Congenital cystic adenomatoid malformations (CCAMs) are theorized to be growing immature lung tissue. Administration of maternal steroids in the mid-trimester may stop the growth or decrease the size of the CCAM, thus increasing normal lung tissue and improving survival in fetuses with large CCAMs. This is a prospective, blinded, randomized trial comparing administration of a single course of antenatal steroids (Betamethasone) to control (i.e., placebo). The primary outcome variable will be incidence of hydrops. One month postnatal survival and relative size of the CCAM as determined by CCAM volume:head circumference ratio (CVR) between treatment/no treatment groups will be secondary outcome variables. Change in size of CCAM will be serially followed for both groups with individual growth curves being plotted prenatally and these will be compared with pathology weigh and volume to evaluate treatment effect. Other prenatal data collected will include: incidence of polyhydramnios, incidence of premature rupture of membranes, incidence of material complications. We will also compare mode of delivery, postnatal respiratory compromise, need for resection in the first week of life, and occurrence of complications during newborn administration