Fetal age assessment

• Fetal age assessment


  Used to generate an estimated delivery date (EDD)
• Estimated date of confinement (EDC)


  Another term for EDD
• Fetal age computation


  1. Using LMP

  2. Conception takes place about two weeks after LMP

  3. Birth normally occurs 40 weeks from the LMP (38 weeks from conception)
• Correct gestational age


  • Aids in planning the mode and date of delivery
  • Gauging fetal growth
  • Suggesting whether or not the pregnancy is progressing normally
• Determination of gestational age in 2nd and 3rd trimester


  1. Obtaining biparietal diameter (BPD)

  2. Head circumference (HC)

  3. Abdominal circumference (AC)

  4. Femur length (FL)

  
• No single measurement consistently dates all fetuses
• Using more than one or two parameters


  Improves the accuracy in assessing dates and fetal size

  
• Other measurements that can be used to assess fetal growth


  • Binocular distance (BOD)
  • Humeral length (HL)
  • Transverse cerebellar measurement

  
• These measurements


  • Help the clinician estimate the fetal age
  • Determine the size and development of the fetus
  • Are an important indicator of fetal health

  
• Early measurements of the embryo


  • Result in the most accurate dating
• Once the gestational age has been determined and confirmed by an early ultrasound, the date should not be changed
• At any given time in a normal pregnancy, variations in a single parameter's prediction of age will usually be less than 10%
• When using a growth chart


  • It is imperative to know the measurement methods for that chart
  • Age estimates will only correlate when using the correct method

  
• Averaging multiple measurements


  1. The preferred method of any given parameter

  2. Each measurement type needs to be a cluster ranging between 2-3 mm

  3. Values that fall outside the cluster are discarded and the remaining measurements are averaged
• Fetal age listed in charts


  • In weeks and days
  • In weeks and tenths of weeks

  
• CRL


  Most accurate measurement
• Sonographer


  • Should use the highest frequency transducer that produces adequate penetration and diagnostic-quality images
  • Linear array and curvilinear transducer, with their larger field of view, make measurements easier to obtain especially in advanced pregnancy
• All fetuses are proportioned differently, any single parameter may not be a specific indicator for fetal age
• This is especially important later in pregnancy when performing serial ultrasounds for fetal growth
• A BPD measured during the final weeks of pregnancy may be measured incorrectly due to molding
• Multiple fetal parameter (MFP) age


  • The average age obtained by using the BPD, HC, AC and FL measurements
  • Determined by Dr. Frank Hadlock and associates to be a more accurate assessment of fetal age

  
• Biparietal Diameter (BPD)


  • Usually easy to obtain
  • Has a distinctive appearance
  • Provides a relatively accurate measurement in normal shaped heads
  • Less reliable indicator of fetal age after 33 weeks due to molding and morphologic variations
  • Measured using leading edge measurements (outer skull to inner skull) in a transverse (axial) plane with a lateral approach
  • Measured across the head from one parietal bone to the other parietal bone
  • Can be measured routinely from 12 weeks gestation

  
• Obtaining the BPD


  1. Start at the base of the cranium and locate the base X formed by the bilateral sphenoid bones anteriorly and bilateral petrous bones posteriorly

  2. Move higher until the thalamus and the cavum septum pellucidum (CSP) appear

  3. The view of the BPD measurement should be made perpendicular to the interhemispheric fissure (falx cerebri)

  4. The measurement should not include any soft tissue of the scalp

  5. Moving the mother may help the fetus to move to a more optimal position

  6. A Trendelenburg position may be helpful when the head is low in the pelvis

  
• Head Circumference (HC)


  • A more accurate measurement than the BPD due to the morphologic variations in the shape of the fetal head
  • A two dimensional HC is more accurate than a one dimensional BPD
  • Measured by tracing around the outside perimeter of the calvarium, using an ellipse or by measuring two perpendicular diameters and calculating the circumference
  • Calculated using the formula (D1 + D2) X 1.57

  
• Occipital-Frontal Diameter (OFD)


  • Obtained at the level of the BPD and HC
  • Obtained by placing one caliper midline at the anterior frontal bone and another is placed midline on the occipital bone
  • Measured outer to outer
• Cephalic Index


  • A useful tool for indicating the shape of the transverse fetal skull
  • Calculated using the formula BPD/OFD x 100
  • Brachycephalic head: CI greater than 85
  • Dolichocephalic head: CI less than 75

  
• Transverse Cerebellar Measurement


  • A commonly used adjunct measurement for gestational age and growth
  • The cerebellum maintains a relationship to the gestational age and it is independent of the shape of the cranium
  • The cerebellum grows at a rate of 1 mm/week between 14 and 21 weeks
  • Measured in a transverse plane at the same level as the cisterna magna

  
• Binocular Measurement


  • Binocular distance (BOD) is the distance from the outer edges of the right and left fetal orbits
  • Can be used as a accessory fetal biometric parameter when the BPD is difficult to obtain

  
• Abdominal Circumference (AC)


  • A valuable indicator of fetal growth because it reflects the development of the abdominal organs
  • Reflects the amount of subcutaneous fat, which has a large influence on fetal weight
  • Obtained by first visualizing the long axis of the fetal spine then turning the transducer 90 degrees at the mid portion of the spine
  • The standard view for obtaining the AC is taken in an axial plane at the level of the stomach, the umbilical vein junction

  
• Binocular distance (BOD)


  The distance from the outer edges of the right and left fetal orbits
• Abdominal Circumference (AC)


  • A valuable indicator of fetal growth because it reflects the development of the abdominal organs
  • It also reflects the amount of subcutaneous fat, which has a large influence on fetal weight
  • Most modern formulas for fetal weight rely heavily on this measurement

  
• Obtaining the AC


  1. Visualize the long axis of the fetal spine

  2. Turn the transducer 90 degrees at the mid portion of the spine

  3. The standard view is taken in an axial plane at the level of the stomach, the umbilical vein junction with the left portal vein, and the transverse spine

  4. The confluence of the umbilical vein with the portal vein forms a "hockey stick" or "J" shape

  5. The AC should be circular and if possible the spine should be to the side

  6. If the veins cannot be visualized, visualizing only the fetal stomach is acceptable

  7. The abdominal circumference plane should not include the heart (too high) or the kidneys (too low)

  8. The lower ribs may normally be observed and if seen, should be symmetric

  9. The measurement can be obtained by tracing around the outside perimeter of the abdomen, using an ellipse or it can be obtained by measuring two perpendicular diameters and calculating the circumference

  10. All measurements are from the outer skin lines and include the surrounding fat

  11. If a measurement is obtained using abdominal diameters, the TAD (transverse abdominal diameter) and the APD (anterior posterior diameter) are obtained, added together and multiplied by 1.57

  12. Acoustic shadowing from the extremities may make it difficult to obtain the AC, it may be necessary to measure based on where the skin line should be
• Fetal long bones


  • Good indicators of fetal growth
  • Not subject to molding or difficult positioning
  • Affected by the genetic pool, tall or short parents tend to have babies with long or short extremities
  • More accurate in the second trimester, because the genetic differences do not exhibit as much as they do in the third trimester
  • At term the FL accuracy is about +/- 2.2 weeks
  • The greatest source of error occurs with a slightly oblique transducer position to the bone rather than along its long axis and does not include the entire bone
• Femur Length


  • Among the most commonly used parameters for estimating fetal age and may be more accurate than the BPD in late pregnancy
  • Can be reliably used after 14 weeks
  • The measurement is actually the measurement of the shaft of the femur, it does not include the femoral head or the distal femoral condyles
• Imaging the femur


  1. Scan down the body to the level of the bladder and rotate the transducer until the longest axis of the femoral shaft is seen

  2. When viewed from the posterolateral aspect, the femur may have a bowed appearance, but this does not affect the measurement

  3. The femur nearest the transducer should always be measured
• Humeral Length


  • The humerus nearest to the transducer is usually easy to find, but the humerus deep to the transducer is often obscured by the fetal ribs or spine
  • The humerus has measurements similar to the femur
  • The humerus is best located by determining fetal lie, then scan the fetal head and move caudally until you see the neck and then the shoulders
  • It is important to make sure that you are measuring the humerus (single bone) and not the radius or ulna (side by side bones)
  • The recommended measurement includes the entire ossified portion to the end of the epicondyle
• Forearm and Lower Leg


  • It may be difficult to measure the distal bones of the extremities because of fetal movement
  • If there is a disparity in measurements of the other parameters, the distal long bones may be used to resolve any conflict in age estimates
  • Measurements of these bones are not routinely obtained, but the bones should be evaluated for normal shape and contralateral symmetry
• Imaging the forearm and lower leg

  1. Once the humerus has been located, follow it to the elbow then turn the transducer to determine if the lower arm is in flexion or extension

  2. The ulna is the larger of the two bones and is anatomically medial in location

  3. If the wrist and hand is rotated, the ulna and radius will be crossed rather than parallel

  4. The lower leg also has two bones, the tibia and the fibula

  5. It can be difficult to image due to activity and position

  6. The tibia is the larger of the two bones
• Fetal Weight


  • One of the most often-sought parameters of fetal growth
  • Low birth weight, or intrauterine growth restriction (IUGR) has been associated with higher incidences of neonatal morbidity and death
  • Macrosomia or large birth weight babies, > 4000 g, are also at risk of maternal and neonatal morbidity
  • Estimation of fetal weight allows the doctor to manage the pregnancy and be alerted if there are any developing problems
  • Most current ultrasound machines calculate the weight based on the fetal measurements
  • Gestational age, sex, maternal height, weight at first visit, ethnic group, parity, and smoking all effect birth weight
  • Sonographers must be very careful when taking measurements used in weight estimates, particularly the AC because it is easy to err on the small side and under estimate fetal weight
• Fetal Parameter Ratios


  • Ratios help to assess fetal proportionality
  • They are used for comparing the relative size of two parameters
  • Many fetal parameters grow at different rates, so it is difficult to compare their sizes directly
  • In a ratio, the so-called standard is the denominator. As the comparative numerator increases, the ratio also increases. As the comparative numerator decreases, the ratio also decreases
  • One of the more commonly used ratios is the HC/AC. As the abdomen becomes larger, relative to the head, the ratio becomes smaller. As the abdomen becomes smaller, the ratio become larger
  • The HC/AC ratio is normally 1:1 at 34 weeks LMP and progressively gets smaller. The normal ratio at 34 weeks is approximately 1.04 and at 39 weeks it is 0.99
  • Other ratios comparing two parameters are also useful