Summary: Generally speaking, doctors mainly measure and calculate the size of the fetus through ultrasound examination. So, what factors determine fetal size? Does the mother have many factors? Or is it more because of his father? Take a look at the following Q&A. Can it help you clear up your doubts? Whether the fetus develops normally in the womb is what expectant mothers are most concerned about. Doctors usually judge whether the fetal growth and development is normal based on the size of the fetus and the number of weeks of pregnancy of the expectant mother.