I agree with ARC that the point of conception of a human egg results in a potential human. In the study of Embryology one discovers that it takes on average 36 hours for the first cell to double and divide. This occurs in the fallopian tube. It is at this point 60% split completely leaving twins (90%of the twin cell does not make it to implantation, thus leaving a 5% twinning rate). Over the next 60 hours, it forms a 64 cell ball called a morrula.
This is the mass of cells to which ARC was referring. At the end of the 5th day on average the mass of cells divide into 4 types of tissues: supporting (which include the placenta and amnionic fluid) structors, Endoderm, Ectoderm and the Mesoderm. Implantation is somewhere between end of this process and the 7th day. A lot happens in the 11 remaining weeks (77days) of the 1st trimester. At the end of this period, the embryo is no longer an embryo. It is a fetus.
Still there are no nerve connections that resemble what makes us human. What is special about humans is the human brain.The next 42 days are extremely important to this aspect of human brain development.This is at the 51/2 month of pregnancy.
The brain comes "alive". That is what is felt on the skin, the fetus is aware and will use this information. At the end of the 4th month 112 days or 28 days of being a fetus, it can feel but the fetus does not process. The functional MRI scans indicate the feeling is not even comparable to already born animals. So the option of choice does not yet exist.
It is over the next 14 days the structures between the middle brain and the outer cortex are finished minimally being connected.Then, traditionally it is called the quickening, the baby makes it self known. Yes, mother and child usually start playing. This is when is does pay off to read to the abdomen of the pregnant woman, the child will be smarter. Education can begin this early. I state based on this knowledge at 51/2 (end of 23 week) the fetus is human and not until.
Barbara J. Houk, M.D., FAPA , Psychiatrist