Okay, let's tackle this question. The question is about what maintains the corpus luteum during the first trimester of pregnancy. Hmm, I remember that the corpus luteum is important for producing progesterone, which is needed to support the endometrium during early pregnancy.

Wait, in the menstrual cycle, the corpus luteum is maintained by luteinizing hormone (LH) from the pituitary. But during pregnancy, once the placenta forms, it takes over producing progesterone. But what happens before the placenta is fully functional? Oh right, in the first trimester, the corpus luteum is maintained by human chorionic gonadotropin (hCG) secreted by the developing placenta. hCG has a similar structure to LH and binds to the same receptors, so it can stimulate the corpus luteum to keep producing progesterone until the placenta can take over.

Let me check the options. The correct answer should be hCG. The other options might include LH, estrogen, progesterone, or maybe FSH. LH is the normal maintainer in the menstrual cycle, but during pregnancy, hCG takes over. Estrogen and progesterone are produced by the corpus luteum itself, but they don't maintain it. FSH is more involved in follicular development. So the wrong options would be LH, estrogen, progesterone, and FSH.

Wait, the question says "principally by secretion of," so the answer is hCG. The clinical pearl here is that hCG is crucial in early pregnancy, and its detection is the basis for pregnancy tests. Also, if hCG isn't present, the corpus luteum would degenerate, leading to a drop in progesterone and possible miscarriage. That's why in ectopic pregnancies where hCG production is impaired, there can be complications. The high-yield fact is that hCG replaces LH in maintaining the corpus luteum during early pregnancy. So the correct answer is hCG.

**Core Concept**
The corpus luteum is essential for progesterone production during early pregnancy. In the first trimester, its maintenance shifts from luteinizing hormone (LH) to a placental hormone with LH-like activity.

**Why the Correct Answer is Right**
Human chorionic gonadotropin (hCG), secreted by the syncytiotrophoblast of the placenta, binds to LH receptors on the corpus luteum. This prevents luteolysis and sustains progesterone synthesis until the placenta can produce sufficient progesterone (~12 weeks). hCG mimics LH’s structure and function, ensuring endometrial support for the developing embryo.

**Why Each Wrong Option is Incorrect**
**Option A:** *Luteinizing hormone (LH)* maintains the corpus luteum in the non-pregnant cycle but is replaced by hCG in pregnancy.
**Option B:** *Estrogen* is produced by the corpus luteum but does not sustain it.
**Option D:** *Follicle-stimulating hormone (FSH)* stimulates follicular growth, not luteal maintenance.

**Clinical Pearl / High-Yield Fact**
hCG is the hormone detected in pregnancy tests. Its absence leads to corpus luteum degeneration, prog

✓ Correct Answer: C. Human chorionic gonadotropin (hCG)