Here is a little info from the Ashton manual....
Adverse effects in pregnancy. Benzodiazepines cross the placenta, and if taken regularly by the mother in late pregnancy, even in therapeutic doses, can cause neonatal complications. The foetus and neonate metabolise benzodiazepines very slowly, and appreciable concentrations may persist in the infant up to two weeks after birth, resulting in the "floppy infant syndrome" of lax muscles, oversedation, and failure to suckle. Withdrawal symptoms may develop after about two weeks with hyperexcitability, high-pitched crying and feeding difficulties.
Benzodiazepines in therapeutic doses appear to carry little risk of causing major congenital malformations. However, chronic maternal use may impair foetal intrauterine growth and retard brain development. There is increasing concern that such children in later life may be prone to attention deficit disorder, hyperactivity, learning difficulties, and a spectrum of autistic disorders