So let me start by answering the question in the title: menstruation and pregnancy are two completely incompatible events. Those who are pregnant do not menstruate, and those who are menstruating cannot be pregnant. There are no exceptions.
However, it is important to emphasize that when we say that pregnant women cannot get their period, we are not saying that pregnant women can not have episodes of vaginal bleeding during their gestation. The point is that this bleeding can have several causes, but, for sure, menstruation will never be one of them.
If you do not believe that the above statement is correct, and if not only think that it is fully possible for a pregnant women to get their period, but can also cite personal cases in which this situation has occurred, I suggest you read this text attentively because I promise to clarify all your doubts.
In order for you to understand the reasons for the total incompatibility between pregnancy and menstruation, we must first explain what menstruation is and how it occurs.
What is menstruation?
The most superficial layer of the internal wall of the uterus is called endometrium. With each menstrual cycle, the endometrium are prepared to receive a possible embryo. This process
occurs through the proliferation of endometrial cells and the development of new blood vessels, which will be responsible for the supply of fluids, oxygen and nutrients for the fetus. The preparation of the uterus for pregnancy occurs every month and begins on the first day of the menstrual cycle, first being stimulated by the hormone estrogen and later by the hormone progesterone.
If we order an ultrasound in the first days of the menstrual cycle, we will see a thin endometrium, with a single layer less than 0.4 cm (0.15 in) thick, and a paucity of blood vessels. As the days go by, hormonal stimulation causes the endometrial cells to proliferate and new blood vessels to be created. In the final phase of the menstrual cycle, the endometrium becomes thick, with 3 layers that are viscous, rich in blood vessels and about 1.5 cm (0,6 in) thick. It is almost 4 times thicker than at the beginning of the cycle.
After ovulation, if the egg (ovum) is not fertilized in about 24 hours, it degenerates and the stimulus for the production of estrogen and progesterone disappears. As there is no longer the possibility of pregnancy during this cycle, there is no reason for the body to keep preparing the uterus for the implantation of an embryo that will not come. With less circulating estrogen and progesterone, the thick wall of the endometrium loses its stimulus to proliferate and its blood supply is suddenly cut off. As a result, the wall of the endometrium begins to collapse, carrying endometrial tissue, mucus, water, and blood.
So, the menstruation is nothing more than the detachment of expressive parts of the inner wall of the uterus which, throughout the menstrual cycle, have been developed to receive an embryo that was never generated.
Therefore, for there to be menstruation, the woman must go through two phases:
1- Increasing production of estrogen, in order to induce ovulation and proliferate the endometrium.
2- Unfertilized egg, as the absence of fertilization and the degeneration of the ovum is what causes the end of the hormonal stimulus which, in turn, leads to the break down of the uterine wall.
Note: When a woman uses birth control pills, she uses hormones to deceive the reproductive system, causing the endometrium to proliferate without inducing ovulation. When the pill is stopped, the endometrium breaks down and the patient gets her period.
Why does menstruation not occur during pregnancy?
When the egg is fertilized, the levels of estrogen and progesterone rise, keeping the endometrium thick and apt to receive the embryo that is about to arrive. Here lies the first reason why a woman can not menstruate when pregnant: a menstruation would mean that the endometrium collapsed and would no longer be able to receive or maintain the embryo that has just been formed.
The second resason is: once the fertilized egg has implanted and is developing in the endometrium, a new hormone begins to be produced: human chorionic gonadotropin, also known by the acronym hCG. One of the many roles of hCG is to “notify” the mother’s body that there is a baby being development in her uterus. This means, among other things, that the ovary is “told” to no longer mature new ovules every month, which, in practice, causes the woman not to ovulate during the entire period in which she is pregnant.
Therefore, since one of the basic requirements for menstruation is ovulation, if the woman does not ovulate or has the hormonal fluctuations of estrogen and progesterone that occur throughout the menstrual cycle, she does not present the necessary stimuli so that she can menstruate. Actually, there is not even a menstrual cycle going on if the woman is pregnant.
However, the main reason for the incompatibility of pregnancy with menstruation is much simpler than the hormonal mechanisms described above. Think carefully about the following situation: the embryo is attached to the wall of the endometrium. The placenta, from which the embryo receives the necessary blood for its development, is also attached to the endometrium. Hence, if the woman menstruates, i.e. the wall of the endometrium breaks down, how could the embryo remain attached in this wall? It couldn’t. Just as it is impossible for a picture to remain hanging on a wall that has been knocked over, an embryo cannot remain attached to the uterus if the inner wall has collapsed.
In short, a pregnant woman does not have any of the stimuli necessary for menstruation to occur. However, even if she did, it would mean the immediate termination of pregnancy because, as soon as she menstruated, the embryo would be expelled along with the endometrial tissue that had collapsed.
Some of you may be thinking: but I know women who bleed during pregnancy. How can this be?
As already mentioned in the introduction to the text, not all vaginal bleeding is menstruation. There are several causes for bleeding in pregnant women, but none of them is menstruation.
Causes of vaginal bleeding during pregnancy
There are several causes of bleeding during pregnancy, and up to one in five pregnant women has at least one episode of vaginal bleeding in the first 12 weeks of gestation. Some of these causes are benign, but others may indicate serious problems during pregnancy, such as threatened miscarriage or ectopic pregnancy.
Among the benign causes of bleeding during pregnancy is the so-called implantation bleeding, which is a mild blood loss that can arise when the embryo is implanted in the wall of the endometrium. This bleeding, though having a very different aspect from menstruation, is often confused with the period, because it usually occurs around the 4th week of the menstrual cycle, exactly when the woman is expecting her menstruation to come.
Bleeding without clinical relevance may also occur after a gynecological examination, or even after sexual intercourse. These type of bleeding occur because, during pregnancy, the vagina and cervix are more sensitive and with increased vascularization. Women with uterine polyps or fibroids may also have episodes of vaginal bleeding throughout pregnancy.
In terms of illnesses, gynecological infections, such as gonorrhea, herpes, chlamydia, candidiasis, or other forms of vaginitis, can irritate the vaginal mucosa, facilitating the occurrence of bleeding.
From the 20th week of gestation, placenta previa (placental implantation in front of the uterine outlet) or premature placental abruption (when the placenta loosens from the uterus before the time of birth) are common and worrisome causes of vaginal bleeding.
Regardless of the cause, all bleeding during pregnancy should be promptly evaluated by an obstetrician. As mentioned above, there is no menstruation during pregnancy. If you are pregnant and are losing blood through the vagina, even in small amounts, do not assume that everything is okay. Contact your doctor and let him or her decide if this bleeding is a cause for concern.