How is using existing science in determining whether to abort a baby due to its sex equal to eugenics?

Eugenics, by definition is the science of improving a human population by controlled breeding to increase the occurrence of desirable heritable characteristics, developed largely by Francis Galton as a method of improving the human race. If you take the ultrasound see it is a girl, you wanted a boy, so you use other science to, <”whoomph”> suck it out and start again, how is that eugenics? If the child was left to be birthed, it would have no enhancements or improvements. If you had three boys, did not want another but saw one was in the oven and decided not to go through with it but give it another try to see if you’d get a girl, why not if the science is there and there is no entity you need to speak to behind your choice?