The Prophet Muhammad (pbuh) said:

"When an embryo passes 42 days, God sends an angel to blow spirit in it, so He creates ears, eyes, skin, flesh, and bones."

Well, I have already talked about embryology in different posts. Look at the steps in this picture:

The steps that the foetus undergoes in the womb to take human shape takes 42 days exactely as the Prophet Muhammad (pbuh) alluded to in his Hadith and as scientists of embryology explained:

During the sixth week after fertilization, the unborn child can respond to local tactile stimulation by reflex movements. At the end of the sixth week, the unborn child is clearly recognisable as a human being by gross morphological observation.

The eye is obvious about 42 days after fertilization.

The eyes are deveveloping rapidly,the ears are developing rapidly, 7th week after conception.

http://www.sfuhl.org/k_appendix_1_sixth_week.htm

Now the question is: why does Islam considers abortion as illegal?

Muslim scolars concluded from the Hadith of the Prophet Muhammad (pbuh) that abortion should be made illegal, especially, after the passage of 42 days, and said that a woman could abort before this time, only when pregnancy could endanger her life or general health. The secret is that the day 42 is the time of the 'blowing' of the soul into the child's body. The soul stirs life within the baby's body and, then, the heart and brain begin its function as it has been noted by modern embryology. Reseachers say that the age of 42 days should make it illegal for a woman to abort her baby, because at this time the baby undergoes a crucial change from the status of a feotus to a full human being, and any abortion in this stage is a murder of human life.