Fetal Age Study Health Article

Definition

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.

Purpose

Biometry is used to determine fetal age. Serial biometry may be performed to assess fetal growth rate.

Precautions

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.

Description

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.

Preparation

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