Author: Sozinova A.V., obstetrician-gynecologist, has been in continuous practice since 2001. March, 2021.
Fertilization (fusion of male and female sex gametes) is a pregnancy that has not yet occurred. Pregnancy is considered to have occurred only after completion of implantation - the introduction of a fertilized egg into the uterine mucosa.
The interval from the moment of conception to the attachment of a fertilized egg in the endometrium is called biochemical pregnancy and cannot be physically felt by a woman or confirmed by additional research methods.
Implantation mechanism
In the process of moving towards the uterine cavity, the fertilized egg is intensively fragmented, gradually increasing the number of cells forming it and turning into a morula (from 16 to 32 cells) or a blastocyst.
The blastocyst is represented by two types of cells: embryoblast (future embryo) and trophoblast (precursor of the placenta).
The movement of the blastocyst into the fetal cavity occurs on average 5–6 days from the moment of fertilization. After the future embryo enters the uterus, the implantation process begins, which includes 2 stages:
1. Adhesion. The future embryo adheres to the surface layer of the uterine mucosa.
2. Invasion. Next, it is directly introduced into the endometrium. To carry out this process, special threads are formed, with the help of which the future embryo grows into the uterine mucosa. At the same time, certain substances are released from these villi that can dissolve the surface layer of the endometrium, resulting in the formation of a fossa called implantation. The embryo is immersed in this depression, and the damaged mucous membrane regenerates, completely hiding it in its thickness. The implantation process does not end there; the villi penetrate deeper into the uterine tissue, which is accompanied by the destruction of its blood vessels and the formation of lacunae filled with maternal blood. Through these lacunae, the embryo communicates with the maternal body (nutrients and oxygen are supplied to the embryo). After implantation has occurred, the pregnancy is considered successful, and the signs that appear in the woman are called doubtful or probable.
Photo: process of adhesion and invasion - formation of threads and ingrowth of the blastocyst into the endometrium
Implantation bleeding: the essence
This phenomenon is not essentially bleeding, since it involves the release of a very small amount of blood. It is called implantation bleeding, since it accompanies the process of implantation of the future embryo into the wall of the uterus, during which the destruction of its vessels occurs with a corresponding outpouring of a certain amount of blood into the vagina.
Implantation bleeding is a physiological process, but occurs in only 20-30% of pregnant women. The absence of this phenomenon does not indicate a pathology of pregnancy, and in many women it goes unnoticed. More often, implantation bleeding is observed in women with the simultaneous maturation and fertilization of several eggs (multiple pregnancy), which is due to the large area of damage to the endometrium during implantation.
With an ectopic (ectopic) pregnancy, implantation bleeding is always absent.
Changes in the body
Attachment is a kind of shock to the entire female body. From the moment of conception, all biological and chemical processes in the body are quickly rebuilt. At the site of the follicle, a corpus luteum forms, which begins to produce the hormone progesterone, which affects the cessation of menstruation and further growth of the endometrium.
The immune system weakens sharply; this is necessary to preserve the life of the embryo, since the immune system may perceive the implant as a foreign dangerous neoplasm and begin to produce antibodies against it. Constant attacks of antibodies can trigger early miscarriage.
Expecting pregnancy involves maintaining a menstrual calendar. Therefore, the question often arises, on what day of the cycle does implantation bleeding occur if fertilization has occurred?
Most often, minor bleeding associated with successful fertilization appears 7–8 days before the expected menstruation, or on the first day of menstruation.
Attachment of a fertilized cell occurs regardless of the day of the cycle, but always occurs after completion of ovulation.
An approximate count of the days of the cycle looks like this:
- Normally, egg maturation occurs on the 14th day of the cycle (with a duration of 30 days).
- The finished egg is released on the 16th day.
- For fertilization, she needs from several hours to 7 days.
- The process of penetration into the uterus and attachment takes from 6 to 12 days.
Based on the results of the calculations, a conclusion should be made: the process of implantation discharge will occur from the 22nd to the 28th day of the menstrual cycle.
When to expect
To calculate the date of onset of implantation bleeding, you need to know the duration of the menstrual cycle. On average, its length is 28 days. Ovulation occurs in the middle of the cycle, that is, on the 14th day, plus/minus one day. Taking into account the viability of the egg (24 hours, maximum 48 hours) and the exact date of unprotected intercourse, it is possible to calculate the date of implantation and the occurrence of implantation bleeding.
The egg enters the fetal sac 5-6 days after conception, where within about 24 hours it determines a place in the uterine wall for implantation. This process takes approximately 40 hours, but it can be reduced to 3 hours or extended to 2 days. Complete immersion of the blastocyst into the uterine mucosa is completed by 7-8 days from the moment of fertilization; therefore, implantation bleeding should be expected to appear within the same period. Thus, bleeding, indicating that implantation has occurred, will appear a week before the next menstruation, in some cases (late implantation) a day before it.
During the IVF procedure, implantation bleeding is observed more often, but it occurs later, 8–10 days after ovulation (egg retrieval, in vitro fertilization, implantation of a blastocyst into the uterus).
What does it look like and when can there be signs of implantation?
Implantation bleeding is scarce - it can be either a few drops, which are almost impossible to see on underwear, or spotting. If a woman does not regularly measure her basal temperature, she will not notice its fluctuations, since they do not affect her general well-being.
Women perceive unpleasant cramps in the lower abdomen, weakness and tearfulness as manifestations of PMS. It should be borne in mind that manifestations of implantation are possible only in the case of unprotected sexual intercourse during ovulation. Only a combination of such factors allows us to talk about conception.
Most often, such discharge occurs 4-6 days after successful conception.
This depends on individual characteristics and the duration of the menstrual cycle. For many women, bleeding during implantation occurs on days 23-26 of the cycle, so most perceive such discharge as a harbinger of upcoming menstruation.
Signs
The discharge indicating implantation is always scanty, in the form of a small “smear”, usually one or two drops on the underwear.
Their color varies from light pink to light brown. Sometimes such discharge is transparent with bloody inclusions. They can last from a couple of hours to 2 days. It is possible that the discharge may stop and then resume, which is explained by a change in the stages of implantation. Important! Implantation bleeding is not characterized by a dark (brown, burgundy) color and blood clots.
Other manifestations
After the blastocyst attaches to the uterine wall, a connection is established between the maternal body and the embryo, which is characterized by the appearance of probable signs of pregnancy caused by the action of progesterone:
- weakness, fatigue;
- mood swings;
- frequent urination;
- engorgement, tenderness of the mammary glands;
- change in taste, nausea;
- odor intolerance;
- low-grade fever.
A number of women, when implantation bleeding occurs, report minor discomfort or nagging pain in the lower abdomen or lower back, caused by the destruction of the uterine lining, the attachment of the blastocyst in it and slight spasms of the uterus. Normally, the intensity of pain does not increase, the discomfort disappears on its own along with the cessation of discharge.
Basal temperature at implantation
When charting the basal temperature, a woman notes a decrease in the temperature curve to 36.8 within one day, which corresponds to the penetration of the blastocyst into the fetal sac and its stay in “free flight”, and is called implantation retraction, noted before the start of implantation.
After the implantation of the future embryo into the uterine mucosa, the basal temperature again reaches 37 degrees and above and remains at this level all subsequent days before the expected menstruation and on the days of its absence.
Pregnancy test
It is recommended to carry out a pregnancy test no earlier than 3 days after the cessation of implantation bleeding or 8-10 days after ovulation (it is optimal to perform the test on days of missed menstruation). But it is possible to obtain a false negative result due to the low concentration of the hormone in the body, therefore, to confirm pregnancy (for example, after IVF), it is necessary to perform a laboratory test for hCG 10 days after the expected implantation bleeding.
HCG in a woman’s body - namely, it is determined by the standard “two stripes” test - begins to be determined after the final immersion of the embryo into the mucous membrane of the fetal sac, as soon as the trophoblast producing human chorionic gonadotropin establishes a connection with the circulatory system of the expectant mother. It is from the trophoblast that the chorion is first formed, and later the placenta, which synthesize hormones responsible for the growth and development of pregnancy. Starting from the implantation period, the level of hCG in a woman’s blood increases every day until the 11th week of pregnancy.
Differences from menstruation
There are several characteristic differences that can be used to distinguish implantation from regular menstruation.
- Volume - when the egg is attached, the discharge is very scanty throughout the entire time, lasting no more than 2-3 days. During menstruation they are abundant and last up to 4-8 days.
- Menstruation is characterized by an uneven character - scanty bleeding turns into intense bleeding and ends with scanty bleeding.
- Menstruation differs from implantation bleeding in time - if the former most often begin on the 27-30th day of the menstrual cycle, then the latter 4-7 days after ovulation (3-8 days before menstruation).
- Implantation is characterized by the appearance of creamy-milky or pink smudges with barely noticeable blood streaks. During menstruation they are a rich red color.
A special feature of menstruation is premenstrual syndrome, which affects many women. It is accompanied by weakness, loss of strength, nervousness, tearfulness and increased irritability. It begins a few days before menstruation and can be accompanied by unpleasant, painful sensations in the lower abdomen, as well as swelling of the mammary glands.
What can implantation bleeding be confused with?
Often, women mistake implantation bleeding for the onset of menstrual flow ahead of schedule. Distinctive features of menstruation:
- if there are pain sensations, they do not disappear, but increase in dynamics;
- dark red discharge;
- the presence of blood clots;
- an increase in the volume of blood released.
Implantation bleeding should be distinguished from discharge caused by the following reasons:6
- background processes of the cervix (pseudo-erosion, cervicitis),
- adenomyosis,
- hormonal imbalances (formation of a functional ovarian cyst).
As a rule, the discharge is dark burgundy or brownish in color and varies in volume (from spotting to heavy).
Implantation and ectopic pregnancy
With an ectopic (ectopic) pregnancy, implantation bleeding is always absent. The occurrence of pregnancy can only be judged by a positive test result and the appearance of dubious signs, including delayed menstruation. Bloody discharge from the genital tract during ectopic pregnancy appears after a delay in menstruation, and can be scanty, moderate, rarely abundant and accompanied by sharp pain in the abdomen, radiating to the supraclavicular region, rectum, and leg. The occurrence of bleeding during an ectopic pregnancy is caused by its interruption (usually tubal abortion or tube rupture) and detachment of the functional layer of the endometrium.
