The process leading up to the birth of a newborn baby can be divided into many steps:

about 1 month before conception: almost all adult males produce thousands of spermatozoa (male germ cells) each second. It would take about 500 of them lined up in a row to total 1 inch in length. They take a month or so to travel from a testicle, through a long tube called the "vas deferens," to reach a small reservoir inside the man's prostate gland. Here, semen (a mixture of spermatozoa and various fluids) is formed. Strictly speaking, beginning of human life happens when a spermatozoon is formed, because: each spermatozoon is alive. If you don't believe this then look at how energetically these little squigglers move under a microscope.
Each spermatozoon contains human dna.

Perhaps one day before conception: the woman ovulates and produces one mature ovum (egg cell). It travels down one of her fallopian tubes towards her uterus. It is about 1/100" in diameter, and is barely visible to the naked eye. It also is a form of human life, for the above reasons.

At conception: one very lucky spermatozoon out of hundreds of millions ejaculated by the man will penetrate the outside layer of the ovum and fertilize it. The surface of the ovum changes its electrical characteristics and prevents additional sperm from entering. A genetically unique entity is formed shortly thereafter, called a zygote. Half of its 46 chromosomes come from the egg and the other half from the spermatozoon. It has a unique dna structure, different from that of the ovum and the spermatozoon. This is the point that most, or all, pro-life groups and conservative christians define as the start of pregnancy. They also define this as the transition from human life (in the form of an ovum and sperm containing human dna) to a human person. The fertilized ovum divides as it passes down the fallopian tubes; it is then called a morula (a.K.A. Pre-embryo).

5 days or so after conception: the morula, now called a blastocyst, has traveled down the fallopian tubes and has started to attach itself to the endometrium, the wall of the uterus (a.K.A. Womb). It continues to be referred to as a pre-embryo.

9 or 10 days after conception: the blastocyst has fully attached itself to endometrium. Primitive placental blood circulation has begun.

12 days or so after conception: the blastocyst has started to produce hormones which can be detected in the woman's urine. This is is the event that all (or almost) all pro-choice groups and almost all physicians (who are not conservative christians) define to be the start of pregnancy. If instructions are followed exactly, a home-pregnancy test may reliably detect pregnancy at this point, or shortly thereafter.

13 or 14 days after conception: a "primitive streak" appears. It will later develop into the fetus' central nervous system. This is the point at which spontaneous division of the blastocyst -- an event that sometimes generates identical twins -- is not longer possible. The pre-embryo is now referred to as an embryo. It is a very small blob of undifferentiated tissue at this stage of development.

3 weeks: the embryo is now about 1/12" long, the size of a pencil point. It most closely resembles a worm - long and thin and with a segmented end. Its heart begins to beat about 18 to 21 days after conception. Before this time, the woman might have noticed that her menstrual period is late; she might suspect that she is pregnant and conduct a pregnancy test. If it is an unwanted pregnancy, she might have arranged and carried through with an abortion.

4 weeks: the embryo is now about 1/5" long. It looks something like a tadpole. The structure that will develop into a head is visible, as is a noticeable tail. The embryo has structures like the gills of a fish in the area that will later develop into a throat.

5 weeks: tiny arm and leg buds have formed. Hands with webs between the fingers have formed at the end of the arm buds. Fingerprints are detectable. The face "has a distinctly reptilian aspect." 1

6 weeks: the embryo is about 1/2" long. The face has two eyes on the side of its head; the front of the face has "connected slits where the mouth and nose eventually will be." 1

7 weeks: the embryo has almost lost its tail. "the face is mammalian but somewhat pig-like." 1 pain sensors appear. Many conservative christians believe that the embryo can feel pain. However, the higher functions of the brain have yet to develop, and the pathways to transfer pain signals from the pain sensors to the brain have not developed at this time.

2 months: the embryo's face resembles that of a primate but is not fully human in appearance. Some of the brain begins to form; this is the "reptilian brain" that will function throughout life. The embryo will respond to prodding, although it has no consciousness at this stage of development. The brain's higher functions do not develop until much later in pregnancy.

10 weeks: the embryo is now called a fetus. Its face looks human; its gender may be detectable.

3 months: the fetus is about 3 inches long and weighs about an ounce. Fingernails and bones can be seen. Over 90% of all abortions are performed before this stage has been reached.

4 months: it is 8" long and weighing a half pound. The fetus' movements may begin to be felt. Its heartbeat can usually be detected.

5 months/21.8 weeks: 12" long and weighing about a pound, the fetus' has hair on its head. Its movements can be felt. An abortion is usually unavailable at this gestational age because of state and province medical society regulations, except under very unusual circumstances. Half-way through this month, the fetus' lungs may be developed to the point where it would have a chance to live on its own.

6 months/26 weeks: 14" long and almost two pounds. The lungs' bronchioles develop. Interlinking of the brain's neurons begins. Some rudimentary brain waves can be detected. The fetus will be able to feel pain for the first time. It will become conscious of its surroundings. State laws and medical association regulations generally outlaw abortions at this stage, except under very unusual circumstances.

7 months: 16" long and weighing about three pounds. Regular brain waves are detectable which are similar to those in adults.

8 months: 18" long and weighing about 5 pounds.

9 months: 20" long and with an average weight of 7 pounds, a full-term fetus' is typically born about this time.

Now, I do not see how any of this supports the logical/philisophical/practical/moral/ethi cal reasons for abortion... Does anyone differ in opinion?