**Core Concept**
The corpus luteum is a temporary endocrine structure in the female ovaries that plays a crucial role in maintaining the early stages of pregnancy. It secretes progesterone, a hormone essential for preparing the uterus for implantation of the fertilized egg and maintaining pregnancy.

**Why the Correct Answer is Right**
The corpus luteum is maintained by human chorionic gonadotropin (hCG) produced by the placenta after implantation. hCG stimulates the corpus luteum to continue producing progesterone, which is essential for the growth and development of the embryo during the first trimester of pregnancy. This hormonal feedback loop ensures the continued production of progesterone and maintains the corpus luteum until the placenta takes over its function around 8-10 weeks of gestation.

**Why Each Wrong Option is Incorrect**
**Option A:** Progesterone itself does not maintain the corpus luteum, but rather it is a result of the corpus luteum's activity.
**Option B:** Estrogen is not directly involved in maintaining the corpus luteum in pregnancy; its role is more related to the growth and differentiation of the reproductive tract.
**Option C:** FSH (Follicle-Stimulating Hormone) is essential for follicular growth and maturation but not for maintaining the corpus luteum.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hCG is produced by the placenta and is detectable in maternal serum around 6-8 weeks of gestation, which is why it's used as a marker for pregnancy in early stages.

**Correct Answer:** hCG. Human chorionic gonadotropin.