|Embryo at 6 weeks gestation|
A sperm and an ovum have only 23 chromosomes each. When they join, they combine their chromosomes to make 46. Ta-da! She's now a one cell embryo. She's a human with her own unique DNA. She's alive (not dead- ie. she develops and grows), and she's human, not a kitten or a camel, because she has human DNA.
She's called a zygote at this point in her embryonic stage and doesn't look like she will when she's older, but she's growing and developing. (She'll continue to do so up until sometime in her teen years, when she will reach her full height.)
But back to our zygote-- About 40 hours after she's formed, her one cell begins to divide. It will go on dividing and when she has 32 cells, she's in her morula stage (on day 4). The next day (day 5) she's already in her blastocyst stage. She now has specialized cells that will become her placenta, umbilical chord, and amniotic sack. Plus she has her stem cells, which will eventually grow into all of her tissues and organs. (In embryonic stem cell research, this is the stage she'd be destroyed to get at her stem cells.) If she lives, she'll reach the uterus by day 7 and she'll snuggle down into the lining (implant).
By day 22 her heart beats.
At 8 weeks she's called a fetus.
At 16 weeks her eyes blink.
At 20 weeks she can suck her thumb and yawn.
At 40 weeks, she's "term" and ready for birth. However, she can survive outside of the womb as early as 24 weeks.
Actually there have have been preemies who survived as as early as 21 weeks. Preemie survival rates vary depending on factors such as gestational age, health, and the medical care available. Because of that last item, medical care available, I think using viability outside of the womb to determine personhood (vs. abortion target) cruel and unfair to children outside of developed countries. A 22 week fetus in the U.S. has a better chance of surviving outside of the womb than one in a poor country, but both children are equal in their worth. Okay, that's my opinion, and I know I said I was just going to look at science. Sorry.
I found this fetal development slide show at medicinenet.com where you can see pictures of babies in the embryonic and fetal stages developing in utero.
So, if you have another idea about when human life begins, other than at fertilization, please tell me about it. Explain your thinking. :)