It's not an opinion.

The whole structure of a sperm or egg, is different than the when they are united. Check out the number of chromozomes in the cells before and after uniting.

As the unit grows, the only difference is the number of cells. The chromozome number remains the same.

The differences have to do with the formation of the organs, BUT, organ formation is already programmed into the DNA, and remains programmed therein throughout life. So really, there is no difference between an embryo and an adult human regarding that he/she is a person.