OBJECTIVE We determined the maternal-fetal transport of immunoglobulin G (IgG) during the third trimester of human pregnancy. METHODS The concentration of IgG and its subclasses (IgG1-4) was determined in sera of blood samples from 38 pregnancies collected at the time of delivery from a peripheral maternal vein (MV) and from the placental umbilical artery (UA) and vein (UV). Gestational age varied between 28 and 42 weeks (WG). RESULTS Whereas placental weight showed a significant correlation with gestational age, the maternal level of IgG and the ratio of its subclasses did not vary with gestational age. At 28-33 WG (n = 15) the mean values in the UA (5.91 +/- 1.53 g/l) and UV (6.41 +/- 1.57 g/l) for total IgG concentration were lower than in the MV (10.74 +/- 2.55 g/l). At the end of gestation (37-42 WG, n = 12), IgG in both UA (11.21 +/- 1.95 g/l) and UV (12.26 +/- 2.06 g/l) exceeded the maternal concentration (9.69 +/- 1.84 g/l). In addition to the significant positive correlation between IgG concentration in the fetal circulation (UV + UA) and gestational age (28-42 WG), an increase in the positive difference between UV and UA at the end of pregnancy indicates that there is a substantial rise in placental IgG transport capacity. CONCLUSIONS All four subclasses IgG1-4 were detectable in the umbilical circulation (28-42 WG). Whereas IgG3 and IgG4 in UA and UV had a similar concentration as in MV and remained unchanged, IgG2 increased with gestation from 0.67/0.74 g/l (UA/UV, 28-33 WG) to 1.29/1.58 g/l (UA/UV, 37-42 WG), but nevertheless remained lower than the level found in the MV (2.65 +/- 1.12 g/l). The main increase in IgG concentration, however, was due to the substantial rise in the transport of IgG1, which increased from 4.37 +/- 1.24 (UA) and 4.94 +/- 1.52 g/l (UV) at 28-33 WG to 8.94 +/- 1.66 (UA) and 10.89 +/- 1.96 g/l (UV) at the end of gestation, which was even higher than the overall IgG concentration in MV.