That's not even true because there are such things as stillbirths etc. even when the fetus is developing in a natural state.

What part of interference is confusing you? There’s nothing in the biological design of a zygote that doesn’t lead to sentience. Interference in the development process is what creates a stillbirth, whether it is genetic mutation, lack of nutrients, abortion etc.

Also sentience absolutely can be objectively measured, we do it with animals all the time.

No we have a scaled concept that is constantly in flux. If you ask 500 bioethicists what sentient means you get 50000 answers. Like love, sentience is a concept, but not one held to any rigidity or objective standards.

Sentient comes from the Latin sentient-, meaning "feeling," and describes things that are alive, able to feel and perceive, and show awareness or responsiveness.

Again these are all subjective descriptions.