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How does passive immunity pass from a mother to a fetus?

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Infographic showing estimated lives saved by childhood vaccinations worldwide from 1974 to 2024, with syringe-shaped bars for each disease. Measles tops the list at 93.71 million lives saved, followed by tetanus, whooping cough, and others, for a total of 153.84 million lives saved. — vaccine Vaccines and the estimates of lives saved over the period 1974–2024.

Passive immunity is transferred from a mother to her fetus primarily through the placenta, which allows for the passage of maternal antibodies into the fetal bloodstream. In humans, this process specifically involves the transport of maternal IgG antibodies throughout the final two-thirds of pregnancy.12

This transfer provides the newborn with temporary protection against pathogens to which the mother has developed immunity. Because these antibodies are not produced by the infant's own immune system, they are eventually broken down and eliminated, leaving the child susceptible to infection once the maternal supply wanes.12

It is important to note that while physicians may recommend certain immunizations for pregnant women to boost this protective transfer, active immunization with certain live viral vaccines must be avoided during pregnancy, as the agent itself can cross the placenta and potentially cause severe fetal complications.1