**Core Concept**
During pregnancy, especially in the third trimester and lactation, the body undergoes significant physiological changes that affect glucose metabolism. The placenta produces various hormones, including human placental lactogen (HPL), which stimulates the mother's pancreas to produce more insulin. However, the placenta also produces other hormones, such as cortisol and growth hormone, which can lead to insulin resistance. This results in increased blood glucose levels, and consequently, glucose appears in the urine.

**Why the Correct Answer is Right**
Glucose is normally present in the urine of pregnant women in the third trimester and lactation due to the increased glucose levels in the blood. The kidneys, which normally reabsorb glucose, are overwhelmed by the high glucose levels and are unable to reabsorb all of it, resulting in glucose appearing in the urine. This phenomenon is known as glycosuria. The increased levels of cortisol and growth hormone during pregnancy also contribute to insulin resistance, further exacerbating the problem.

**Why Each Wrong Option is Incorrect**
**Option B:** Lactose is not present in the urine of pregnant women. Lactose is a disaccharide found in milk and other dairy products, and its presence in the urine would indicate a problem with lactose metabolism, such as lactose intolerance.

**Option C:** Galactose is not typically present in the urine of pregnant women. Galactose is a simple sugar that is a component of lactose, but it is not normally found in the urine unless there is a specific problem with galactose metabolism, such as galactosemia.

**Option D:** Fructose is not typically present in the urine of pregnant women. Fructose is a simple sugar that is found in many fruits and vegetables, but its presence in the urine would indicate a problem with fructose metabolism, such as hereditary fructose intolerance.

**Clinical Pearl / High-Yield Fact**
It's essential to note that glycosuria in pregnancy is a normal physiological phenomenon and does not necessarily indicate the presence of diabetes mellitus. However, if glycosuria persists or is accompanied by other symptoms, further evaluation is necessary to rule out gestational diabetes.

**✓ Correct Answer: A. Glucose**