First and most importantly, JD is right, this is a moral issue and not scientific.  If you want to get technical, once fertilization occurs (a haploid sperm + a haploid egg cell come together to form a diploid zygote with 46 chromosomes) that zygote has the equivilent DNA of a fully developed human.
But everyone that is not pro-choice must put themselves in the place of a pregnant woman.  Pregancy is not simply "having the baby".  The consequences of having a child when a woman is not "ready" are devastating to the woman and the child.  You must understand that a woman's body will be physically altered for 40 weeks.  Often young girls can even be physically harmed by the pregancy.  Smaller girls have know to have longterm lower back problems for example.
Following those 40 weeks a woman must give birth.  Something that we as men cannot even comprehend.  I was present when my sister gave birth.  Many people will say that they know what it is like, but it's completely different when you are there in person.  Even with the medical advances of today, it is still extremely painful and stressful.  If i had time I'd scan the hormone chart from my human phys textbook.  It is the highest level of hormones ever active in the body at one time.  Not to mention that many girls that aren't "ready" are more likely to have a cesarean section.
Complications during birth can risk the life of the mother and the child.  Even after the birth, the mother is physically disabled for a time period.  And this does not even touch on the social aspects of the birth.
I love the idiot that mentioned puting the child up for adoption.  As of September 2003, over 500,000 children were left in temporary foster care in the US alone.  That does not even include the ones not in foster care, that are living in orphanages.
http://faq.acf.hhs.gov/cgi-bin/acfrightnow.cfg/php/enduser/std_adp.php?p_sid=GkEhnZ5i&p_lva=&p_faqid=47&p_created=1001634224&p_sp=cF9zcmNoPSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9OSZwX2NhdF9sdmwxPTEwJnBfY2F0X2x2bDI9MTUmcF9wYWdlPTE*&p_li=
Another problem facing the adoption issue is the mass amounts of foreign adoptions.
http://www.google.com/search?hl=en&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=adoption&spell=1
Foster care is never good for these children.  Most simply bounce around from home to home.  This is because some parents aren't willing to fully adopt a child.
Getting back to the effects of the birth on the mother.  Here is a very good analysis:
http://www.brook.edu/comm/policybriefs/pb05.htm
Statistics show that the mother and child will both most likely suffer the reprocussions of the birth for the rest of their lives.  When a young girl become pregant out of wedlock, she normally has to take a break from highschool or college.  Sadly a lot of the time many never go back to finish, quite frankly because they can't.  They have the financial responsibility of caring for a child.  Also, a birth out of wedlock often cause a great rift between the mother and her family.
And most of the time the father has limited involvement.  The journal above about "shotgun marriages" is very interesting.
This is why it should be the woman's right to choose.  The effect on men is practically nothing compared to that of the mother.  I'm sorry if I put society and statistics ahead of the bible.  Have fun in heaven I guess.