The fractional absorption of calcium (FA-Ca) was measured using a dual non-radioactive Ca isotope technique in 26 control women, 49 women in the last trimester (36 weeks) of pregnancy and 31 of these women in established (20 weeks) lactation. The ratio of the two non-radioactive Ca isotopes was measured, by high precision thermal ionisation mass spectrometry, in urine 12-24 hours after administration and was used to calculate FA-Ca. This is the first study to clearly show that FA-Ca is significantly elevated in late pregnancy but not in established lactation, when compared with control women.