Which is the most likely reason that a doctor might require a pregnant woman to get ultrasounds more frequently?

Let's start from the beginning.

Ultrasounds are a type of radiology equipment. Ultrasounds(sonography) can be used to see images inside of the body, including kidneys, uterus and many other internal organs.

Ultrasounds are commonly used for pregnant individuals.another common use is to ee the brain and hips in infants

There can be many complications in a pregnancy. Ultrasounds can determine the sex of the fetus, see if there are amino bands around the limbs( which can cause limbs to fall off, etc. Most ob/gyns want to keep the pregnant individual calm and healthy as well of the fetus.