The reduced accuracy of prediction of gestational age after 20 weeks must be appreciated.At any gestation, if the ultrasound fetal measurements of each parameter are not in agreement, the reason for this difference should be evaluated.Percentile Example: Out of 100 babies, a reading of forty percent (this is the percentile value) indicates that the baby is smaller than sixty other babies and larger than forty other babies. A value that reads below 50% indicates that the measurement is lower than the average.Thus, a value that is above 50% indicates that the measurement is above average.The accuracy rate is very low, many women are pressured into interventions that do more harm than good, and even the mere PREDICTION of macrosomia (large baby) alters the way physicians perceive and treat labor.
This should include at least two fetal measurements (usually biparietal diameter (BPD) and femur length (FL)) plus a consideration of additional parameters such as head circumference (HC), occipitofrontal diameter (OFD), abdominal circumference (AC) and humerus length (HL).
The abdominal circumference is measured at the level of the liver and stomach, including the left portal vein at the umbilical region.
If we assume a fetus is growing normally, biometric measurements are determined by gestational age, and we can estimate the gestational age and thus the due date.
If measurements taken after the first trimester are within one week of the gestational assessment taken from menstrual dating then the ultrasound assessment of gestational age confirms the menstrual dates.
If the ultrasound measurements are in agreement and differ from menstrual dates by more than one week prior to 20 weeks a new estimated due date should be calculated and recorded.