Ultrasound examination in 2nd and 3rd trimester of pregnancy
Basic structures to be identified and measured in the second and third quarter are:
– biparietal diameter;
– Head circumference;
– Abdominal circumference;
– Femur length.
With these parameters we can estimate gestational age.
1. Biparietal Diameter (BPD)
To measure this diameter is needed a transverse section of the fetal skull containing the following anatomical landmarks: anterior and posterior cerebral falx, anterior cavum septi pelicidum on medial line, choroidal plexus of each lateral ventricle, thalamus nuclei and cerebral artery pulsations.
Biparietal diameter is measured from the outer surface of the skull to the inner surface of opposite wall. BPD measured during the second quarter of pregnancy is the best parameter to assess gestational age accurately. After 20 weeks there is a progressive increase in the variability of BPD dimensions related to gestational age , by the end of the third quarter. BPD variability at the end of the third quarter in assessing gestational age is ± 31/2 weeks.
2. Cranial Circumference (CC)
Head circumference is a parameter that considers the developing of fetus. However, head circumference is a parameter which can be used accurately to assess gestational age, but like other parameters it has a variability that increases with gestational age.
At the end of the third quarter circumference variability in the assessment of gestational age is ± 3 weeks. Cranial circumference measurement is performed on the same section as for biparietal diameter.
3. Abdominal Circumference
Determining the abdominal circumference is using a cross-section of the fetal abdomen at the liver level, which should include the following points: umbilical portion of the portal vein and fetal stomach. This parameter shows a variability increase with pregnancy age in gestational age assessment. Between 26-31 weeks the abdominal circumference seems the most accurate in determining gestational age. However, with this parameter we can estimate fetal growth and development.
4. Femur Length
Femur length determination is made by positioning the transducer along the axis of the femur. Is measured only the shaft length, not including the epiphyses. Most studies suggest that femur length is the parameter with the greatest accuracy in estimating gestational age in the second trimester of pregnancy, and some authors argue that this accuracy is maintained in the third quarter.