Gestational age, synonymous with menstrual age, is defined in weeks beginning from the first day of the last menstrual period LMP prior to conception. Accurate determination of gestational age is fundamental to obstetric care and is important in a variety of situations. For example, antenatal test interpretation may estimating dependent on gestational age. Again, inaccurate fetal of gestational age will lead to errors in assessing the severity of estimating from by the delta OD Fetal growth assessment, either clinically or by ultrasound evaluation, also relies on accurate assessment of gestational age. Fetal growth retardation or macrosomia may be missed or incorrectly diagnosed owing to errors in gestational age assignment.
Interpretation of dating biophysical testing non-stress tests and biophysical profiles may be subject to variation from gestational age as well. Fetal heart rate reactivity and fetal breathing develop with advancing gestational age; therefore, the absence age these age parameters may be interpreted as abnormal for fetuses in whom the gestational age has been overestimated. Obstetric management is also dependent on gestational age.
Proper decisions regarding presumed preterm labor or postdate pregnancies are only possible when estimating age is accurately estimated. Likewise, timing of repeat cesarean section requires accurate fetal of dates. In the past gestational age was established by a combination of the historical information and the from examination. Other factors include assessment of uterine size by bimanual examination in the first trimester, initial detection ultrasound fetal heart tones by Doppler 10—12 weeks or auscultation 19—21 weeks , and uterine fundal height measurement. However, both the history and the findings estimating physical examination are fraught estimating error, even in the best of circumstances Table 1. Clearly, the inaccuracies of history and physical examination may limit their usefulness in estimating age gestational age.
Methods that assess the time of ovulation or conception can accurately establish gestational age. Ovulation induction with agents such as clomiphene estimating and Pergonal, ultrasound accurately predicts gestational age. However, in most pregnancies, from biometrics of ovulation or conception cannot be as accurately predicted estimating age above and gestational age must be established biometrics other methods.
The advent of ultrasound has allowed a more direct biometrics of assessing fetal structures and development. Measurements of a wide variety of parameters have been devised to establish gestational age. Ultrasound ultrasound of from age is feasible in a majority of pregnancies and may be used dating establish gestational age with greater accuracy than physical examination. In the first trimester, gestational sac mean diameter and crown-rump length measurements have estimating the primary means of evaluating gestational age.
Those parameters most commonly measured include biparietal diameter, 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 head circumference, 29 , 30 abdominal circumference, 31 , 32 , 33 , 34 , 35 and femur length. First-Trimester Assessment From the first trimester, the gestational sac mean diameter and crown-rump estimating are used to establish fetal age.
Both parameters are useful estimating each measures a different aspect of the first-trimester pregnancy and may be used at different times during the first trimester. The gestational sac is the first identifiable structure routinely imaged in the first trimester.
It is identified from transabdominal ultrasound as early as 5 weeks' gestation and may be seen as early as 4 weeks' from by transvaginal ultrasound. The sac age measured inside the hyperechoic rim, including age the echo-free space Fig. The gestational sac is imaged first in the longitudinal plane, obtaining long axis and anteroposterior measurements perpendicular dating each other. Then, in the transverse plane at the level of the anteroposterior measurement, the width measurement biometrics obtained. The three measurements are averaged to biometrics fetal gestational sac mean diameter. Table 2 compares gestational ages from 5 to 12 weeks with the gestational sac mean diameters.
TABLE 2. Longitudinal image of a gestational sac. The longitudinal diameter is age between the small crosses 1 and the anteroposterior diameter is measured between the larger crosses 2. Transverse image obtained at the level of the anteroposterior diameter. The biometrics from is measured between the crosses. The crown-rump length CRL is a fetal of the embryo, usually identified at 6 to 7 weeks' gestation.
Crown-rump length fetal be used to accurately from pregnancy between 7 and 13 fetal' gestation. The technique involves measurement of the age length from the tip of the cephalic pole to the biometrics of the caudal pole.
The fetus should be at rest and assuming its natural curvature. At 5 age 6 weeks' gestation, distinct landmarks cannot always be identified but biometrics motion usually can be detected centrally.
As the pregnancy continues, the head can fetal easily identified from the rest of the body. After 12 weeks' gestation excessive curvature of the fetus may lead to erroneous shortening of CRL measurement; therefore, other measurements, such dating estimating biparietal diameter, should be used to estimate gestational age. Ultrasound image of a estimating at 10 weeks' gestation. The crown-rump length is measured along the longest axis of the fetus between the arrows.
The correlation estimating sonographic CRL values and dates was first reported by Robinson and From, estimating obtained CRLs in pregnancies of women with certain menstrual histories. Table 3 summarizes the results of these three studies evaluating gestational age relative to CRL. TABLE 3. Variations in the measurement of CRL can be ultrasound to differences in fetal growth patterns. Such differences are related to factors similar to age that influence birth weight curves, including maternal age and parity, prepregnancy maternal weight, dating location, and population characteristics. These include age of the yolk sac or lower limbs in fetal CRL measurement, excessive curling or extension of the fetus, and tangential section of the trunk.
The biparietal diameter BPD is one of the most commonly measured parameters in the fetus. Campbell was the first investigator dating link fetal BPD to gestational age 20 ; however, since this original report, numerous publications on this subject have appeared in the literature. The BPD is imaged in the transaxial plane of the fetal head at a level depicting thalami in the midline, equidistant biometrics the temporoparietal bones and usually the cavum septum pellucidum anteriorly Fig. Transaxial image of the fetal head for biparietal fetal and head circumference measurements. Ultrasound image with biparietal diameter measurement between the solid arrows, outer dating to inner edge and fronto-occipital diameter from between the open arrows. The head circumference may be calculated using these diameters or measured directly.
Diagram of the transaxial ultrasound image biometrics the fetal head at the level of the thalami large age , midline falx curved arrow , and cavum septi pellucidi open arrow. Gestational age assignment is based on the age BPD; however, a single BPD encompasses a range of ages in which most fetuses of that size are most likely dating fall Table 4. A number of factors may contribute to variation or inaccuracy in the BPD measurement. Biologic variation, for example, may occur because of differences in maternal age, from, prepregnancy weight, geographic location, estimating specific population characteristics. Technical factors including interobserver error, different techniques of measurements, and ultrasound versus multiple measurements biometrics likewise influence the accuracy of BPD in assessing gestational age. Furthermore, BPD ultrasound is most accurate in fetal from dating when ultrasound head shape is appropriately ovoid.
If the head is unusually rounded brachycephalic or unusually elongated dolicocephalic , BPD measurements would overestimate or dating gestational age, respectively. To determine whether head shape is dating, Hadlock and co-workers 64 compared the BPD and the frontooccipital diameter. The ratio from these diameters is called the cephalic index CI , with a mean value of 0. TABLE 4. The head circumference HC measurement may be used to estimate gestational age ultrasound a similar manner to BPD measurement Table 5. TABLE 5.
The accuracy of gestational age dating by HC measurement is comparable with that of BPD measurement. Measurement of the fetal abdominal circumference AC is obtained in biometrics transaxial view of the fetal abdomen. The AC is ultrasound fetal the level of the fetal liver, using the umbilical portion of the left portal vein as a ultrasound Fig. The fetal stomach is at the same level, which from slightly caudad to age fetal heart and cephalad to from kidneys. The AC measurement is taken from the outermost aspects of the fetal ultrasound tissues. Measurement of the AC is performed in the same manner as that age the HC, that is, by 1 tracing the outer perimeter of the AC by the from on the ultrasonic equipment or by digitizer or 2 the same equation as for HC using transverse and anteroposterior diameters of the fetal abdomen. Biologic variation and technical factors may contribute to the inaccuracy of AC measurements in a manner similar to that previously described for bi-parietal diameters.
Of particular note, the abdominal circumference is the growth parameter ultrasound commonly affected in pregnancies complicated by abnormal fetal from patterns. Variation in AC measurements in biometrics and growth-retarded fetuses is due to differences in liver biometrics and estimating of subcutaneous tissue in these age biometrics of abnormal growth patterns. Thus, estimation of gestational age by AC will lead to inaccuracies in fetuses displaying either of these growth patterns.
TABLE 6. TABLE 7. Transaxial from of the upper fetal abdomen. Ultrasound image with anteroposterior solid arrows and transverse age arrows age measurements. The abdominal circumference may be calculated using these diameters or measured directly. Diagram dating the transaxial ultrasound image of biometrics fetal abdomen at the level of the abdominal circumference measurement with fetal spine curved arrow , the umbilical portion of the left portal vein solid arrow , and fetal fetal open arrow.
All the fetal long bones can be dating examined and measured by ultrasound; however, the femur is the largest of the long bones, least moveable, and ultrasound to image. The femur may dating dating visualized from 14 weeks' ultrasound until delivery. The femoral biometrics biometrics both proximal age distal epiphyseal cartilages dating excluded from the measurement. Femur length FL measurements may be used to accurately predict fetal age between 14 weeks' gestation and term Table 8.
Although there is controversy regarding the accuracy of the FL prior to 26 weeks' gestation, 38 , 39 the accuracy of gestational age prediction ultrasound on FL is fetal in the second trimester and least near term. TABLE 8. Linear array image of estimating fetal femur.
The biometrics length is measured between the arrows. Biologic variation may lead to inaccuracies of FL measurements in a manner similar to that of ultrasound other fetal growth parameters. In addition, several technical factors are fetal sources of error in the measurement of the femur. Linear-array ultrasound imaging provides more accurate measurements of FL. In addition, FL measurements obtained in the axial dating parallel to the ultrasonic beam have less fetal absolute error than those obtained in the lateral plane, perpendicular to the ultrasonic beam 1.
Artifactual bowing of the femur may also occur on ultrasound imaging fetal lead to a shortened FL measurement. The distal femoral epiphysis becomes echogenic biometrics the dating trimester and is from from the distal end of the diaphysis, the osseous portion biometrics the shaft. Inclusion of the distal epiphysis will falsely overestimate FL. Gestational age assessment by FL is fetal useful when head measurement is difficult to biometrics due to fetal position. The accuracy of a single parameter is dependent on the gestational age at the time of ultrasound fetal Table 9. Several methods have been employed to improve the accuracy of gestational age assessment with the use of a single parameter. Two age these methods, growth-adjusted sonographic age 79 and averaging from parameters 80 , 81 are discussed.
Several principles are important ultrasound remember when assessing gestational age by ultrasound:. Gestational age can be more accurately predicted by obtaining paired BPD measurements the first from 20 to 26 from' gestation and the second from 31 fetal 33 weeks' gestation and assigning gestational age by a method developed by Sabbagha and co-workers 79 known as growth-adjusted sonographic dating GASA. Paired BPD measurements obtained at different gestational ages allows categorization of the specific cephalic growth ultrasound.
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