What does the fertilized egg look like and what size should it be normally?

An egg must fertilize in order for a new life to begin. At this early stage, this tiny structure is almost invisible to the unaided eye, but it has the potential to grow into a fully formed baby. Parents who are interested in tracking the early stages of pregnancy may find it fascinating to learn about the appearance and development of a fertilized egg.

The egg is minuscule at the time of fertilization, roughly the size of a grain of sand. Despite its tiny size, this one cell has all the genetic material required for the baby’s development. It will rapidly alter over time, proliferating and dividing in order to get ready to implant in the mother’s uterus.

Early in the pregnancy, knowing the size of the fertilized egg and its development can be comforting. It’s an amazing journey that starts with something so little, but so powerful.

Appearance and structure

By the third week of its development, if we count from the day of fertilization, the so-called fertilized egg is formed. This is a rather complex "structure" of a round shape, which consists of an embryo, a yolk sac, a chorion and an amnion. The amnion is the inner membrane of the fetal sac. It produces amniotic fluid – a special nutrient medium in which the embryo and other embryonic structures are located. The chorion is the outer membrane. It contains villi with which the fertilized egg is attached to the endometrium of the uterus. The yolk sac is a "provolvular warehouse" that contains nutrients. It looks like a small yellowish pea, located between the chorion and the amnion at the site of the umbilical cord.

The fertilized egg can only be examined starting in the fifth week of pregnancy, at which point its size is large enough to be seen on ultrasonography. Stated differently, it is only visible a week or longer after the next menstrual delay.

The color of the fetal shells is grayish, the shape is oval or rounded. Since the shells are quite elastic, under the influence of different factors (for example, uterine tone), the fetal egg can change shape, but when these factors are eliminated, it quickly returns to the original appearance. The embryo looks like a small strip in it. The presence of one fruit egg does not guarantee that one child will be born. In the case of monozygous twins, embryos develop in one fruit egg. If two fruitful eggs are found, this means that a woman is not waiting for twins who are similar to each other and having one floor, but twins, each of which will have a separate “house” – a fetal egg, a placenta during intrauterine development.

When pregnancy develops, the fertilized egg is typically found in the upper third of the uterine cavity. Because placenta previa, which forms at the point where the chorionic villi attach to the uterine endometrium, can be either complete or partial, if it is located low, it can seriously complicate the course of pregnancy. The actual process, known as implantation or nidation, starts about a week after fertilization.

Sizes by week

The size of the ovum in the early stages of pregnancy is the main parameter by which the doctor can judge how the baby is developing. The embryo is still very small, it is not possible to measure it and its individual parts, but the growth rate of the ovum is a very informative indicator of the development of pregnancy as a whole. The size of the ovum indicates not only development, but also compliance with certain obstetric terms. The fact is that at the very beginning of pregnancy, when the embryo is just appearing, there is not much difference in height and weight. It is much later that children in the mother"s womb begin to grow differently, in accordance with their genetic program (some are tall, others are small). In the meantime, all babies develop almost identically, so the growth rate of the fetal egg is almost the same. Errors and the range of values ​​in the diagnostic tables are associated with the probability of late implantation, as well as with other factors that can affect the size of the fetal egg, but do not pose a threat to the development of the baby.

To measure, a unique method is employed. The ultrasound diagnostician visualizes the fetal egg on the monitor and draws a straight line through it so that the ends of the segment are at opposite positions on the fetal sac’s inner shell. We refer to this size as average internal diameter, or SVD. The very first determines this size. Next, it is supplemented with the embryo’s own coccygeal-parietal size. The yolk sac’s dimensions are also thought to be significant.

If it is not visualized in any way, that is very bad. Even if it is noticeable and the size is within normal limits, there is still no assurance that the pregnancy will go smoothly or that the baby will be healthy.

Table of correspondence of sizes of the fetal egg.

Therefore, it is deemed to be entirely typical if a woman is discovered to have a fertilized egg that measures 4-5 mm at 5 obstetric weeks, which is one week following the start of the delay. Furthermore, a 20 mm fetal egg at 7 obstetric weeks is perfectly normal. Finding a difference in the terms’ sizes could point to specific diseases. However, a lag should be interpreted as a significant deviation; for instance, the fetal sac measures 4-5 mm at 7 weeks of pregnancy. Let’s talk about the pathologies that affect fetal eggs and their prognoses.

Pathologies

There’s no need to freak out when the doctor tells you that the fetal egg has been found, but it’s elongated and malformed. The uterine muscles’ increased tone is usually the cause of this; once it is removed, the fetal membranes will assume their normal shapes. There are numerous ways that medicine can reduce elevated tone and stop miscarriage before it starts. The following issues are among those that can be found with an ultrasound examination.

Hypoplasia

This is an anomaly in which the development of the fetal membranes lags behind the growth rate of the embryo itself. The fertilized egg, therefore, differs from the embryo in size and timing. Based on the diameter of the fetal sac, the doctor puts only 7 weeks, and based on the size of the embryo – 9 weeks. The reasons for which hypoplasia occurs are multifaceted. This may be taking antibiotics in the early stages, flu or acute respiratory viral infection suffered in the initial stages of pregnancy, hormonal imbalances in the woman"s body (endocrine diseases, hormonal stimulation suffered as part of the IVF protocol), as well as fetal malformations. The prognosis, alas, is unfavorable. In most cases, the embryo becomes too cramped in the small membranes and dies. A frozen pregnancy occurs.

A fertilized egg that fails to grow or grows too slowly causes the hCG hormone in the pregnant woman’s blood to rise insufficiently because the chorionic villi are unable to perform their functions, which include producing the hormone required for conceiving a fetus.

Hydatidiform mole

A gross and total anomaly in which the embryo does not develop, but the chorionic villi grow and turn into a mass of small bubbles resembling bunches of grapes. With a complete mole, the embryo is completely absent, with an incomplete mole, the embryo and other structures of the ovum may be present, but cannot develop normally. The reasons for this phenomenon are as a female reproductive cell. If a sperm fertilizes an oocyte devoid of DNA, this is the pathology that develops. Only the paternal chromosomes are doubled, such an embryo is not viable in principle. If one egg is fertilized by two sperm at once (which happens, although rarely), an incomplete hydatidiform mole will be formed.

The woman’s gonads may develop cysts as a result of the overgrown chorionic villi producing an excess of hCG, which will be "off the charts." However, the mole can develop into chorionepithelioma in 17–20% of cases, making it dangerous for reasons other than this. This tumor is malignant, meaning it spreads rapidly to other parts of the body.

Upon detection of a hydatidiform mole, the uterine cavity is cleared either by curettage (scraping the uterine cavity) or vacuum aspiration (virtually an abortion).

Anembryony

In this pathology, the fertilized egg is present and grows, but the embryo is completely absent from within. Another name for the anomaly is "empty fertilized egg syndrome." After 6-7 weeks of pregnancy, when the doctor cannot see the embryo or hear the baby’s heartbeat, this is discovered during an ultrasound. In as many as 80% of cases, severe genetic disorders present during conception result in anembryony. Other acute viral diseases and the woman’s flu may also be contributing factors. Both endometriosis and an untreated bacterial infection of the genital tract can result in anembryony.

The condition is more common in female residents of areas with unfavorable radiation environments. Furthermore, women with metabolic disorders—particularly those involving deficiencies and abnormalities in progesterone production—are frequently affected by the pathology.

The woman is advised to have multiple control ultrasounds spaced a few days apart if anembryony is suspected. In the event that suspicions are validated and the embryo remains invisible, curettage or vacuum aspiration are carried out.

False ovum

In terms of diagnosis, this is one of the most challenging cases. There is a marked growth lag and a fertilized egg found in the uterus, which is definitely not appropriate for the term. Furthermore, unlike in the case of empty ovum syndrome, it is not possible to find an embryo within it. The sneakiness, however, is not in this; rather, it is in the likelihood that a second fertilized egg is growing outside the uterus, resulting in an ectopic pregnancy.

Low location

In the event that the fertilized egg is discovered below the upper third of the uterus, close medical supervision is necessary. However, it’s too soon to make judgments. During pregnancy, the uterus grows larger, allowing the fetal egg to "migrate" above. In this case, observation is all that is necessary if it develops normally based on gestation.

Amniotic partition

Such a pathology is found in about one case for one and a half thousand pregnancies. Amnion forms cereals – the partition is formed inside the fetal egg. This, of course, requires careful observation from doctors. The reasons for the development of anomaly have not been fully studied, but doctors tend to believe that cereals are formed due to damage to the fetal egg at the very early stages of development. It is quite possible to endure and give birth to a child during a partition inside the fruit shells, but the birth of a child with crevices (“wolf mouth”, “hare lip”) is not excluded. The limbs of the baby can also suffer due to prolonged compression. Sometimes it leads to necrosis of the limbs and their subsequent amputation after the birth of the child.

Children born after an intrauterine stay in a bubble with a septum frequently have feet deformities called valgus. The occurrence of such unfavorable results is 12–15%. The child is carried by the other women without experiencing any severe health consequences.

Furthermore, the septum does not have to stay in place the entire pregnancy. Because the septum is so thin, it can easily tear, even if it was found on one ultrasound, it might not be there on the next.

Large fetal egg

Early on, an excessively large fetal egg may be a sign of several diseases affecting the fetus and the pregnancy. Excess size is frequently a sign of a frozen pregnancy; frequently, it is accompanied by fetal heart rhythm abnormalities and the embryo itself is behind schedule in terms of normal size.

At five or six weeks, a small increase in the ovum may suggest the visualization of one egg, but it’s also possible that it contains two embryos (monochorionic twins, twins). In this situation, an hCG blood test is typically performed, and both embryos are examined with a follow-up ultrasound a week later.

Retrochorial hematoma

Blood can build up between the chorion and the endometrium when the chorion partially separates from the uterine wall, resulting in a hematoma. Such pathology typically shows up as mild lower abdominal pulling pains and the appearance of bloody discharge from the genitalia.

The size of the hematoma determines the prognosis. If discharge is seen, this is a good sign because it means that blood is leaking out and is decreasing. The pregnancy will develop entirely normally in the future.

A total detachment of the ovum may happen (or has already happened) if the hematoma grows but there is either no discharge or a lot of it. Under such circumstances, the pregnancy cannot be saved. Retrochorial hematomas typically occur in women who are extremely anxious, constantly stressed, have endometriosis, hormonal imbalances, or other reproductive system disorders. Detachment can also result from unreasonable medication taken without a doctor’s permission and excessive physical activity.

It can be comforting for expectant parents to know the typical size and appearance of a fertilized egg. The fertilized egg is initially invisible without a microscope. It is a tiny cell the size of a pinpoint.

The egg changes significantly during development. By the time it gets to the uterus, its diameter usually measures between 0.1 and 0.2 millimeters. It’s just starting to grow at this early stage and will keep developing into an embryo.

A healthcare provider’s routine examinations can provide more detailed information regarding the development and growth of the fertilized egg. During the early stages of pregnancy, these check-ups give parents peace of mind by ensuring that everything is proceeding as it should.

A fertilized egg, also known as a zygote, is a microscopic cell that typically has a diameter of 0.1 to 0.2 millimeters and signifies the beginning of human life. It has a small, spherical appearance, and both its size and form are important markers of normal early development. The egg starts to divide quickly in the early stages following fertilization as it gets ready to implant in the uterus and develop into an embryo.

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