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A crucial pathway in the pathogenesis of pestivirus infection is the transplacental infection of the foetus. It occurs with all three viruses (BVDV, BDV & CSFV). Cattle that are not immune to BVDV are susceptible. About 40-50% of heifers, at the time of their first insemination, are seronegative and therefore vulnerable. Acute infection of dams in early pregnancy will usually result in foetal infection. If the dam is PI, then there is 100% vertical transmission from PI dams to their foetuses. The outcome of foetal infection depends on the age of the foetus. Infection in the first trimester (up to 110 days) results in either abortion or the birth of persistently infected calves. During the second trimester, infection can cause congenital damage of which the CNS and musculo-skeletal lesions are particularly noticeable (eg hypocerebellar atrophy). The foetus is sufficiently developed to withstand an infection during the last trimester and will be born actively immune to BVDV.