Ultrasound is a key diagnostic tool used to monitor the development of the fetus and detect any potential health issues. If an ultrasound shows that the fetus has a smaller head than expected for its gestational age, it can be caused by a variety of factors. In some cases, the baby may still be growing normally and be healthy, while in other situations, it could indicate a medical condition or developmental concern. Regular ultrasound scans help doctors track the fetus's growth and ensure it is developing as expected.