Milestones And Survival Rates Of Embryos
There are no abrupt steps at which failure to meet a milestone causes an immediate drop in survival rate. Rather, a milestone not met will likely eventually lead to embryo death or may sometimes result in short-lived birth defects such as trisomy or anencephaly. The following is a very approximate smooth curve for embryo/fetal survival.
First-week data is not readily available, so attrition curves have to be estimated. The over-all rate of success might be as high as 1/2, but the curve shown here is fit to the apparently more likely number of 1/3.
First, as described under Embryonic Development / Fertilization & Meiosis, the microtubules holding chromosomes in the pre-egg cell are vulnerable to damage during the long wait before fertilization or the high metabolic activity during the last month building up to ovulation. The resulting trisomies and monosomes are fairly common chromosomal errors in which growth from fertilization is often shout lived but for which death may not occur until a few days after birth.
Then too, a lot of preparation must happen in the ovary, especially during the final days and hours before ovulation. Abundant cytoplasm swollen with stores of organelles, complex metabolic compounds and mRNA must be accumulated. If fertilized, the ovulated egg will be required to sustain itself and be able to divide into 64 or more smaller cells without receiving nutrition until it finally reaches the uterus on day three.
It is estimated that a quarter to a third of ovulated human eggs are deficient either in chromosomal or cytoplasmic maturation.
There are additional milestones for egg success, too. Fertilization is certainly required, but two additional hurdles that may fail are implantation and then by day 24 neural tube folding and similar growth requirements.
Blastocyst formation and gastrulation are often-cited, but since these stages are not particularly failure prone, they should perhaps not be counted as milestones for survival of fertilized eggs. These are highly significant structural changes. The blastocyst stage marks the beginning of cell specialization. Gastrulation marks the end of possibility of twinning and final commitment to the direction along which cells will develop.
And there are many other steps and phases during human reproductive development that, because of high success rates, are not ranked here as egg-survival milestones. Perhaps most phenomenally impressive is the threading of nerve fibers through long distances of tissue to connect the correct end points. Often publicized is reflexive response to touch in vitro by only seven to eight week old fetuses and the eye opening and rubbing that can be seen in womb with ultrasound at 12 weeks. These are events that occur with high success rate in all higher life forms and primates and so are not counted here as milestones for the regeneration of human life.
The highest attrition of all is for sperm. A man produces several trillion of them in his lifetime, most reabsorbed. And he releases some 200 million at once for just 200 to make it up the fallopian tubes. Tens of sperm usually reach the egg and try to penetrate it, but all die trying except for the first. The low likelihood of a sperm surviving is not counted here as a separate milestone for egg success other than the obvious requirement for fertilization.
So it is suggested that a list of milestone requirements for fertilized eggs should include perhaps five events: maintenance of chromosome integrity, cytoplasm preparation for ovulation, fertilization, implantation, and development through to neural tube folding.