Accurate determination of the fetal gestational age is crucial. It checks the compliance of the fetal development with the average standards, calculates the period of tests and genetic screenings, and also determines the estimated date of birth.
How is the fetal gestational age calculated?
In medical practice, pregnancy is measured by an obstetric period, also called gestation. According to this method, the countdown is from the first day of the last menstruation of a woman. To calculate the fetal gestational age, it is necessary to subtract two weeks from the obstetric period. It is assumed that this is the actual age of the embryo from the moment of its 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 beyond it. 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 by an example. That is, for a pregnancy of 4 weeks, the embryonic period will be 2 weeks.
Why gestational age is not always reliable
There are cases when the duration of the menstrual cycle differs from the standard 28 days. Such a situation is not uncommon at all, normally the cycle length can be 21-35 days. The standard calculation formula is not suitable here - in a woman with a cycle of 35 days, conception should occur on the 21st, and not on the 14th day. With an obstetric gestational age of 5 weeks, the embryonic period will be not 3 weeks, but two. Such an error can lead to errors in the choice of tactics of pregnancy by a gynecologist.
A woman may not remember the start date of the cycle in which the pregnancy occurred. There is a situation when conception occurs 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 observe the intrauterine development of the fetus, focusing on its compliance with the averaged indicators. If necessary, treatment is prescribed. A woman goes on maternity leave in 30 obstetric weeks to prepare for the appearance of the baby. The estimated date of the onset of physiologically timely delivery (PD) is also determined. It ranges from 37 to 42 full obstetric weeks.
Genetic fetal screenings aimed at identifying malformations and congenital genetic abnormalities should be done at the exact time. Otherwise, the diagnostic value of the study is lost - the results can be either false positive or false negative.
Calculation Methods
There are several methods to calculate the fetal gestational age. It should be noted that an absolutely reliable method does not exist, the probability of a small error is always there. The following methods are applied:
- a blood test for hCG - in the early stages of pregnancy, this type of study is the most reliable;
- ultrasound - starting from 9 weeks of gestation, it is recommended to rely on the information obtained during the ultrasound;
- gynecological examination - the doctor may suggest the true gestational age during a manual examination on a chair and when measuring the height of the bottom of the uterus.
The most reliable data are 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 fetal term for hCG analysis
The hCG hormone, or human chorionic gonadotropin, is also called the pregnancy hormone. In the blood of men and non-pregnant women, this hormone is absent or is contained in a small amount (less than 5 IU / l). But as soon as conception begins, the cells of the embryo chorion, which subsequently transform into the placenta of the fetus, begin to actively produce hCG.
The hormone is designed to regulate metabolic processes by which a woman’s body begins to actively prepare for gestation. A blood test can determine the growth of hCG within 7 days after conception. For the same purpose, pharmacy rapid tests for hCG in 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 Pregnancy
An ultrasound should be performed at least three times during pregnancy. The dates are 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 determined quite accurately.
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. Up to 6 weeks, the embryo is too small, it is impossible to accurately measure KTP. After the 13th week, more important fetal development parameters appear.
Determination of the term at the gynecological examination
An experienced doctor already at 4-5 weeks of gestation can assume the onset of pregnancy. The uterus of a woman on a manual examination is enlarged, takes a rounded shape. The nature of the secretions of the cervical canal, the color of the mucous membranes of the vagina and external genitalia changes.
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 higher.
In addition to the height of the bottom of the uterus, the abdominal circumference of a woman is measured at the level of the navel. But this is a secondary indicator, which is inherent in the error due to the initial composition of the pregnant woman. More fat women will have a larger abdominal circumference. 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.
Gynecologist's advice on determining the fetal term
Nevertheless, there are some tips from gynecologists, based on which you can try to determine the fetal period. When determining hCG, it is recommended to pass an analysis in dynamics several times with an interval of 3-4 days. The growth dynamics of the hormone 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 from the 9th to the 13th week of gestation. It is extremely important to undergo ultrasound screening in the period of 11-13 weeks of gestation. Before this period, the embryo is too small, and after signs of malformations may not be visible. That is why it is important when establishing the fact of pregnancy to register in the antenatal clinic until the 12th week.
There is a "first knock" method - determining the fetal gestational age by first moving a 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 impulse is felt earlier, at the 18th week. The diagnostic value of this method is doubtful, because due to physiological characteristics, the period when a woman first feels fetal movements can vary greatly. Moreover, this method is not at all suitable for early pregnancy.
It should be remembered that the data of any medical study should be interpreted by a doctor. You should not make hasty conclusions if the results obtained are confused or distressed by something. Only a gynecologist will correctly compare all the nuances identified during the examination, make a diagnosis and prescribe treatment. Attentive attitude of a woman to her health, maintaining a calendar of menstrual cycles and an early appeal to a antenatal clinic during pregnancy will help give birth to a healthy baby.