The objectives of this study were to determine the profiles of plasma estrogen concentrations during the first trimester of gestation and to correlate them with the origin of the embryos and fetal number. Pregnancies were induced either by artificial insemination (AI) on the day of estrus (day 0) or by transfer of 1 or 2 blastocysts produced in vitro (IVF-ET) on day 7. Five singleton-bearing cows impregnated by AI, and 6 singleton and 6 twin pregnancies induced by IVF-ET were used in the experiment. Blood was collected by jugular venipuncture at 2 to 5-day intervals until day 100. The concentrations of plasma estrone, estradiol-17β and estrone sulfate were measured by using a radioimmunoassay. Concentrations of all the estrogens fluctuated at low levels until day 50 in all the groups. Thereafter, the concentration of estrone sulfate gradually increased, whereas those of estrone and estradiol-17β remained at basal levels until day 80. The concentration of estrone sulfate after day 50 was affected by the day of gestation and the number of fetuses. After day 80, the concentration of estrone sulfate increased drastically, coinciding with the increases in the estrone and estradiol-17β concentrations. The rate of increase in the concentration of estrone sulfate during days 80 to 100 was the highest among all the estrogens. In the singleton pregnancies, the concentrations of estrone and estrone sulfate between days 80 and 100 were higher in the artificially inseminated cows than in the IVF-ET cows. In the pregnancies induced by IVF-ET, estrone sulfate levels were higher in the twin-bearing cows than in the singleton-bearing cows during days 50 to 80 and 80 to 100. These results suggest that the concentrations of estrogens in bovine peripheral blood plasma could be applied for monitoring fetoplacental development in the first trimester of gestation.