This question has taken on major proportions of debate and controversy, between supporters of Pro-life and groups of Pro-choice. To most, the answer is distinctly clear about when life begins. It begins at conception, when the sperm fertilizes the egg.
Dr. Landrum Shettles, sometimes called the "Father of in vitro fertilization" notes about life's beginnings: "Conception confers life and makes that life one of a kind." Continuing on his comments about the Supreme Court ruling on Roe vs. Wade he says, "To deny a truth (about when life begins) should not be made a basis for legalizing abortion."
Dr. Albert Liley, the renowned physiologist known as the "Father of Fetology" has stated, "Biologically, at no stage of development can we subscribe to the view that the unborn child is a mere appendage of the mother. Genetically, the mother and baby are separate individuals from conception."
The baby is autonomous of the mother and has been demonstrated successfully since the introduction of test tube babies. The ovum or the egg is fertilized with the sperm inside a laboratory facility. The zygote, a fertilized egg, begins to divide and grow looking like a bag of marbles even though it may be countries away from the mother.
This halts the viewpoint that the fertilized egg (or pregnancy tissue, as Planned Parenthood calls it) is an appendage of the mother. If any other part of the mother's anatomy is removed, such as an appendix, it will not continue to live. But the fertilized egg has shown its autonomy in survival outside the mother's body, and has further shown its autonomy implanted into another woman. The fertilized egg does not take on the surrogate mother's characteristics, but develops according to its own DNA make up.
Within four days of fertilization, the sex of the zygote can be determined through microscopic techniques. Soon after one week of fertilization, the zygote contains the beginning of all major body structures. Fetal circulation occurs by three weeks - complete with heart and major blood vessels coursing through its quarter-inch frame. And at five weeks, all basic body systems are developing, including the brain and nervous system enabling the fetus to feel pain.
When the zygote reaches the eighth week, it is called a fetus for the remaining of the fetal development. The fetus at twelve weeks of growth shows tooth buds, nail beds, and genitals. By thirteen to sixteen weeks, the arms and legs are complete. There is air on the head, the skeletal system shows up on x-ray film and has breathing movements. By the twentieth week, the baby has eyebrows, tiny nipples, sucks its thumb, and holds its own umbilical cord.
If the sexual gender of a fertilized egg can be determined within four days of conception, then this pregnancy tissue is more than an appendage of the mother. It is the beginning of a life.
Read also this resource material from Princeton University: Life Begins At Fertilization
"Before I formed you in the belly I knew you, and before you came forth out of the womb I sanctified you." (Jeremiah 1:5)
"For I know the thoughts that I think toward you, says the Lord, thoughts of peace, and not of evil, to give you hope in your latter end. You shall call on me, and you shall go and pray to me, and I will listen to you. You shall seek me, and find me, when you shall search for me with all your heart." (Jeremiah 29:11-13)
"I will give thanks to you O Lord, For I am fearfully and wonderfully made. Your works are wonderful. My soul knows that very well." (Psalms 139:14)