NEW YORK (Reuters Health) - Despite the fact that pregnant women are advised to opt out of the smallpox vaccine, the U.S. Centers for Disease Control and Prevention (CDC) said Thursday that 103 women have either received the vaccine while pregnant or conceived shortly thereafter.

In all cases, women received the vaccine when they did not know they were pregnant, conceived within 28 days after being vaccinated, or were in close contact with a person who had been recently vaccinated.

However, since the program began, almost 70,000 women of reproductive age have received the vaccine. Consequently, the number of accidentally exposed pregnancies is much lower than would be expected if there were no attempts to screen and educate women about the risks of vaccine exposure in pregnancy, the report indicates.

"I think this clearly shows we've got a very effective screening program and education program," CDC spokesperson Glen Nowak told Reuters Health.

A developing fetus exposed to the smallpox vaccine is at risk of fetal vaccina, an infection that affects internal organs and causes skin lesions, and can cause premature birth and death.

Only around 50 cases of fetal vaccina have ever been reported globally, three of which occurred in the U.S. Those cases were reported in 1924, 1959 and 1968.

Nowak noted that the CDC has received two reports of miscarriage among the six non-military women exposed to the vaccine in pregnancy. He added that miscarriage occurs in between 16 and 31 percent of pregnancies in general, so it was unclear whether the vaccine was responsible.

"We don't have any reason to believe that the vaccine had a causal relationship" to the miscarriages, Nowak said.

He added that the CDC is continuing to monitor the other pregnant women, most of whom have not yet delivered.

He explained that the CDC recommends that vaccine administrators offer pregnancy tests to any woman of reproductive age who is not sure whether she's pregnant. In addition, women receive educational materials that "clearly state that women who are pregnant should not get the smallpox vaccine," and should wait 28 days after vaccination before conceiving.

However, no pregnancy test can pick up a pregnancy that is less than two weeks along, and in some cases, accidentally exposed pregnant women may have received a negative pregnancy test before being vaccinated, Nowak said.

And given that vaccine administrators cannot prevent women from becoming pregnant soon after being vaccinated, he noted that it might be impossible to prevent all accidental vaccine exposures in pregnant women.

The rate of exposure in pregnancy "may very well be as low as we can make it," Nowak said.

He added that the CDC is working to lower the rate further by enforcing the message that women should wait 28 days after vaccination before becoming pregnant.