After ascertaining that intrauterine growth and maturation continue when pregnancy is prolonged with progesterone in Sprague‐Dawley rats this technique was combined with the teratogens chlorcyclizine, vitamin A, and β‐aminopropionitrile (BAPN), to study the effects of 2 to 3 extra days of intrauterine growth on various abnormalities. Females were killed on day 21 or, if treated with progesterone, on day 24 of gestation. Untreated animals served as controls. In the day‐24 chlorcyclizine group, edema and hydronephrosis were either as severe as or more marked than in the day‐21 group and the testes remained near the kidneys rather than descending deep into the pelvis. Vertebral calcification, usually not present on day 21, had occurred by day 24. These findings suggest that edema, hydronephrosis, and undescended testes were probably permanent defects, whereas lack of calcification was due to delay in occurrence. The smaller dose of vitamin A produced facial abnormalities ranging from marked underdevelopment of the mandible to relative prognathism in day‐21 fetuses but mostly prognathism in day‐24 fetuses; whereas the higher dose produced severe underdevelopment of the jaws and decrease of skeletal calcification in young of both age groups. Severe limb malformations, noted in day‐21 fetuses after high doses of vitamin A, were not grossly altered by prolonging pregnancy. BAPN‐induced limb malformations and gibbus deformities noted on day 21 were still present on day 24. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company
