Fig. 1. Sketch of fetal blood levels estimated from cord blood.⁴⁵.

These curves represent predicted fetal plasma after maternal treatments with the 3 mg betamethasone phosphate (Beta P) plus 3 mg betamethasone acetate (Beta Ac) mixture given 24 h apart (red curve), 3 mg dexamethasone phosphate (Dex P) given every 12 h (orange curve), and 3 mg Beta Ac component (blue curve) based on measurements in a sheep model with the characteristics of drug levels that we will discuss. The light gray area on the figure is our estimate of a good target for fetal plasma levels for lung maturation. Issues for dosing are: 1. The range for minimal dose for fetal lung response. 2. Need for high peak exposure. 3. Nature of exposure—duration and continuous for a durable response. Redrawn from Ballard and Ballard.⁴⁵