Effect of oestrogen on pregnancy of guinea-pigs

Pregnant guinea-pigs were injected with oestrogen at different stages of gestation. After injections on Days 17 and 18 and on Days 25, 26, 32 and 33 peripheral maternal progesterone concentrations were lower than in control animals. Histology showed deleterious changes in the corpora lutea. After injections on Days 10 and 11 the maternal progesterone concentrations remained in the normal range for some time in 12 of 16 guinea-pigs and the corpora lutea appeared histologically normal. Oestrogen therefore affects the pregnancy differently at different stages of gestation.