Why would it be necessary to only show a drawing, that's to try and trivialise and would be deceptive.

Drawings don't inherently "trivialize". Maybe the art style could, but artist's depictions can be very serious; for example, they are used to depict the proceedings of, and are sometimes even used in, courts of law.

It wouldn't be deceptive unless it's really, really good photorealism--otherwise, the audience can tell something is a drawing. In fact, you've managed to identify the key here--deception. How can you think a drawing could deceive the audience but not think that video footage without a disclaimer, that looks like it could actually be the thing being described, wouldn't? It's absurd.

The child shown is at the age and development of children that can be legally aborted though?

But it wasn't aborted. And they didn't say so. It's like trying to rile up sentiment against an accused murderer by instead showing the mangled body of someone who was hit by a bus, or trying to provoke a reaction against animal euthanasia by showing instead a dog that died of some awful-looking disease instead--whatever analogy you prefer--and saying, "Well, it could have happened, they're the same age."

If you really don't think your audience can understand a verbal or written description, if you really think you can't commission appropriate artwork, why not just say or put in text that it's a stillbirth, but at the same gestational age you're talking about? Why hide the truth? If it's really about helping your audience understand, why not actually help them understand?

It's not about fostering a rational understanding. It's about manipulating emotions, and it's hard to do that when you're honest and up-front and use disclaimers. These people wanted to show footage that would provoke an emotional reaction. They didn't have it, so they subbed in other footage--and didn't say so. That's deception to further their agenda--fraud.