It takes about 5-10 days for a future embryo to implant in the uterus (it's not called an embryo until it implants), and I've always pictured this phase as a lone cell floating down the fallopian tube, just vegetating waiting to implant, so I was surprised to find out that as soon as conception happens, a zygote's cells begin dividing and arranging appropriately. All the while the zygote is traveling towards the uterus, this new human is growing. Once a new human is put into motion, it's all about the business of growing--there is no hanging around idly waiting to find out whether or not it can really get the show on the road, so to speak, depending on its ability to burrow into the womb for nourishment. Growth is already happening.
It's always hard for people to understand others who are not like themselves; people of different religion, race, even gender, are puzzling, and understanding the significance of a zygote or an embryo is no exception. It's especially hard to accept something that doesn't look like us as one of us. But as Pope Paul IV said, "how could a human individual not be a human person?".* From conception, a zygote is human, there is no other species it can be assigned to. Some would argue that while it is human, it is not a person and therefore is expendable, but there is no wisdom in this. There is no biological benchmark for a tiny human that clearly proclaims 'okay, NOW I'm fully a person and have rights'. All the benchmarks people have tried to come to consensus over as to when a human being constitutes a human person that should not be killed--end of the first trimester, viability, etc.--are arbitrary. It's like trying to tack up our own blueprint of development over the one God created in the beginning. We can't figure out the mind of God. We can't. But we can know that from the beginning, God has a plan for every life, and to intentionally disregard the sacredness of his plan is a grave moral error.
For you created my inmost being;
you knit me together in my mother's womb.
I praise you because I am fearfully and wonderfully made;
your works are wonderful,
I know that full well.
My frame was not hidden from you
when I was made in the secret place.
When I was woven together in the depths of the earth,
your eyes saw my unformed body.
All the days ordained for me
were written in your book
before one of them came to be.