Probability of conception is low

Chances of achieving pregnancy are critically dependent on

the timing of insemination

Here is why:

Any woman has a 90% chance to be healthy at the time the baby-making sexual intercourse is occurring; the fertilization rate could then be intuited to average also 90%.  But it does not because of the inherent 25% loss to early embryonic mortality [EEM] or miscarriage, spontaneous abortion, so that a successfully inseminated healthy female has a significantly decreased chance of successful pregnancy.

As a consequence, the probability of achieving pregnancy is critically dependent on whether the insemination (natural or artificial) occurs at the right time (i.e. during the so-called fertile window). Contemplate the reason why this Bronzino picture of Allegory of Venus is so small.

Here is how critical this timing is for healthy women: Even if the probability of determining the insemination time correctly were 90%, the resulting probability of successful pregnancy from any one particular insemination event would be only 55%. 

Get this! Only 55% under perfect conditions. This is because the probability of pregnancy is the combination of four individual probabilities:  90% x 90% x 75% x 90% = 55%.

That is, the statistical formula for the probability of successful conception of pregnancy multiplies the probabilities of being in good health, of successful insemination, of not miscarrying the conceptus, and of the probability of correct timing of the conceptive intercourse. Thus [P stands for probability of]:

P-health x P-fertilization x P-non-abort x P-insemination timing = P-pregnancy

For example, a 60% success rate of correct timing brings the overall rate of pregnancy down to a mere 36%, and this goes down to a mere 30% if the correct timing probability is only 50%, in healthy fertile couples.

But then, even a quick search for data on EEM (Early Embryonic Mortality] suggests that human EEM is likely much higher than the above-considered 25%, possibly even as high as about 83% (“only one embryo in six survives to term”), and certainly appears likely around 50% in healthy women. (Different studies come up with different results.)

Hence the probability of pregnancy is lowered from the approximate 36% or 30%, and it can be much lower if the timing of insemination (intercourse) is off, if the probability of correct insemination timing is low. See the adverse effect of wrong timing of the attempt to get pregnant (Insemination timing probability) on the probability of success (Pregnancy probability) in the following table.

Probability of pregnancy as a product of four probabilities:

Health	Fertilization	Non-abort	Insemination timing	Pregnancy probability
.9	.9	.15	.5	.06
.9	.9	.15	.9	.11
1	.95	.55	.5	.26
1	.95	.45	.5	.21

This page is an excerpt from the bioZhena’s Weblog post “Difficult conception tied to pregnancy complications – addressed” (https://biozhena.wordpress.com/2010/05/25/difficult-conception-tied-to-pregnancy-complications-addressed/ )