If a mother with herpes transfers antibodies to the fetus in utero, how long do the antibodies last in fetus for protection after birth?

Roughly 6 m: When maternal antibodies are present at birth, the levels start to decline rather quickly and are usually not detectable by 6 months. But remember, they are already declining the entire time after birth, so exposure at 3 or 4 months may lead to infection.

May not protect: Maternal genital herpes is a problem.Periodic flares can result in viral sheading and if this happens during delivery the virus can pass to the newborn.A neonatal encephalitis may occur within weeks with devistating consequences.Some will plan a c-section delivery prior to labor to avoid this possibility.A thorough discussion of the issue with the OB is worth having.