From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Mon May 27 2002 - 14:04:06 MDT
In list discussions regarding cloning, abortion, overpopulation,
infanticide, etc. the topic of the failure rate for human
conceptions comes up and we (I?) do some handwaving with
regard to the actual rates.
Well, ask the universe, and sooner or later it delivers.
Nature Science Update has a fascinating article about
chimaerism and mosaicism in humans:
Helen Pearson, "Human genetics: Dual Identities" (2 May 2002)
http://www.nature.com/nsu/020429/020429-13.html
And interestingly enough it referenced this article:
Boklage CE, "Survival probability of human conceptions from fertilization
to term", Int J Fertil 1990 Mar-Apr;35(2):75, 79-80, 81-94
http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1970983&dopt=Abstract
Abstract:
"Preterm death of the human conceptus is common. A consistent biphasic
pattern in the rate of loss from biochemical pregnancy detection to term
suggests that most wastage occurs prior to clinical recognition. After
simple adjustments for varying methods, existing data show that at least
73% of natural single conceptions have no real chance of surviving 6 weeks
of gestation. Of the remainder, about 90% will survive to term. IVF
conceptions do nearly as well as natural pregnancies after clinical
recognition, but poorly before, despite selecting apparently normal
embryos for transfer. Reasons may lie in the uterus more than the embryo
itself. Multiple pregnancies may constitute more than 12% of all natural
conceptions, of which number about 2% survive to term as twins and about
12% result in single births. In all of these situations, simple equations
for exponential decay in a mixture of two populations can accurately
describe the distribution of those deaths in time."
So there you have it -- hard numbers. My only comment is that if
natural conception and development is so darn difficult, it explains
a large chunk of why cloning is so problematic.
Robert
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