During pregnancy, what happens to electrolytes and osmolality?

• A. Increase
• B. Decrease
• C. No change
• D. Fluctuate

Answer: (B)

During pregnancy there is a substantial expansion of extracellular fluid and plasma volume due to hormonal changes that promote water and sodium retention to support the fetus. This dilution effect lowers the concentration of circulating electrolytes and reduces serum osmolality slightly. In a healthy pregnancy, electrolytes largely stay within normal ranges, but their measured values may appear lower because of the increased total body water. So the best description is a decrease in electrolytes (dilution) and osmolality.