Wednesday, October 7, 2009

Statistics

Statistics . . . sounds riveting I know. Just thought it would be interesting to run a few numbers. It's been a long time since I was in a statistics class, so if I get the numbers wrong and you're a math whiz who can correct me, feel free.

If you've had a late pregnancy loss due to an umbilical cord problem, you've no doubt tried to find the answer to this question: What are the odds of it happening again in a subsequent pregnancy?

First, let's review some basic information and figures we'll use:

Types of umbilical cord accidents that can lead to pregnancy loss - Abnormally short cord, abnormally long cord, hypercoiling, hypocoiling, single umbilical artery, nuchal cord, nuchal loop, torsion, body loop, true knot, marginal insertions, velamentous insertion, vasa previa, cord prolapse, constriction, and monoamniotic twins

Frequency of late pregnancy loss - The odds of a second trimester loss are about 3% and the risk of a loss in the third trimester is less than 1%. I'm going use the 3% second trimester rate.

Late losses caused by umbilical cord accidents (UCA) - According to Dr. Jason Collins, 20% of all stillbirths are caused by UCA. That means the risk of late pregnancy loss due to UCA is .6%.Now we're ready to run some numbers. First, let's assume that UCA is totally random - just "bad luck". Every woman would have an equal chance (.6%) of suffering a UCA with each pregnancy progressing beyond the first trimester, regardless of her past history. [PLEASE NOTE: Since it's impossible to pin down an exact number for either the frequency of late pregnancy loss or the proportion of them due to UCA, I've used numbers on the high end of the studies I've found. I'm being pessimistic here and the real chances of a loss or losses are likely even lower than indicated by the numbers below.  Remember, we will only consider pregnancies that are beyond the first trimester.]

Risk of losing one pregnancy to UCA: 1 in 167
Risk of losing two pregnancies to UCA: 1 in 27,889
Risk of losing three pregnancies to UCA: 1 in 778,000
Risk of losing four pregnancies to UCA: 1 in 605 million

According to these numbers, you are more likely to be murdered (1 in 18,000) than you are to have two late UCA pregnancy losses. Your risk of being struck by a lightning (1 in 576,000) is much greater than the chance of having three late UCA losses. And you're more likely to be die from a shark attack (1 in 300 million) than have four late UCA losses.

"Bad luck", indeed. It would be wonderful if the numbers above represented the true risk of UCA loss recurrence, but now assume that late losses due to UCA are more than just random "bad luck".  Let's consider the research of Dr. Jason Collins, which puts the UCA recurrence rate at 1 in 7.  Keep in mind that this figure only means that there's an issue with the cord, NOT necessarily that it's a fatal problem.  But since I have no research on the proportion of affected babies who survive, Ill run the numbers using the very pessimistic 1 in 7 figure:

Risk of losing next pregnancy to UCA: 14% or 1 in 7
Risk of also losing a third pregnancy to UCA: 2% or 1 in 49
Risk of also losing a fourth pregnancy to UCA: 0.3% or 1 in 343

Even when using this frighteningly high recurrence rate, the odds are still very much in your favor for a successful future pregnancy. But what if for reasons unknown, some women (like our case study moms) have even worse odds, perhaps because of a genetic or physiologic condition, that gives them a dismal UCA loss rate of 50%. In a group of 100 such women, we would expect to see the following results if each woman had four pregnancies going beyond the first trimester:

6% of the women would have four babies and no losses
25% would have 3 babies and one loss
38% would have 2 babies and two losses
25 % would have 1 baby and three losses
6% would have four losses

37.5% would have two losses in a row
12.5% would have three losses in a row
6.25% would have four losses in a row

So, if you've lost a baby to a UCA, should you try again? We here at Cradles and Graves say yes - but demand specialized care! And let us know what the outcome is.

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