The prevalence of obesity is increasing, and the origins of obesity and metabolic dysfunction may be traced back to fetal life. Currently, overweight pregnant women are advised to substitute sugar-sweetened beverages with diet drinks containing artificial sweeteners. Recent evidence suggests that the consumption of artificial sweeteners during pregnancy increases the risk of obesity in the child, but the mechanism is unknown. We hypothesized the transportation of artificial sweeteners across the placenta into the fetal circulation and the amniotic fluid. We included 19 pregnant women who were given an oral dose of acesulfame, cyclamate, saccharin, and sucralose immediately before a planned caesarean section. Nine women were included as controls, and they refrained from an intake of artificial sweeteners. The maternal and fetal blood and amniotic fluid were collected during the caesarean section, and concentrations of artificial sweeteners were measured using mass spectrometry. We found a linear relationship between the fetal plasma concentrations of artificial sweeteners and the maternal plasma concentrations, with adjusted coefficients of 0.49 (95% CI: 0.28–0.70) for acesulfame, 0.72 (95% CI: 0.48–0.95) for cyclamate, 0.51 (95% CI: 0.38–0.67) for saccharin, and 0.44 (95% CI: 0.33–0.55) for sucralose. We found no linear relationship between amniotic fluid and fetal plasma concentrations, but there were positive ratios for all four sweeteners. In conclusion, the four sweeteners investigated all crossed the placenta and were present in the fetal circulation and amniotic fluid.