The rubella virus is a potent teratogen, embryotoxin and fetotoxin. While the conceptus may be infected at any stage of gestation, the risk of consequent fetal damage is dependent on the period of pregnancy at which infection occurs. The "older" the fetus at the time of infection, the lower the incidence and severity of congenital anomalies.
At least 90% of infants born to women infected with rubella before 11 weeks of pregnancy will have congenital heart disease, be deaf, or both. The incidence of congenital anomalies drops sharply for infection during weeks 11 to 12, and is virtually nil after 20 weeks of pregnancy.
Congenital rubella is a notifiable disease, and while the number of miscarriages due to rubella infection is not known, the incidence of congenital rubella over the last decade in Australia has been determined at 1-2 cases in every 10,000 live births.
Maternal infection with rubella is diagnosed on conventional serological grounds by detection of specific IgM antibodies in maternal serum or by seeing appropriate changes in the levels of IgG. Since the ramifications of fetal infection with rubella are gestational age dependent, management of the pregnancy complicated by rubella is directed by the gestational age of the fetus at the time of infection. The high incidence of congenital heart disease and/or deafness in the infants born to women infected with rubella before 12 weeks gestation warrants the offer of elective termination of pregnancy.
Legally, termination of pregnancy remains an option for women up to 20 weeks gestation, although the medical indication for termination on the grounds of maternal infection with rubella diminishes rapidly after 13 weeks gestation. Women whose pregnancies are complicated by rubella infection after 20 weeks gestation can be given strong reassurance regarding the lack of any fetotoxic or teratogenic effects due to the virus.
The difficulty in managing pregnancy complicated by infection with rubella in the first and early second trimesters is in determining which fetuses are going to be significantly damaged by vertical transmission of the virus. It is not useful clinically to sample fetal tissues or blood to look for the rubella virus, as this will not indicate damage which may be associated with handicap, such as whether the infant with be deaf or blind.
Sonographic assessment of the fetus is sometimes useful. Fetal echocardiography will identify up to 60% of fetuses with a structural abnormality of the heart. Serial sonographic assessments will permit determination of fetal growth velocity and identify those fetuses with retardation of intra-uterine growth. Hydrops fetalis following maternal infection with rubella is also identifiable using ultrasound examination. This information is useful in counselling and preparing for birth, as cardiac abnormalities and growth retardation are both indicative of serious fetal infection and herald a poor prognosis.
Hydrops fetalis due to rubella infection is universally fatal and will result in either stillbirth or neonatal death. Unfortunately, however, many of rubella's effects on the developing fetus, including cataracts, glaucoma, retinopathy, microphthalmia, deafness, meningo-encephalitis and hypotonia are not detectable antenatally using currently available techniques. Furthermore, childhood associations of congenital rubella, including behavioural disturbances, mental retardation, thyroid abnormalities, progressive rubella encephalopathy and diabetes mellitus, may not be accompanied by any identifiable antenatal stigmata.
Therefore, in continuing pregnancy complicated by rubella infection in the first and early second trimesters, the effects of the virus on the developing fetus, if any, may only be apparent at the time of birth or on the basis of serial childhood neurodevelopmental assessments.
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