The Bible doesn't give us a scientific explanation of when life begins, but the Bible writers clearly had a conception (please excuse the pun) of life before birth. I am quoting below a few texts for you to consider. Notice that many of them talk about being created in the womb. If you are relating the beginning of life to the creation of Adam, remember that Adam was created as an adult and never spent time in the womb, whereas our creation begins with a tiny fertilized egg. I don't believe that the fact that God breathed life into Adam should be extrapolated to mean that life begins only when a baby can breathe on its own. Unborn babies still have the breath of life from God even though they do not literally breathe on their own; their breath comes through their mothers.
Anyway, that would not be clearcut either. People would still disagree on whether that meant when the baby's lungs are fully developed or when the baby is actually born and takes its first draft of real air. Would a viable premature baby who can't breathe on its own yet but has to be given artificial respiration actually be alive, or could we kill it?
JOB 31:15 Did not he who made me in the womb make them?
Did not the same one form us both within our mothers?
PS 22:9 Yet you brought me out of the womb;
you made me trust in you
even at my mother's breast.
PS 22:10 From birth I was cast upon you;
from my mother's womb you have been my God.
PS 139:13 For you created my inmost being;
you knit me together in my mother's womb.
ISA 44:2 This is what the LORD says--
he who made you, who formed you in the womb,
and who will help you:
Do not be afraid, O Jacob, my servant,
Jeshurun, whom I have chosen.
ISA 44:24 "This is what the LORD says--
your Redeemer, who formed you in the womb:
ISA 49:5 And now the LORD says--
he who formed me in the womb to be his servant
to bring Jacob back to him
and gather Israel to himself,
for I am honored in the eyes of the LORD
and my God has been my strength--
JER 1:5 "Before I formed you in the womb I knew you,
before you were born I set you apart;
I appointed you as a prophet to the nations."
If God knows us even before He forms us in the womb, how can anyone presume to have the right to thwart His plans for us?
JER 20:13 Sing to the LORD!
Give praise to the LORD!
He rescues the life of the needy
from the hands of the wicked.
JER 20:14 Cursed be the day I was born!
May the day my mother bore me not be blessed!
JER 20:15 Cursed be the man who brought my father the news,
who made him very glad, saying,
"A child is born to you--a son!"
JER 20:16 May that man be like the towns
the LORD overthrew without pity.
May he hear wailing in the morning,
a battle cry at noon.
JER 20:17 For he did not kill me in the womb,
with my mother as my grave,
her womb enlarged forever.
JER 20:18 Why did I ever come out of the womb
to see trouble and sorrow
and to end my days in shame?
This is kind of a poetic lament, but it shows that Jeremiah had the idea that God could have killed him in the womb.
HOS 12:2 The LORD has a charge to bring against Judah;
he will punish Jacob according to his ways
and repay him according to his deeds.
HOS 12:3 In the womb he grasped his brother's heel;
as a man he struggled with God.
LK 1:39 At that time Mary got ready and hurried to a town in the hill country of Judea, 40 where she entered Zechariah's home and greeted Elizabeth. 41 When Elizabeth heard Mary's greeting, the baby leaped in her womb, and Elizabeth was filled with the Holy Spirit. 42 In a loud voice she exclaimed: "Blessed are you among women, and blessed is the child you will bear! 43 But why am I so favored, that the mother of my Lord should come to me? 44 As soon as the sound of your greeting reached my ears, the baby in my womb leaped for joy. 45 Blessed is she who has believed that what the Lord has said to her will be accomplished!"
What I see consistently throughout these verses is that God guides us and is involved in our lives not only from birth but even in the womb, when He first creates us, and He has plans for our us even before He makes us, even before our parents know about us.
Therefore, I believe that life begins at conception and that to intentionally destroy that life is murder. Any other definitions of when life begins--when the egg implants, when the heart starts beating, at three months or six months, when the fetus is viable, at birth--seem like rationalizations to justify something unjustifiable.