**Core Concept:**
Estrogen is a group of steroid hormones that play a crucial role in maintaining the female reproductive system, regulating menstrual cycles, and promoting the growth and maintenance of bone tissue. The production and secretion of estrogen are influenced by the hypothalamic-pituitary-gonadal (HPG) axis.

**Why the Correct Answer is Right:**
Estrogen is mainly produced in the ovaries during the follicular phase of the menstrual cycle. As menstruation ends, the corpus luteum, a temporary structure formed from the ruptured follicle, starts to secrete large amounts of estrogen and progesterone to prepare the uterine lining for potential implantation. This rapid rise in estrogen levels in the blood is due to the corpus luteum's secretion.

**Why Each Wrong Option is Incorrect:**
A. Ovarian estrogen production remains active during menstruation, not ending with it.
B. The placenta is a source of estrogen during pregnancy, not menstruation.
C. Testosterone is the primary hormone responsible for the development of female secondary sexual characteristics and not estrogen, which is produced in the ovaries.
D. Thymus gland is not a source of estrogen; it produces thymosin and other proteins involved in immune system development.

**Clinical Pearl / High-Yield Fact:**
Understanding the hormonal changes during menstruation and the role of the corpus luteum is crucial for students studying human physiology, gynecology, and obstetrics. The sudden rise in estrogen levels can lead to symptoms such as nausea, vomiting, and breast tenderness, which are often reported by women during the early stages of pregnancy, but also indicate an ectopic pregnancy or hormonal imbalance. It is essential for medical students to differentiate between these conditions based on clinical presentation and other diagnostic tools.