Final answer:
During the menstrual cycle, estrogen causes water retention by mimicking the action of aldosterone, which leads to the conservation of water and sodium.
Step-by-step explanation:
Women have a tendency to retain water during part of the menstrual cycle because estrogen mimics the action of aldosterone. Progesterone, which is also involved in the menstrual cycle, can bind to aldosterone receptors, leading to the retention of water and sodium in the body. This mechanism is part of the complex hormonal interactions that occur during the menstrual cycle, with aldosterone playing a key role in regulating the balance of fluids in the body.