Final answer:
Two ratios, the Crown-Rump Length (CRL) and the Biparietal Diameter (BPD), are measured during an ultrasound to estimate fetal age, with the technique being preferred due to its safety and effectiveness in monitoring fetal growth and development.
Step-by-step explanation:
In order to determine fetal age, two crucial ratios are measured during an ultrasound: the Crown-Rump Length (CRL) and the Biparietal Diameter (BPD). The CRL is the measurement of the length of the fetus from the top of the head (crown) to the bottom of the buttocks (rump), typically used up until around 14 weeks of gestation. The BPD measures the distance between the sides of the fetal head and is generally used from about 14 weeks until birth.
These measurements are vital as they provide an estimation of gestational age, which is important for determining the expected date of delivery (EDD) and assessing fetal growth and development. The ultrasound technique is ideal for studying fetal growth and development because it is non-invasive, provides excellent image resolution, and is safe for both mother and fetus. Unlike X-rays, ultrasound does not use ionizing radiation, which can be harmful.