Ultrasound has become an integral part of obstetrics and it is difficult to imagine managing a pregnancy without this important examination method. But with the growing popularity of ultrasounds, problems appeared: they began to be abused, and the more and more often ultrasounds are performed, the more the practical value of this method of examination is lost. The quality training of ultrasound technologists and ultrasound doctors with specialization in obstetrics, that is, diseases and abnormalities of fetal development (ultrasound perinatology), still suffers. There are not enough specialists all over the world. Interpretation of survey results is often difficult due to differences in reference values in different medical institutions, as well as in different countries.
Not so long ago, a study was conducted in Canada to assess the accuracy of measuring the thickness of the placenta, and it turned out that the measurement error by different doctors of the same medical institution was 1-3 cm, and among European doctors these discrepancies were even greater. The human factor plays an extremely important role in the conduct of the survey, but so far the increase in the qualification level of medical personnel is slow and inefficient.
Although the majority of pregnant women trust their doctors, more and more often they have to double-check the results of the examination and the rationality of the prescribed treatment. In the countries of the former Soviet Union, many ultrasound specialists intervene in the process of pregnancy management, giving recommendations and even prescribing treatment, which is not practiced in most countries of the world. The worst option is when the conclusion completely contradicts the indicators of measurements of fetal size and other parameters of pregnancy. Many ultrasound doctors practice creating a diagnosis “from nothing”: against the background of normal indicators, imaginary diagnoses of some kind of violations are put in prison. A number of diagnoses are not generally recognized in modern obstetrics, for example, “hypertonus of the uterus”. Therefore, having received a “terrible conclusion” and a negative prognosis in case of refusal of treatment (usually a lot of ineffective drugs-fuflomitsins), many pregnant women dive into the Network in search of ultrasound examination data in order to compare their results with the “Internet” ones. The most frequently asked question in such cases is: “How dangerous is it and can I bring a pregnancy?”
The work of ultrasound specialists is now simplified by the fact that many indicators are compared with reference values automatically – through programs embedded in ultrasound machines. The task of the ultrasound specialist is to make measurements correctly and accurately. Since the fetus is small, especially in early pregnancy, the process of measuring different parts of the ovum and the fetus can be difficult. A small error of just 1 mm can create a stressful situation when interpreting indicators and choosing the tactics of pregnancy. In addition, we should not forget that many reference values were obtained on the basis of data from small clinical studies several decades ago, at the dawn of the use of ultrasound in obstetrics. Therefore, now there is a revision of indicators of norms and deviations, taking into account the physiological changes of the progressing pregnancy and the growing fetus, depending on the period.
Nevertheless, many pregnant women need accurate information that will help them evaluate the results of an ultrasound scan without the help of a doctor. Often women ask what type of ultrasound is best done: transvaginal (tv) vaginal sensor or transabdominal (TA) abdominal sensor? There is no difference in the choice of sensor – it all depends on the skill of the doctor and his ability to measure correctly. With the progress of pregnancy, if necessary, use a combination of sensors.
Another mistake of doctors and pregnant women is constantly observed when calculating the duration of pregnancy. Remember: there is one type of gestational age – obstetric and it is always expressed in weeks and days, not months. Sometimes, especially in the first weeks of pregnancy, it is called a menstrual period. Fetal term does not exist! Ultrasound specialists make mistakes when they tell women that the period is determined by the size of the embryo, which means that it is supposedly different from the period of the menstrual period (on the first day of the last menstrual period with regular menstrual cycles of 26-30 days). Any ultrasound machine calculates the period, adjusted for the duration of menstruation and the size of the ovum and the embryo, and it is always obstetric term. The date of expected birth is also calculated by obstetric gestational age.
Ultrasound in the first trimester is carried out with the aim of:
- confirmation of uterine pregnancy;
- confirmation of live or frozen pregnancy;
- confirmation or exclusion of ectopic pregnancy;
- determine the duration of pregnancy;
- as part of prenatal genetic screening.
There are three phases of pregnancy, which can and should be controlled by ultrasound picture of the stages of development of the ovum, the embryo and the fetus.
- The conception phase, or concept, – the first 3-5 weeks. It starts from the moment of conception – approximately 2 weeks from the first day of the last menstrual period – and the appearance of the ovum, which is not always possible to detect using ultrasound during this period.
- Embryonic phase – 6-10 weeks, when it is already possible to detect the embryo.
- Fetal phase – from 10-12 weeks, when the embryo becomes a fetus and the process of laying and the initial development of all organs and organ systems is completed.
Basic ultrasound parameters of a normal first-trimester pregnancy
Early pregnancy is confirmed by the presence of three important structural units:
- gestational sac;
- yolk sac;
- fetal (or fetal) pole.
Additional signs of uterine pregnancy: detection of a double decidual bag and a symptom of a double bubble (ultrasound terms that people can not remember without medical education, but which it is important to focus on when conducting ultrasound in the early stages).
Fetal egg, or gestational (fetal) bag (GM)
The presence of the fetal sac is the first ultrasound sign of pregnancy. Up to 5 weeks of pregnancy (from the last day of menstruation), the presence of an embryo in the fetal egg is most often not possible. By determining the size of the gestational sac, gestational age can be delivered with an accuracy of 1 week (+/- 1 week). When an embryo is detected, the size of the ovum to determine the duration of gestation is of no practical value.
In obstetrics use two indicators for determining the duration of pregnancy by measuring the size of the ovum:
1) the diameter of the gestational bag – only one measurement is carried out;
2) the average diameter of the gestational bag – determine the internal diameter of the ovum in three dimensions and calculate the average.
A fruit egg can be seen in:
- 4 weeks and 3 days from the first day of the last menstrual period with a TV ultrasound, and its diameter is usually 2–3 mm;
- 5–6 weeks from the first day of the last menstrual period with TA ultrasound, and its diameter is about 5 mm.
Calculation of the term for the size of the ovum:
Menstrual gestational age = mean diameter of the ovum (mm) + 30 or 35 (if the diameter is less than 16 mm)
For example, an average diameter of 5 mm, gestation period = 5 + 30 = 35 days, or 5 weeks.
Normal growth rate
A fetal egg grows on average by 2 mm every 2 days from the 4th to the 9th week of pregnancy, but such indicators are usually not used to determine pregnancy progress, but are used only to confirm the diagnosis of its regression (fading).
What to look for
If the embryo cannot be detected when the size of the ovum is 16–24 mm, it is necessary to suspect a frozen pregnancy or an empty fertilized egg and repeat the ultrasound scan after a week.
The size of the ovum more than 25 mm and the absence of the embryo speak in favor of pathological pregnancy (frozen, empty ovum).
If the size of the ovum decreases in the presence of a live embryo after 9 weeks, you may be suspicious of water deficiency, although the determination of the level of amniotic fluid is usually carried out not earlier than 18–20 weeks.
Before the appearance of the embryo using ultrasound, you can see such an important structure of the ovum as the yolk sac. The yolk sac is a 100% confirmation of uterine pregnancy. When ectopic pregnancy in the uterus, you can detect education, something resembling a fertilized egg, which is a specific reaction of the endometrium to pregnancy.
The yolk sac is between the amnion and the chorion (two formations of the ovum, from which the fetal membranes and the placenta are formed) – in the chorionic space.
Normally, with an average fetal egg diameter of 5 mm, the size of the yolk sac is up to 6 mm (on average, 3-5 mm). The maximum size of the yolk sac is observed at 10 weeks and is 5–6 mm. When laying and developing organs of the embryo, the yolk sac is partially used in the formation of the intestine.
Amnion is the membrane (membrane) inside the fetal egg, which contains the fruit. Consider the fetal membrane can be up to 12 weeks of pregnancy with a fetus size of 5-7 mm. Completion of shell formation occurs by 12–16 weeks.
Kopchik-parietal size (KTR, CRL)
The coccyx parietal size is the longest embryo length, but significant errors are possible in measuring the size of the embryo. In the short term, when the limbs are not clearly visible, it is more difficult to see the tailbone. By measuring this size of the embryo, it is possible to deliver a more defined period of pregnancy – up to 4 days.
Using ultrasound, an embryo with a size of 1–2 mm can be seen when the size of the ovum is 5–12 mm at 5–6 weeks of gestation. During the day, the size of the embryo increases by 1 mm. The difference of 5 mm between the size of the embryo and the ovum is considered the minimum standard for pregnancy.
Calculate the duration of pregnancy
- Menstrual period in weeks = KTR (cm) + 6 (with KTP less than 1 cm)
- Menstrual period in weeks = KTR (cm) + 6.5 (with KTR more than 1 cm)
- Menstrual period in days = KTR (mm) + 42 (with KTR up to 84 mm)
The embryo / fetal heartbeat can be detected at 6 weeks gestation when the embryo is larger than 2 mm, which is a reliable sign of live pregnancy. However, in embryos up to 5 mm (TV ultrasound) and 9 mm (TA ultrasound), the heartbeat can be overlooked, so in such cases it is desirable to perform a second ultrasound in 3-5 days.
Up to 6 weeks of pregnancy, the fetal heart rate is 100–115 beats per minute. Within 6–9 weeks, the frequency increases and reaches a maximum at 8 weeks – 144–159 beats per minute. Starting from 9 weeks, the frequency slowly and slightly decreases.
Slow heart rate is called bradycardia. However, it can be a sign of a fading pregnancy.
Bradycardia is prescribed if:
- heart rate is less than 80 per minute with CTE less than 5 mm;
- less than 100 beats per minute with KTR 5–9 mm;
- less than 110 beats per minute with a CTE of 10–15 mm.
Collar zone (collar space, VZ, NT)
The collar zone is a collection of lymphatic fluid between the skin and soft tissues of the embryo behind the neck (in the collar area), the thickness of which is a prognostic indicator of chromosomal abnormalities of the fetus. It is measured at 11-14 weeks (up to 13 weeks 6 days) – this is part of the prenatal genetic screening of the first trimester, which also includes the determination of biochemical markers in the mother’s blood.
Since the size of the neck fold depends on the size of the embryo, it is very important when measuring to fit within the gestational age – then the measurements will be not only accurate, but also of practical importance. That is, they do this with a CTE of 45–84 mm (11–14 weeks). Such strict measurement frameworks are associated with the characteristics of anatomical changes in a developing embryo, especially the lymphatic system and the exchange of lymphatic fluid.
Normal sizes OT – up to 3 mm. Sizes of 5 mm and more at 16–18 weeks are considered pathological, 6 mm and more – at 19–24 weeks.
Unlike the collar space, which is measured in the first trimester, the thickness of the neck roll is measured in the second trimester. The neck roller is the distance between the outer edge of the occipital bone and the skin of the neck of the fetus. In many cases of chromosomal abnormalities, especially those accompanied by malformations of the cardiovascular system, a disturbance of lymph exchange occurs and it accumulates in the region of the cervical ridge. Measurements are carried out at 16–20 weeks (usually – as part of an anatomical ultrasound), and dimensions greater than 6 mm are abnormal.
The presence or absence of the nasal bone, as well as its length, are recognized as markers for trisomy 21 (Down syndrome). The length is measured at 11–14 weeks – this is part of prenatal genetic screening. The absence of a bone or its length less than 2.5 mm may be signs of Down syndrome.
There are other ultrasound signs that are used by experts to clarify the normal development of pregnancy or abnormalities, including missed pregnancies, empty fetal eggs and others.
Starting from the second trimester (after 14–16 weeks), all indicators of measurements of certain parts of the fetus, as well as various indices, which are calculated on the basis of these indicators, are entered into graphs having baseline percentile curves (95%, 50%, 5%), which were obtained by statistical processing of data from numerous measurements of fruits in the past to determine the normal indicators of growth and development of a given fruit. Creating individual graphs of important indicators of ultrasound allows you to see the child’s development over time and to evaluate it more correctly.
For pregnant women, most of whom have no medical education, it is always important to know whether the gestational age of an ultrasound scan is the same as that expected for menstruation or the day of conception (not necessarily to the day and hour) where the fertilized egg is placed, including in relation to the walls uterus (bottom, posterior wall, anterior wall, inner pharynx), how many fetal eggs and embryos are there, whether it is alive or fading.
However, it is very important not to independently compare these results with the data of numerous sites, because the lack of ability to correctly interpret these data will lead to false conclusions and unnecessary stress.
Remember: a healthy pregnancy does not require numerous ultrasounds and frequent monitoring!