2024 Author: Priscilla Miln | [email protected]. Last modified: 2023-12-16 00:21
Accurate determination of fetal gestational age is extremely important. It checks the compliance of the development of the fetus with average standards, calculates the time for passing tests and genetic screenings, and also determines the expected date of birth.
How the fetal gestational age is calculated
In medical practice, pregnancy is measured by obstetric term, also called gestation. This method counts from the first day of a woman's last menstrual period. To calculate the embryonic gestational age, it is necessary to subtract two weeks from the obstetric period. This is assumed to be the actual age of the embryo since conception.
This calculation is based on an average menstrual cycle lasting 28 days. In the middle of the cycle, ovulation occurs - the maturation of the follicle in the ovary and the release of the egg forits limits. During this period, the woman's body is ready for conception. In the event of pregnancy, the embryonic period will be exactly two weeks less than the obstetric one. This can be illustrated with an example. That is, for a pregnancy at 4 weeks, the embryonic period will be 2 weeks.
Why gestational age is not always reliable
There are times when the duration of the menstrual cycle differs from the standard 28 days. This situation is not at all uncommon, normally the length of the cycle can be 21-35 days. The standard calculation formula is not suitable here - for a woman with a cycle of 35 days, conception should occur on the 21st, and not on the 14th day. With an obstetric gestation period of 5 weeks, the embryonic period will not be 3 weeks, but two. Such an error can lead to errors in the choice of pregnancy management tactics by a gynecologist.
A woman may not remember the start date of the cycle in which the pregnancy occurred. It is not uncommon for conception to occur a few months after the previous birth, before the hormonal background returns to normal and a regular cycle is established. In such cases, it is simply impossible to correctly calculate the obstetric period.
Why is it important to know the exact gestational age
Doctors monitor the intrauterine development of the fetus, focusing on its compliance with average indicators. If necessary, treatment is prescribed. A woman goes on maternity leave at 30 obstetric weeks to prepare for the arrival of the baby. It also determines the expected date of onset of the physiologic altimely delivery (PDR). It varies from 37 to 42 complete obstetric weeks.
Fetal genetic screenings aimed at identifying malformations and congenital genetic abnormalities should be performed at a precise time interval. Otherwise, the diagnostic value of the study is lost - the results can be both false positive and false negative.
Calculation methods
There are several ways to calculate the embryonic gestational age. It should be noted that there is no absolutely reliable method, the probability of a small error is always there. The following techniques are applied:
- blood test for hCG - in the early stages of pregnancy, this type of study is the most reliable;
- ultrasound examination - starting from 9 weeks of gestation, it is recommended to rely on the information obtained during the ultrasound;
- gynecological examination - the doctor can guess the true gestational age during manual examination on the chair and when measuring the height of the uterine fundus.
The most reliable data is obtained by comparing the results obtained by different methods. Below it is proposed to consider in more detail how to determine the embryonic gestational age using these methods.
Determination of the embryonic period by the analysis of hCG
The hCG hormone, or human chorionic gonadotropin, is also called the pregnancy hormone. in the blood of men andin non-pregnant women, this hormone is absent or contained in small amounts (less than 5 IU / l). But as soon as conception occurs, the cells of the chorion of the embryo, which later transform into the placenta of the fetus, begin to actively produce hCG.
The hormone is designed to regulate metabolic processes, with the help of which the woman's body begins to actively prepare for pregnancy. A blood test can determine the rise in hCG as early as 7 days after conception. For the same purpose, pharmacy express tests for the content of hCG in the urine are used.
But if a pharmacy test strip determines the very fact of pregnancy, a clinical blood test reveals the quantitative content of the hormone. By the level of hCG in the blood, you can calculate the embryonic gestational age by weeks, since until the 9th week it grows in proportion to the growth of the embryo.
Ultrasound gestational age
Ultrasound examination should be carried out at least three times during pregnancy. The timing is set by the gynecologist. But if necessary, this method can be used to determine the exact embryonic period. There are norms for the development of the embryo and the growth of the fetal egg, with the help of which the gestational age is fairly accurately determined.
The main parameter is the coccyx-parietal size (KTR). This is the distance from the coccyx to the crown of the embryo, which has diagnostic value in the period of 6-13 weeks. Before 6 weeks the embryo is too small for sureit is impossible to measure KTR. After the 13th week, more important parameters of fetal development appear.
Determination of the term at the gynecological examination
An experienced doctor already at the 4-5th week of gestation can suggest pregnancy. The uterus of a woman on a manual examination is enlarged, acquires a rounded shape. The nature of the discharge of the cervical canal, the color of the mucous membranes of the vagina and external genital organs are changing.
At later dates, starting from the 18-19th week, the height of the uterine fundus is determined quite accurately. So, at the 20th week, the bottom of the uterus is about 2 cm below the navel, and at the 28th week - 2 cm above.
In addition to the height of the bottom of the uterus, the circumference of the woman's abdomen is measured at the level of the navel. But this is a secondary indicator, which is characterized by an error due to the initial complexion of the pregnant woman. In more complete women, the circumference of the abdomen will be larger. Therefore, gynecologists pay more attention not to specific numbers, but to the growth rate of the circumference - normally, from the second trimester of pregnancy, it is about 1 cm per week.
Advice from gynecologists on determining the embryonic term
However, there are some tips from gynecologists, based on which you can try to determine the embryonic period. When determining hCG, it is recommended to take an analysis in dynamics several times with an interval of 3-4 days. The dynamics of hormone growth will help not only to accurately determine the embryonic period, but also to assess the pace of development of the baby.
The most reliable ultrasound determines the embryonic period in the period from the 9th to the 13th week of gestation. Extremely importantundergo ultrasound screening at 11-13 weeks of gestation. Before this period, the embryo is too small, and after the signs of malformations may not be visible. That is why it is important to register at the antenatal clinic before the 12th week when establishing the fact of pregnancy.
There is a "first knock" method - determining the embryonic gestational age by the first movement of the baby in the stomach. It is believed that women who are pregnant for the first time feel it at the 20th week. During the second pregnancy, the first push is felt earlier, at the 18th week. The diagnostic value of this method is doubtful, since due to physiological characteristics, the time when a woman first feels fetal movements can vary greatly. In addition, this method is not at all suitable for early pregnancy.
It should be remembered that the data of any medical study must be interpreted by the doctor. Do not make hasty conclusions if the results obtained are somewhat embarrassing or upsetting. Only a gynecologist will correctly compare all the nuances identified during the examination, make a diagnosis and prescribe treatment. A woman's attentive attitude to her he alth, keeping a calendar of menstrual cycles and early contacting a antenatal clinic when pregnancy occurs will help to give birth to a he althy baby.
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