## **Core Concept**
The hormone responsible for maintaining the corpus luteum during early pregnancy is crucial for sustaining the pregnancy until the placenta takes over. This hormone supports the corpus luteum in producing **progesterone**, essential for pregnancy. The key player here is a hormone produced by the trophoblastic cells.

## **Why the Correct Answer is Right**
The correct answer, **human chorionic gonadotropin (hCG)**, is produced by the trophoblastic cells of the embryo. It plays a critical role in maintaining the **corpus luteum** by binding to the **luteinizing hormone (LH) receptors** on the corpus luteum, thereby stimulating it to continue producing **progesterone**. This action is essential during the early stages of pregnancy before the placenta develops enough to take over the production of necessary hormones.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **estrogen** levels do increase during pregnancy and are crucial for fetal development and maternal health, estrogen itself does not directly maintain the corpus luteum.
- **Option B:** **Progesterone** is the hormone produced by the corpus luteum (and later by the placenta) that is essential for maintaining pregnancy, but it is not produced by the trophoblastic cells to maintain the corpus luteum.
- **Option D:** **Relaxin** is a hormone involved in relaxing the pelvic girdle and softening the cervix during pregnancy, but it does not play a role in maintaining the corpus luteum.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that **hCG levels** are often used as a marker for pregnancy and can be detected in blood and urine. The level of hCG typically doubles approximately every 48 hours during the first trimester, which is an indicator of a healthy pregnancy.

## **Correct Answer:** .