Hormone with no change in levels in menstrual cycle?

Ans. is 'a' i.e., Activin Hormonal regulation of the menstrual cycleMenstrual cycle involves the interplay of ovarian, pituitary and hypothalamic hormones.At the beginning of the cycle, the estrogen level is very low and progesterone level is negligible.Therefore, the pituitary and hypothalamus are free from the negative feedback effect of estrogen and progesterone.Hence the hypothalamus secretes GnRH and the anterior pituitary secretes FSH and some LH.FSH stimulates follicular development.Ovarian follicles secrete estrogen.Estrogen tends to suppress FSH secretion, but progressive follicular development ensures a rise in estrogen level.Around day 11 of the cycle, the estrogen level crosses a ceain threshold.Above this threshold level. estrogen has a positive feedback effect on LH secretion, and to some extent also on FSH secretion.This leads to LH and FSH surge.The LH surge is essential for ovulation.After ovulation has taken place on day 14, the corpus luteum stas developing under the influence of LH.Corpus luteum secretes estrogen, progesterone and inhibin.The combined effect of these hormones is to suppress the secretion of FSH and LH by negative feedback effect on the anterior pituitary, and to a lesser extent also on the hypothalamic GnRH secretion.Absence of suppo from FSH and LH leads to regression of the corpus luteum.Regression of the corpus luteum leads to a sharp decline in estrogen and progesterone levels.Withdrawal of suppo from estrogen and progesterone leads to menstrual bleeding.Thus one cycle ends and the next cycle begins.