Serial Physiological Changes During Pregnancy

Abstract

0224 Only a few studies have examined the mechanisms and time sequence of changes in blood biochemistry at rest and during exercise in early human pregnancy. This information is urgently needed to provide guidelines to safely prescribe and monitor physical activity throughout pregnancy. PURPOSE: To accurately describe the time sequence of changes in plasma osmolality, electrolytes, and proteins at rest and during exercise in healthy pregnant women. METHODS: Serial testing was conducted on healthy, moderately-active women (n = 11) between the 7th and 20th weeks of pregnancy. Subjects were studied at rest and during steady state cycling at a work rate corresponding to approximately 60% of aerobic working capacity. Measurements included plasma proteins, electrolytes, the strong ion difference ([SID]), and osmolality. Changes between the 7th and 20th week of gestation were analyzed using a mixed model linear regression procedure. Comparisons were also made with pre-conceptual data from the same subjects and with measurements in late gestation from earlier studies from this laboratory (J Appl Physiol 88: 149, 2000; MSSE 34: S119, 2002). RESULTS: Pregnancy-induced reductions in plasma osmolality, electrolyte concentrations and [SID] occurred within the first 7 weeks of pregnancy. Plasma albumin concentration was reduced by 7 weeks gestation, and continued to decrease gradually between 7 and 20 weeks. CONCLUSIONS: The findings support the hypothesis that primary changes in plasma osmolality, electrolytes, and [SID] are essentially complete by the 7th week. Conversely, changes in maternal blood volume (as reflected by reductions in plasma albumin concentration) continue until late gestation. In accordance with Jenning's hypothesis (Can J Physiol Pharm 10:815, 1998) reductions in plasma osmolality and [SID] may contribute to the establishment of respiratory alkalosis in early pregnancy. Supported by the Ontario Thoracic Society, OTS Block Term Grant Funding and NSERC.