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Is that possible that ovulation happens in the time of actual menstrual haemorrhage in human females - let say in a case that woman has 21 day cycle and her menstrual period is 7 days?

If yes, does it means that in this particular case the woman has lower chances to conceive?

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Ovulation and menstruation don’t happen in normally cycling women at the same time. A basic outline of the hormonal cycle that triggers these events will make this clear.

Proliferative (a.k.a follicular*) phase
Beginning after menses (when the endometrium is thinned), the hypothalamus produces GnRH which stimulates the anterior pituitary to produce LH and FSH. These in turn stimulate ovarian follicles to develop. A dominant follicle produces estradiol which causes the endometrium to thicken (proliferate).

At a certain level of estrogen (actually estrogen/progesterone ratio), the feedback on the hypothalamus flips from a negative to a positive feedback loop. Thus, there is a GnRH followed by a LH surge. The latter triggers ovulation. Note that at this time the endometrium is stable due to relatively high estrogen levels.

Secretory (a.k.a. luteal*) phase
After ovulation, the high levels of LH trigger formation of a corpus luteum from the tissues left behind after ovulation. The corpus luteum makes progesterone. This hormone triggers a change in the endometrium from a proliferative to a secretory state. Progesterone also provides negative feedback to the hypothalamus and anterior pituitary, maintaining low levels of GnRH, LH, and FSH, so no new dominant follicles develop at this time.

If pregnancy does not occur, the corpus luteum will eventually (10-12 days) degenerate and stop producing progesterone. It is this abrupt drop in progesterone that triggers the sloughing of the endometrial lining. You can see these hormone shifts in an illustration like this:

menstrual hormones

The decline of the corpus luteum is correlated with a decline in serum levels of ovarian hormones including progesterone, estradiol, and inhibin A. Release from negative feedback provided by these hormones at the level of the hypothalamus and pituitary permits FSH to rise, and the cycle begins again.

You should now be able to see that:

  • Around the time of ovulation, the uterine lining is not fully developed and is stable due to the hormonal milieu. Menstruation does not occur.
  • Around the time of menstruation, FSH and LH are suppressed in a way that is not conducive to ovulation.

In theory, yes, of course there would be a lower chance of initiating a viable pregnancy (implantation rather than conception is the most obvious problem) were the endometrial lining to be unstable at the time of ovulation. The problem of luteal phase deficiency is along these lines. In this condition, the corpus luteum does not produce adequate progesterone during the luteal phase to develop the endometrial lining in such a way as to support a healthy pregnancy. However, ovulation and menstruation are still time-separated events for the reasons outlined above.

*Note that the first term is with respect to the endometrium; the second is with respect to the ovary.

Abbreviations:
GnRH - Gonadotropin Releasing Hormone; LH - Luteinizing Hormone; FSH - Follicule Stimulating Hormone

References
1. Anatomy & Physiology, Connexions Web site. Illustration is also from here.
2. Jerome Strauss, Robert Barbieri. Yen & Jaffe's Reproductive Endocrinology. September, 2013. Saunders.

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  • $\begingroup$ Thanks for the answer, I don't understand why one can clearly say that actual hemorrhage is time separated from hormonal activity of the ovary and hypophysis - what feedback do provide endometrium itself to be independent in any way from the system? If it is not the "last" organ in the chain we should know how it itself influences changes in the chain... $\endgroup$
    – Ilan
    Oct 3, 2014 at 6:12
  • $\begingroup$ I took a look at the google search from the comment just deleted. It seems to agree with this conclusion. Quoting the two sources on the front page I see that directly address this, "women who have regular menstrual cycles typically will not ovulate during their period." and "It is impossible to ovulate while having a period." I tried to provide some background on why that is so. $\endgroup$
    – Susan
    Oct 3, 2014 at 6:37
  • $\begingroup$ I added a paragraph to try to better elucidate my summary of the feedback mechanism (which is extremely complex, beyond my top-of-the-head knowledge, and way too voluminous to include here in full detail). To my knowledge, it is most at the level of the ovary-->hypothalamus/pituitary rather than the uterus/endometrium. Women after a hysterectomy may still cycle regularly. $\endgroup$
    – Susan
    Oct 3, 2014 at 6:38
  • $\begingroup$ So, we have a case of a woman with short cycle: let say 21 days + long periods ie 7days. From this we know that her ovulation is 14 before the ovulation (it's well known "fact"), thus her ovulation can come on the day 7 the last of her periods when the endometrium did not ready to implantation. So, this particular women probably has lower chances to become successfully pregnant that women with longer cycle and shorter periods. $\endgroup$
    – Ilan
    Oct 3, 2014 at 6:42
  • $\begingroup$ Perhaps. Remember, though, that ovulation and implantation are time-separated events. If conception occurs ~24h after ovulation and the embryo implants another ~3-4 days later, so there is some window for the progesterone from the corpus luteum to act on the endometrium. Also, a 21 day cycle in a normally cycling woman is unusual, and it would be even more unusual for it to be associated with a long menses since the rising sex hormones prior to ovulation will stabilize the endometrium. $\endgroup$
    – Susan
    Oct 3, 2014 at 6:49

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