Immunizations during pregnancy
by Carrie Koval, MS, September 1998

It is recommended that women not conceive a pregnancy within three months of an MMR or varicella vaccine. Unfortunately, there are many cases in which women are vaccinated, unaware they are already pregnant. In these cases, what are the risks, if any, to the developing fetus?

It has been well established that contracting natural measles, mumps, or rubella during pregnancy poses certain risks. Measles and mumps infections may increase the risk of premature labor, stillbirth, or perinatal death. Mumps contracted in the first trimester may also increase the risk of ear, eye, and urogenital anomalies in the fetus. If a woman contracts rubella in the first trimester of pregnancy, the risk of the fetus developing congenital rubella syndrome (CRS) is relatively high. CRS is characterized by heart defects, cataracts, deafness, postnatal growth retardation, and developmental delay, among other anomalies. When maternal rubella infection is during the first 12 weeks of gestation, the risk for CRS in the fetus is reportedly up to 80%. This risk drops to about 50% if the maternal rubella infection is between 12 and 16 weeks gestation. Generally, the later in gestation a rubella infection occurs, the smaller the risk of CRS in the fetus. Deafness is the most common abnormality seen if rubella is contracted during the third trimester.

The MMR vaccine contains live attenuated virus. Reports to the Center for Disease Control of MMR vaccines used before or during pregnancy fail to show an increased risk for congenital defects. Data collected in the United States, Germany, and the United Kingdom demonstrate an observed risk of congenital defects, including CRS, of zero following immunization with the MMR vaccine. Over a period of ten years, the manufacturer of the MMR vaccine has compiled information on over 700 women who were immunized shortly before or during pregnancy. None of the newborns examined had findings of CRS or congenital defects related to the MMR vaccine.

There have been cases of live virus being isolated from placental tissue following vaccination with the MMR vaccine during pregnancy. It should therefore be noted that a theoretical risk as high as 2% for congenital defects, including CRS, still exists when the vaccine is given shortly before or during pregnancy. Based on the theoretical risk for congenital defects, the American College of Obstetricians and Gynecologists considers the vaccine contraindicated during pregnancy.

Maternal varicella infection during pregnancy has also proven to be teratogenic. The risk for congenital varicella syndrome in the fetus depends on when in pregnancy the maternal infection occurred. Several studies have indicated that the risk for congenital varicella syndrome with maternal infection during the first 12 weeks of gestation is 0.4-2%. If maternal infection is between 13 and 20 weeks gestation, the risk is estimated to be 2-3%. Maternal varicella infection after 20 weeks gestation is associated with a <1% risk for congenital varicella syndrome, although an infection close to delivery may manifest as varicella infection in the newborn. Congenital varicella syndrome is characterized by skin scarring, eye abnormalities, growth retardation, microcephaly, limb malformations, and developmental delay among other abnormalities.

As with the MMR vaccine, the varicella vaccine is contraindicated during pregnancy and women are advised to wait three months to conceive after immunization. The manufacturer of the varicella vaccine, Merck, has developed a registry for women who received the varicella vaccine three months prior to pregnancy, or anytime during pregnancy. If you have a patient who you would like to add to this registry, you can call the Pregnancy Registry for VARIVAX at 1-800-986-8999. Written reports can be sent to: Merck Research Laboratories, Worldwide Product Safety & Epidemiology, BLA-31, West Point, PA 19486. Annual reports of the status of the registry will be sent to all health care participants. To date, reports have been received on 251 pregnant women. Data compiled from the first 100 cases show no cases of congenital varicella syndrome when the VARIVAX vaccine was given 3 months before or anytime during pregnancy. There did not appear to be an increased risk for other congenital defects. Conclusions about the risk of the vaccine cannot be drawn until more data is collected.

If a patient has had an MMR or varicella vaccine during pregnancy, she should be counseled about possible risks to the fetus. The actual risk observed in several studies, however, should offer some reassurance. When dealing with this or any teratogenic exposure, it is important to remind the patient of the background risk for congenital defects of about 3% in all women. A careful ultrasound examination may also be helpful to rule out congenital defects for which the fetus may be at risk.