Actually just found this info which has been helpful:

• corticosteroids vary with respect to their ability to cross the placenta; dexamethasone and betamethasone cross the placenta readily while 88% of prednisolone is inactivated as it crosses the placenta
• use of corticosteroids for prolonged periods or repeatedly in pregnancy is associated with intra-uterine growth retardation (IUGR) in the foetus
• no evidence of an increased incidence of IUGR following short-term treatment, such as prophylactic treament for neonatal respiratory distress syndrome
• no convincing evidence that systemic corticosteroids cause an increased risk of congenital abnormalities such as cleft lip or palate
• prednisolone is excreted in small amounts into breast milk. However, systemic effects in the infant are unlikely with doses of up to 40mg of prednisolone or equivalent; infants should be monitored for adrenal suppression if the mothers are taking a higher dose

Reference:

1. Current Problems in Pharmacovigilance (1998), 24, 9.