Fetal Age Study
A fetal age study, also called biometry, is part of an obstetric ultrasound exam. The study uses ultrasonography to take measurements of several fetal anatomic structures, compare the results to expected values, and convert that information into an estimated gestational age of the fetus.
As imaging technology has improved resolution of the fetal structures, care must be taken that the measurements start and end at the correct locations. The skill and experience of the ultrasound technologist are a critical component in obtaining reliable results. In general, ultrasonography dating of a pregnancy is more accurate than dating using the date of the mother's last menstrual period.
Accurate dating of a pregnancy is the foundation on which obstetrical management relies. The date of the mother's last menstrual period is often used to estimate delivery date, but it is accurate to within only about two weeks. By comparison, the biometric crown-rump measurement of the six to 12-week fetus provides the most accurate measurement, within three days of true gestational age. First-trimester biometric measurements may be obtained using either an abdominal or transvaginal transducer. Transvaginal ultrasonography can visualize fetal structures about one week in advance of abdominal sonography. Since the fetal structures are better visualized in the second trimester than in the first, a routine ultrasound is usually performed during that period, usually between 16 and 20 weeks gestation. Early sonography may be performed if a first-trimester pregnancy appears to be threatened, or if there is maternal bleeding, pelvic pain, or a concern that the pregnancy may be ectopic. Biometric dating may be used to determine the best time to perform chorionic villi sampling or maternal alpha-fetoprotein levels.
Since many pregnancies are not dated until the pregnancy is more advanced, different structures are measured to obtain fetal age, depending on the suspected gestational age. As gestational age increases, normal variability between fetuses increases. To some degree, then, biometric accuracy decreases with advancing gestational age. After about the twelfth week of gestation, the fetal position and movements make crown-rump measurements less accurate. Other anatomic structures will then be measured. From 12 to 18 weeks gestation, biometric measurements are usually accurate to within a week of gestational age. After 26 weeks gestation, fetal variability has greatly increased and the dating provided by biometric measurement is less accurate, but the method is still used. By late in the third trimester, measurements are accurate only within a two-to three-week window. In general, once the crown-rump measurement is no longer reasonable to obtain, the averaging of several anatomic measurements is more accurate than the use of one measurement alone. Results outside of the expected outcome should be further evaluated, to check for fetal anomalies.
The four most common measurements taken during biometry are the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). Because normal fetal variability is present between the 10th and 90th percentiles, the basis for comparison is usually the 50th percentile. However, imaging technology is sufficiently advanced so that almost any fetal structure can be measured and compared against normal ranges.
Fetal growth is based on the fetal size in relation to the expected gestational age. Normal fetal size and weight values are expected between the 10th and 90th percentiles. Fetal size or weight below the 10th percentile is considered small for gestational age (SGA). A fetus with measurements above the 90th percentile is considered large for gestational age (LGA), and may be an indication of fetal response to maternal gestational diabetes. The diagnosis of an SGA or LGA fetus impacts the future obstetric management of the pregnancy, and assists in avoiding complications related to these conditions. Imaging technology software can estimate fetal weight using the measurements obtained for size and their relation to fetal mass. Estimated fetal weight may be used to check for fetal growth, or may be used in cases of fetal surgery or when the fetus needs to receive medication or a blood transfusion.
Biometry is a non-invasive sonographic fetal assessment tool. In early pregnancy, the mother may need to drink water prior to the test, without urinating until after the test has concluded. A full bladder is necessary to allow for transmission of the sound waves. As pregnancy advances, sufficient amniotic fluid is present so that fluid consumption prior to the test is not required. In order to
Gestational diabetes—Diabetes that first occurs during pregnancy and resolves once the pregnancy is terminated.
Transducer—The external device or probe used in conjunction with the ultrasound machine. Applied on the outside of the abdomen with a special gel, it bounces sound waves into the area being visualized, and then sends the return waves back to the computerized ultrasound machine for interpretation and visualization on the monitor. A transvaginal transducer is inserted into the vagina in order to be closer to the structures being evaluated.
Trimester—The average length of pregnancy is 280 days. The pregnancy is often divided into three trimesters, as different fetal development takes place in each of the three time periods. The first trimester extends to 14 weeks, the second through 28 weeks, and the third until 40/42 weeks, or term.
perform the ultrasound, a transducer is placed externally to the mother's abdomen. Use of a special gel allows the transducer to glide over the surface of the abdomen during scanning. Because the abdomen is exposed, privacy should be provided either with the use of a curtain or with a closed door. The mother should be asked if she would like her partner or support person with her during the test. A comfortable room temperature and warmed transducer gel assist in putting the mother more at ease. A towel or cloth placed over the mother's clothes protect them from becoming wet from the gel.
When the test is completed, the abdomen should be wiped dry of the transducer gel. If the mother needed to consume water prior to the test, she may need to urinate. She may require assistance in getting up from the ultrasound table. If non-reassuring test results have been conveyed to the mother, she may need a health care professional to stay with her for emotional support and to answer questions.
Because a fetal age study is noninvasive and observational, no complications are anticipated. If the measurements were not obtained carefully, the wrong gestational age will be determined. Interventions undertaken based on inaccurate findings could be considered a complication or risk of the procedure.
The biometric results obtained are compared with norms for size and age, in order to estimate fetal age. Results outside of the normal range may indicate an SGA or LGA fetus, and require additional evaluation. Biometry may be used later in pregnancy when the fetal age is unclear. In this case, serial measurements may be taken after a two-to three-week period. The fetal growth during this time period can be compared against normal growth rates to estimate the gestational age. Because of rapid periods of fetal growth, each gestational week is also broken down further into the number of days. For example, the fetus's weight and length at 27 1/7 weeks is different from that for a fetus of 27 6/7 weeks.
Health care team roles
The fetal age study is usually performed by an ultrasound technologist in the imaging department. Non-routine biometry testing may provoke maternal anxiety over her baby's well-being. The ability of the technician to provide a calm, professional environment can help put the mother at ease.
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