1) Fertilized Egg Attrition: The human reproductive system does not work by getting everything right, but instead by having a chance of getting things right and then culling out the mistakes, lots of them. 1/2 to 2/3 or more of fertilized eggs are culled by nature. On average, for women of all ages and health, trying to get pregnant causes the loss of about 4 to 8 times as many embryos as abortion.
2) Predestination: Half of these losses are because of cytoplasmic or chromosomal deficiencies at the time of ovulation. Fertilization may initiate cell division but can not repair lethal deficiencies. We all come from a fertile egg.
3) Identical Twins: Embryo splitting is a creation event that occurs independent of and several or up to 14 days after fertilization. Only about one in eight twins survives.
4) 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. Chimera have mixed blood type, and in half of cases are hermaphrodites. Prenatal ultrasound can detect the dual genitals.
5) Induced Pluripotent Stem Cells: iPS cells are now made by adding a mix of mRNA to the cytoplasm of adult skin cells. That's a mix for making proteins, and if it is the right mix, the resulting proteins program the cell's genes so as to switch the cell from adult to embryonic.
Mice have been grown by replacing the natural embryonic cells inside blastocysts with iPS cells from adult skin cells. The blastocyst shell becomes the placenta, the iPS cells the fetus.
6) Cloning: The process is mechanical, requiring no a priori understanding or manipulation of gene programming, and so is a simpler way of using the genes from an adult cell to establish new life. The nucleus of a mature but unfertilized egg is removed and replaced with the nucleus out of a non-sex cell from an adult. Backward programming of these genes from adult to embryonic then occurs naturally by not yet fully understood chemistry in the nurturing environment of the mature egg. The method has been demonstrated for humans with high success rate to the blastocyst stage for the purpose of generating stem cell lines, but its main objective is control and economics in animal husbandry.
7) Epigenetics: The human genome project found that humans do not have a large number of genes. So form of life must be shaped by an additional inherited higher level or epi genome that controls gene activity, especially during fetal development. It turns out that this epigenome can be altered during life by external influences, even depending on exposures related to behavior, and then be inherited. Green in the figure indicates difference in epigenetics on a chromosome from identical twins. Left is at age six. Right is at age sixty six.
8) Onset of Egg Attrition: Gonad stem cells function throughout life in males, but in females only until the 7th month of fetal development at which time she has about seven million pre-egg cells. At birth this number is down to two million and at puberty half a million. Then during adulthood fifteen eggs begin forming each month with just one completing maturation and ovulation.
9) Birth Attrition: Losses within the first couple days of birth are significant, about one in 1000 for anencephaly alone, and the median survival times of live-birth trisomy 13 and 18 are respectively 7 and 14.5 days. Anencephaly is caused by a non-genetic error during embryonic development perhaps induced by trace methanol in the mother's diet. Trisomies are due to a starting error in the number of chromosomes before fertilization.
10) Down Syndrome: A trisomy fetus has an extra third copy of one of its chromosomes. This error occurs naturally because of the up to decades long wait between beginning and completion of the final cell divisions to get down to a single set of 23 chromosomes in an egg. An egg sometimes ends up with two or none instead of one copy of a chromosome, which combined with a sperm chromosome, means three or one instead of two. Nature culls monosomies almost immediately and trisomies usually before birth.
Trisomy 21, though, fails to self abort 28% of the time. One in 800 born in the U.S. has this Down syndrome, some 5,500 cases 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 screening followed by amniotic genetic testing, and 80 % of women informed of such a diagnosis choose not to carry to term. Some view that choice as aiding nature's natural culling process.
11) Viable Ovulation: Women not on birth control pills produce about 400 mature eggs during their lifetimes. With a quarter to a third or more of these non-viable, a women produces something less than 300 viable eggs in her lifetime, and only a third or so of those during the period when she is most likely to bear children. Men in contrast produce several trillion sperm in a lifetime, almost a thousand times as many as the number of people on earth and, although microscopic, enough end to end to go five times around the world or reach half way to the moon. So in nature's scheme it is the eggs that are important. Reproduction is from one of the 100-300 eggs viable at the time of ovulation.
12) Adopted Embryo Attrition: Freezing is damaging to any tissue, and many freeze-stored embryos are deceased. Only about half are viable after thawing, and of those only about a quarter to a third survive after implantation.
13) High Multiple and Premature Births: Triplets have a significant probability of malformation or disability. For quadruplets, the likelihood that at least one will suffer blindness, deafness, cerebral palsy or mental retardation is up to 50 percent, and the chance of death or severe malformation is still higher for septuplets or octuplets. In 2001 a mother chose to cary septuplets to term, the delivery cost alone being one million dollars.
Single prematures caesarian delivered are twice as likely to be hospitalized for breathing or bloodstream problems at 37 instead of 39 weeks and 20% more likely if just days early. The leading problem is undeveloped lungs, but brain and other damage can occur if prematures are very young. With modern medical interventions, survival rates in 2010 including those with disability, for 25, 24 and 23 week old premies were 63%, 41% and 20%. The US rate of prematures increased 36% from 1980 to 2010 with the US now having one of the highest rates in developed nations, one in eight.
14) Abortion Rates: 60 percent of women who have an abortion already have children. By groups surveyed in the US, the procedure is used by 29% of Catholics, 27% of ultraconservatives and 17% of the rest of the population. The Catholic Church spends one to five million dollars a year in the US lobbying against contraception and abortion.