It's true, an acorn isn't an oak tree. But all this really shows is that an infant isn't an adult. An acorn is the immature stage, and the tree is the mature stage, but both are oaks. An acorn can become an oak tree, but it never becomes an oak. It is already a complete oak, even at the infant stage.
The confusion can be cleared up by asking a simple question: "What kind of seed is an acorn?" There's only one answer: an oak. Ask the same question at every stage of growth-seed, sprout, sapling, or tree-and the answer is always the same.
"But the unborn doesn't look like a human being."
Sure it does. It looks exactly as a human being should at this stage of his growth. In our experience we are familiar with only certain phases of human development: newborn, child, adolescent, and adult. The reason why we're reluctant to call a developing unborn a human is because he looks so different from the stages of growth we're familiar with.
This teaches us an important lesson. Living things never look the same at one stage of development as they do at another. That's what it means to develop biologically: constant change according to a predetermined growth pattern.
Stand firm in Christ,
Koukl, Greg. Precious Unborn Human Persons. Pages 18-19.