What people do know: (from 1880s embryology)
Fertilization is a unique defining moment. So human life begins then.
What people don't know:
1) Identical Twins: The embryo splitting is a creation event that occurs several to many days after fertilization. Only about one in eight twins survives.
2) Chimera: Though rare, the opposite of twinning is now known to sometimes occur, the fusing of two separately-fertilized embryos into a single embryo. This is in a sense an anti-creation event, the ending but not death of an individual. They have mixed blood type, and in half of cases they are both sexes in one individual.
3) High Attrition: The human reproductive system does not work by getting everything right in the first place, but instead by having a chance of getting things right and then culling out the mistakes, lots of them. Some 1/2 to 2/3 or more of fertilized eggs do not survive to advanced fetuses, the attrition being highest before but also continuing after implantation. Trying to get pregnant causes the death of far more embryos than taking the pill. The pill prevents egg formation, giving nothing to ovulate or to fertilize.
4) Predestined Attrition: About half the time the reason for embryo failure is due to cytoplasmic or chromosomal deficiencies in the egg at the time of ovulation. There is predestination here. Many eggs that become fertilized are predestined before fertilization to become embryos or fetuses that soon die.
5) Down Syndrome: Trisomy eggs are a normal occurrence, except that trisomy 21 becomes abnormal when it fails to self abort like the others, some 5,500 cases of Down syndrome being born in the US in 2006. The probability of occurrence is higher if the mother has problems metabolizing folic acid and increases dramatically with any mother's age. Down syndrome can now be detected with high reliability in the first trimester using ultrasound and amniotic genetic screening, and 80 % of women informed of such a diagnosis choose eugenic abortion.
6) Stem Cell Line Research: Development of stem cell lines is separate from medical use of stem cells. For stem cell lines, pluripotent stem cells have recently been generated from adult human skin cells. The researchers are beginning to learn how to control the extra-nuclear cellular chemistry that controls gene expression and are achieving steps in the direction of running the clock backwards on embryonic growth, from adult cell to embryonic cell.
7) Cloning: Many types of mammals have been reproduced by asexual cloning, the process of manually removing the nucleus of a fresh unfertilized egg and replacing it with the nucleus form an adult cell. This has now been demonstrated for humans with high success rate to the blastocyst stage for the purpose of generating stem cell lines.
8) Conception Message Conveyance: The message, "The beginning of life is at conception," must have been conveyed sometime after microscopes became available and researchers found that living things are made of cells.
References are at EggInfo.info