Great question! I love bringing up biology in the abortion debate, because it really is on our side.
However, it’s important to clarify what the pro-choice person means when they say the embryo isn’t a person. Many people will agree that embryos are biologically alive and human, but will not consider them a person with rights and value. If that’s the case, you have to switch gears and talk about the philosophy of human rights. Our basic argument is that all humans should have equal human rights, including the preborn. If that isn’t the case, then that means one group of humans can deny human rights to another group, and that’s never a good idea.
If they are arguing biologically, there’s a lot you can use. EHD.org is a good website for prenatal development facts with textbook and journal article sources. They also have videos of different developmental stages.
Dr. Maureen Condic is also an awesome source. She’s a professor of Neurobiology and Anatomy, and has written a lot of great stuff. Here’s a PDF of an article she wrote on how we know a zygote is a new human being.
These are good sources to back up your point. But here are three basic questions that include both the biological and philosophical arguments:
1. If the embryo is growing, isn’t it alive?
2. If it has human parents, isn’t it human?
3. If living humans like you and me have human rights, then why don’t they?
I recommend asking these questions one at a time and getting an answer to each one before moving on.