Observation by a gynecologist of the development of pregnancy from an early date is the key to a successful birth and the health of the unborn baby. Several informative criteria help the doctor assess the dynamics of gestation. In the first trimester of gestation, the most important of them is the coccyx-parietal size (KTR).
How to measure KTR of the embryo
KTR is the length of the fetus in millimeters from the toe to the coccyx. You can obtain data on the CTE of the embryo on an ultrasound scan from 6 to 13 weeks of pregnancy. Up to 6 weeks, the embryo size is still too small to be measured using an ultrasound probe. During this period, you can only confirm the presence of pregnancy, if a fetal egg is visualized in the uterine cavity. At approximately 6 obstetric weeks and 3 days, the CTE size reaches 7 mm. The embryo is already clearly visible on ultrasound, the heartbeat is determined, sometimes it is already possible to measure the heart rate.
In order to find out KTR, an ultrasound specialist must label the measurement from the top of the head to the tailbone when the embryo is in a calm, relaxed state. The uterus is scanned in several planes, the largest value of the coccyx-parietal distance is selected. The data obtained are interpreted using a special table of KTR norms during pregnancy by gestational weeks.
Why is it necessary to measure KTR, and not fetal growth
The distance to the coccyx is measured, and not full growth, because the limbs of the embryo at this time are still too small. It is impossible to correctly evaluate embryo growth up to 8–9 obstetric weeks. In addition, a natural sitting position for the baby in the womb is sitting, with legs pressed under it. Therefore, at a later stage of pregnancy, fetal growth is not measured.
To correctly measure the full growth of the child, an ultrasound specialist would first have to determine the CTE, the length of the tibia and lower leg, and then add all the results together. Such calculations have no diagnostic value and are impractical, because the rate of increase in CTE is directly proportional to the growth of the fetus and quite accurately allows you to establish the embryonic gestational age.
How does KTP change by week of pregnancy
There are indicative KTR values for gestational age approved by the medical community. The table of KTR of the fetus by week is given to the average indicators recommended by the World Health Organization. Specialists rely on these data when conducting ultrasound studies in pregnant women.
Determining the gestational age using CTE is the most accurate research method. The error in the numbers in the norm with KTP determination is only 3-5 days. If the deviation from the expected period is more than 5 days, you can suspect an incorrectly established obstetric gestational age or problems in the development of the embryo. Establishment of the embryonic gestational age by measuring CTE is possible with an accuracy of 90%. This method is much more reliable than determining the gestational age by the date of the first day of the last menstrual cycle.
Ultrasound screening and KTR values in the first trimester
If the pregnancy is uneventful, the first ultrasound screening is prescribed between 11 and 13 weeks of pregnancy. The study allows to identify malformations and signs of chromosomal abnormalities, as well as evaluate the development of the fetus. The norm of KTP at 12 weeks of gestation is 48-65 mm. The optimal period for undergoing ultrasound screening of the first trimester of pregnancy is 12 obstetric weeks and 3 days.
Correspondence of the KTR of the fetus to normal values is a positive factor in assessing the course of pregnancy. In addition to KTR, at a screening of 12 weeks, the length of the nasal bone and the size of the collar space are of great diagnostic value. Up to a full 12 weeks, the embryo grows by about 1 mm per day. From 13 weeks, all the organs of the fetus are already fully formed, the baby begins to quickly gain in height and weight. The growth rate of KTP increases to 2-2.5 mm per day.
Why KTP may be more than normal
Small deviations from the norm should not worry the future mother of the baby. Measurement errors by the ultrasound machine allow deviations from the norm within one week. With KTR, the norm is more than one week ahead of schedule, the doctor may suspect the following factors:
- incorrect pregnancy period - the woman will be sent for a second ultrasound in 7-14 days;
- late ovulation;
- features of fetal development (high growth, large physique).
Observation in dynamics allows you to get the most complete picture of the condition and development of the fetus. Clarification of the gestational age is necessary to determine the timing of some studies and to set the estimated date of birth. In general, advancing KTP growth is not a dangerous sign.
What to do if KTR is less than normal
When KTR differs from the size established by the norm in a smaller direction by more than one week, a woman is prescribed 1-2 repeated ultrasounds. Sometimes an additional blood test for the hormone hCG is required. Slow fetal growth may indicate an alarming pathology, especially if the heart rate is lower than average by more than 40 beats per minute.
In the complete absence of a heartbeat and motor activity of the fetus, a frozen pregnancy is assumed. The fact that pregnancy has stopped developing is evidenced by the absence of fetal growth on repeated ultrasound. In this case, it is necessary to curettage the uterine cavity as soon as possible. The death of a fetus for a pregnant woman is very stressful and requires tactfulness from medical personnel. But procrastination with surgical intervention can lead to infertility and even death of a woman.
Nevertheless, far from always the deviation from the normal KTR indicates a pathological course of pregnancy. The cause may be an erroneously established obstetric term. An incorrect determination of the gestational period occurs often, especially if the woman’s menstrual cycle is irregular. A physiologically small growth of the unborn child is also possible. In this case, upon repeated observation, the CTE of the fetus will increase by 7-14 mm.
What CTR may require treatment
When there is an increase in KTR on the second measurement, but it is below normal (less than 1 mm per day), the doctor may suggest a threat of termination of pregnancy. This may occur due to improper adaptation of the female body to the bearing of the fetus, under the influence of external and internal causes, due to congenital genetic abnormalities of the embryo. To support pregnancy, the following drugs can be prescribed:
- progestogens to maintain the necessary level of the hormone progesterone;
- hCG hormone injections for better fetal development and preparation of the uterine cavity for bearing;
- antispasmodics for relaxing the muscles of the uterus;
- drugs that stimulate blood supply and oxygen saturation of tissues.
Timely identified problems and the appointment of drug therapy can normalize the course of pregnancy. Therefore, a woman should register with the antenatal clinic as soon as pregnancy is confirmed, and visit her gynecologist in accordance with his recommendations.
Since when is KTR no longer informative
Starting from 13-15 weeks, data on the CTE of the fetus is no longer informative. Other criteria for the development of the future baby become more important. From the second trimester, parts of the body are studied in detail, such as the length of the bones of the limbs, the biparietal circumference of the head, the length of the nasal bone and the size of the collar space. The internal organs are also examined in detail, the structure of the heart and brain are evaluated, and the blood flow and lumen of large vessels are checked in Doppler mode.
These parameters cannot be evaluated in the first trimester, as the fetus is still too small. But by 16 weeks, the whole body is perfectly visible with a sonography sensor. In this connection, after a full 15 weeks of pregnancy, KTR in an ultrasound study of pregnant women is no longer measured.
Who can interpret the KTP results
Only the specialist should interpret the results of KTP measurements obtained using ultrasound research. Based on the coccyx-parietal size, a doctor observing a pregnant woman can draw conclusions about the successful course of pregnancy and the development of the fetus. In the case of a significant deviation of the CTE from the average values, it is necessary to conduct a number of additional studies to establish the cause.
In addition to the norm of KTR of the fetus by weeks of pregnancy, there are other features that determine the dynamics of the development of the embryo. Therefore, any appointment should be made by a gynecologist observing pregnancy. He has enough information to assess the totality of all factors that affect the course of pregnancy.