenital toxoplasmosis, 1981–2004: the national collaborative Chicago-based, congenital toxoplasmosis study.” Clinical Infectious Diseases 42.10 (2006): 1383-1394.