Vertical transmission HBV cannot cross the placenta. However, hepatitis B can be transmitted from a pregnant carrier mother to her fetus when invasive procedures such as amniocentesis are carried out. However, it has some implications for prevention strategies since babies who are infected prior to birth cannot be protected by vaccination. More commonly, hepatitis B can be transmitted from a carrier mother to her child when the newborn infant comes into contact with its mother's infected blood during delivery for example when the umbilical cord is cut. It may also occur shortly after delivery, for example, through minor scratches during breast feeding.
Perinatal transmission is particularly important in high endemicity areas.
It is estimated that about half of all the chronic HBV carriers in South East Asia were infected by this route. The mechanism of this mode of HBV transmission is not fully understood, but it is thought that the vehicle is small amounts of infected blood or saliva coming into contact with open wounds. These may just be minor scratches or abrasions on the skin or mucosal surfaces. Infection may be spread by direct physical contact, for example during play or sports activities, or through activities such as sharing toothbrushes or razors. Horizontal transmission accounts for a significant proportion of hepatitis B infections among children, adolescents and young adults in areas of intermediate to high endemicity.