**Core Concept**
Insulin resistance in pregnancy is a physiological adaptation that occurs due to the increased demand for glucose by the fetus and the placenta. This is mediated by various hormonal changes, including the placental hormones that oppose the action of insulin.

**Why the Correct Answer is Right**
Insulin resistance in pregnancy is primarily caused by the placental hormone human placental lactogen (hPL). hPL is produced by the placenta and has a role in promoting fetal growth by stimulating the mother's pancreas to produce more insulin. However, hPL also has an anti-insulin effect, leading to insulin resistance in the mother's body. This insulin resistance is a normal adaptation to ensure that the mother's body supplies enough glucose to the fetus. Additionally, the placental hormone cortisol also contributes to insulin resistance by stimulating the breakdown of glucose to produce energy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the underlying cause of insulin resistance in pregnancy. While it is true that insulin resistance is a common feature of gestational diabetes, it is not a complete answer to the question.

**Option B:** This option is incorrect because it is a consequence of insulin resistance, not the cause. Hyperglycemia is a result of the body's inability to effectively use insulin, but it is not the underlying reason for insulin resistance in pregnancy.

**Option C:** This option is incorrect because it is a general term that encompasses many causes of insulin resistance, but it does not specifically address the unique physiological changes that occur in pregnancy.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that insulin resistance in pregnancy is a normal adaptation to ensure fetal growth, but it can lead to gestational diabetes if not managed properly. Monitoring glucose levels and maintaining a healthy diet and lifestyle are crucial in preventing and managing gestational diabetes.

**Correct Answer:** D. Human placental lactogen (hPL) and cortisol.