Question

Why can a woman not get pregnant again during pregnancy?
A. A woman ovulates during pregnancy, but the oviducts are plugged with protective mucus to prevent sperm from entering.
B. High levels of hCG in women's bodies kill sperms.
C. A woman cannot have intercourse during pregnancy.
D. High levels of estrogen and progesterone, secreted by the corpus luteum or placenta during pregnancy, inhibit the secretion of gonadotropins, and prevent ovulation.

Answer 1

Progesterone and estrogen are the female reproductive hormones and are required to maintain the pregnancy. These two hormones, specifically progesterone, exert negative feedback on the secretion of GnRH and gonadotropins from the hypothalamus and anterior pituitary respectively.

Complete answer:
To answer this question, we should know the events that occur in the ovaries and uterus during the menstrual cycle and pregnancy.
To start, let's quickly recall the events that occur during the menstrual cycle. FSH and LH together called gonadotropins, trigger the maturation of follicles and ovulation respectively.
The most important point is that ovulation is required for the fertilization as it releases the secondary oocyte from the ovaries. The secondary oocyte is then propelled into the uterine duct where fertilization occurs. If ovulation does not occur, secondary oocytes would not be available for fertilization and pregnancy would not be established.
LH also stimulates the transformation of the ruptured follicle, after ovulation, into the corpus luteum. The corpus luteum is a yellow colored structure that serves as a temporary endocrine gland and secretes progesterone and estrogen. However, it secretes a relatively copious amount of progesterone.
Now, what is the effect of progesterone? During the secretory phase of the uterine cycle, progesterone triggers the proliferation and vascularization of the uterine endometrium. The endometrium is the layer of the uterine wall into which the embryo is implanted after fertilization and leads to the pregnancy. Therefore, progesterone prepares the endometrium for the pregnancy.
Here, one should remember that if progesterone levels fall, as it occurs in the absence of fertilization due to the disintegration of the corpus luteum, the hypothalamus would start secreting GnRH which in turn would trigger the anterior pituitary to secrete gonadotropins. Gonadotropins would begin the new ovarian cycle in the ovaries and the uterus would experience menses. However, if fertilization has occurred, the ovarian cycle and menses should be prevented so that the implanted embryo is not lost with the menstrual flow.
This is achieved by maintaining the higher levels of progesterone throughout the pregnancy. Progesterone prevents the hypothalamus to secrete GnRH and in the absence of stimulation from the GnRH, the secretion of gonadotropins from the anterior pituitary is inhibited. The overall result is no follicle development, no ovulation, and no menses. The corpus luteum is the source of progesterone during the first three months of the pregnancy after which the placenta serves the same function. To maintain the corpus luteum during the first trimester, the developing embryo secretes hCG (human chorionic gonadotropin) which in turn does not allow the degradation of corpus luteum.

So, the correct answer is D. High levels of estrogen and progesterone, secreted by the corpus luteum or placenta during pregnancy, inhibit the secretion of gonadotropins, and prevent ovulation.

Note: There are many things that a student needs to know to understand this question and the given options. First, GnRH is a gonadotropin-releasing hormone. It triggers the anterior pituitary to secrete FSH and LH. FSH and LH are together called gonadotropins since they target gonads (testes and ovaries) in both males and females. Estrogen alone exerts positive feedback on the secretion of FSH and LH and causes LH surge by the mid-follicular phase. However, the higher levels of progesterone and estrogen together inhibit the secretion of these hormones.