So I am sitting down in earnest today, having gained some valuable feedback from blogland, to review yesterday's decision. This is a customary part of my decision-making procedure - I need to be thorough in order to move on without regret over my choice. I am getting a bit stuck on the stats, however - the bottom line is the patient leaflet doesn't contain enough of them to be useful to me, so I need actual stats, and I also need someone to explain to me how these stats are calculated.

The problem is the results of the screening test are presented as a risk, not a diagnosis. A risk of greater than 1 in 300 is considered "high", but in fact what this means is your baby is probably fine. For every 300 patients with a 1 in 300 risk, one is going to have an abnormal baby, and two hundred and ninety nine are going to be AOK. That means anything more than a 0.3% risk of abnormality is considered "high risk". In other words, unless they're more than 99.7% sure you have a normal baby, they're going to recommend invasive testing. If someone walked up to me and said, "There's a 99.7% chance your baby is just fine!" I'd be elated, I wouldn't be saying, "Oh no! Better stick a needle into the amniotic sac, risking miscarriage!"

In fact, even if your chance is as scary as 1 in 5, your baby is still probably ok. Now you have an 80% chance of a healthy baby, and a 20% chance of a defect - quite a high chance, to be sure, but people announce whole pregnancies and start buying nursery furniture based on less certainty than that. You have to get down to a less than 1 in 2 chance of a defect before you can say your baby is probably affected, and I'm not saying that can't happen to us, but it's extremely unlikely we'll get results that clear-cut from our screening.

Am I right so far?

The clinic claims they can pick up 90% of Trisomy 21 cases using the combined screening method and their chosen cut-off level (stats are not offered for other abnormalities, but the study below indicates similar sensitivities for other abnormalities). That's pretty good, but still leaves a 10% chance your affected baby will be passed through unnoticed. Ten percent is high. Unreassuringly high. I'm not sure I can find peace of mind in 90%.

They also quote a 5% false positive rate - ie. 5% of all people who opt for prenatal screening will be unnecessarily distressed by their results. But what's the true positive rate for that population? What percentage of people who line up for screening will end up getting bad news? They don't say, so I've used the figures from this study, which screened a maternal population ranging from 15-49 years in age (with the average woman being 34 years old), and calculated that we are a good five times more likely to receive a false positive than a true positive. A full 83.4% of people in the high risk category have nothing to worry about, compared to 99% of the unscreened population. Am I right? I mean, I know it's only one study, and someone please tell me if I'm going wrong, but you see what I mean?

The point is, if we do prenatal screening we are five times more likely to be unnecessarily distressed than to find an abnormality. And even if we're "reassured" are we actually going to find it reassuring given 10% of cases slip through the net? What do we gain from screening? Best case scenario, instead of being 99% sure our baby is ok, which we are now, except we're not but that's just general paranoia, we get to be, say, 99.75% sure our baby is ok. Not an overly tempting gain. Is it worth it?

Well, there's a small chance we could find an important abnormality now, rather than in, say, 3-6 months' time. However, in order to actually diagnose such an abnormality, we would almost certainly need to submit to more invasive testing, which carries a risk of miscarriage of a perfectly healthy pregnancy. This is a very small risk, but nevertheless. If we wait and find out later, we need to submit to the risks of pregnancy and delivery, which I am obviously prepared to take on under normal circumstances, and I am even prepared to take them on under less than ideal circumstances, but not for a baby I know, for sure, won't make it. Of course, these are also very small risks, but nevertheless.

Perhaps I'm more comfortable with the risks of pregnancy and delivery because I squared myself with them before we first started trying, so it's the devil I know, but in any case, there it is*. Having given it twenty-four hours and written it out fully, I find myself unswayed. Unless you can think of somewhere I've gone wrong? I still have a couple of days to finalise my decision.

*You can add in the risk of delivering and having to raise a disabled child if you want to complicate matters and depending on your view of termination under such circumstances, but I can't seem to get much of a grasp on the idea of delivering and raising a live child at all, so this whole argument tends to slide right off my back.

**After pubmeding my way through some abstracts, I have decided to ignore any difference in meaningfulness of PAPP-A levels based on method of conception. A lot of studies do show a significantly higher rate of false positives with this test in assisted conception - some say high responders are more prone to it, others say it's ICSI, others say FET. Other studies say there's no difference to speak of at all. Sounds suspiciously like the answer isn't clear enough to enter into the above argument.


Geohde said...

Bea, it boils down to the simple fact that no screening test is perfect, and as you rightly point out, your a priori risk is low.

If you aren't planning to risk the diagnostic test, many would argue against screening anyway, because a +ve result of increased risk adds worry which may be baseless, and without acting on it you just can't know.

Every decision is individual.

But I think you're number crunching is very pretty nonetheless :)


Rachel Inbar said...

I lost track a bit with the false positive rates, but I guess that in any case I looked at it a bit differently.

I thought that if my initial risk is 1:153 and then I went for the NT scan where my risk was calculated as 1:975, then that's good enough for me... I did feel that I wanted to be fairly sure that the fetus is healthy and that I didn't miss something that would have been easy to catch early on, which is why I went for the NT scan - to be reassured... (Also note that the scan cost me all of $25.) At a younger age, I don't think this would have been very important to me.

My doctor says that Israel is leaning toward changing the recommendation that all women over 35 go for amnio & that only women who are determined to be at high risk go.

You've got your head on your shoulders. Go with what feels right for you!

TeamWinks said...

Now that you've done all the logic for me, I won't have to think about it when my turn comes around! :-) I agree with you Bea. I'd skip it. That's just me though.

serenity said...

I'm with you, the logic and the statistics seem pretty reasonable to me.

But I'm not completely objective, either, since we also declined the early screening. Personally, I just didn't want to worry needlessly.

We also decided with our OB that, IF something came up on our big ultrasound - there was a soft marker, etc - then we'd get the screening bloodwork done.

I think it's whatever you're most comfortable with.

Piccinigirl said...

Again, I think that you need to go with your gut, you're right: why worry about something that might be a false positive anyway?
I am ashamed to admit that I did the tests because I was asked if I wanted to and because I didn't want to disappoint my dr's or the karma gods, I did it. However, when the NT screen found us unable to do the bloodwork associated, I made a decision not to worry about it, the u/s looked great and I left that hospital saying "well if there is something wrong, I will have no one to blame but me"
At 16 weeks when it was time for more b/w I did it because my dr was saying "we can take the blood tonight" ...."ok, strap me in and let's just get it done".
I will tell you with every u/s , I am still scared, I am still nervous that they will find something wrong with the boys, but I am relying on my gut to tell me how far we go with invasive testing , for us the worry about a CVS or an amnio just wasn't worth the sleep we wouldn't be getting.

I believe that with all you know and are educating yourself with, that your decision not the right one.

Heather said...

Plus you have to weigh in the emotional effects...

Also, does it matter? Do you plan on aborting if there is something wrong?

If you aren't going to abort and there is nothing you can do about it now - why worry yourself?

I prefer to stick my head in the sand.

I think the medical community likes to come up with more ways to get money out of us sometimes. I think most of these "tests" are great for some people but they hand them out like band-aids nowadays. And all of us infertiles have paid our share.

Heather said...

Have some oreos and milk - just don't forget you have eaten oreos and milk.

I'm just sayin...

Jess said...

If you won't do the amnio for a chance (we woulnd't), I wouldn't do the other stuff. I mean...

It's that things are probably ok. Things are probably fine. And for US, we wouldn't have terminated ANYWAY for such things as downs, so we didn't have the testing.

It SOUNDS like to ME that you'd be better off personally NOT doing the testing. By your numbers and posting. But in the end, of course, it's got to be you who decides.

Lut C. said...

Someone meticulously picked apart the logic of deciding whether or not to do a amnio. Unfortunately, I can't remember who. :-/
I thought perhaps Thalia, but I can't find it in her archives. I remember it was someone with a degree in statistics.

From what I remember, the problem was indeed that there are many false positives.

Anonymous said...

I like your logic. I don't know if I could have gotten that technical on my own and if I ever manage to get pregnant myself, I will be back to read this post.

If it were me, the only way I would consider terminating a pregnancy for medical reasons would be if the child had absolutely no chance at survival. If a baby had Down's, then the baby would have Down's. If there is anything really major, it would be caught during routine ultrasounds. If the stress of false positives and the 'what ifs' is too much, then I say skip it.

Karaoke Diva said...

We skipped the triple test with my son and with this pregnancy we're skipping the nuchal translucency testing and most likely the triple test again. Like you said, you're not in a high risk group (neither am I) so why risk a pregnancy filled with worry and dread? You have to do what's right for you, but I'd hate to see you worry unnecessarily. You've already said that you would not terminate a child with special needs and your chances of a truly terminal child are scarce so why put yourself through that?

That's my $0.02. :-)

Aurelia said...

I'm sorry, I didn't mean to prompt this much stress!

Basically, ignore any and all of the 16 week screening stuff. It is so inaccurate and has such a high rate of false positives, it is being delisted by OHIP. My Doc refuses to order it.

For me, I never look at risks, etc, because they are all based on lots of old original research that only gathered stats on older women having chromosomal abnormalities. Younger women like me who have chromonsomally damaged babies are considered an aberration by those stats, because they simply don't count the younger women who miscarry the babies earlier.

I look at hard numbers and measurements, period.

I'm emailing you the guidelines that the Society of Obstetricians and Gynecologists of Canada now officially use. They recommend that all women regardless of age be offered non-invasive screening, like the NT scan, and the Papp-A. Simply because so many women were suddenly finding out something sad at 18 weeks, and having to wait for ages afterwards for final results, and then give birth to a dead child.

I said it before, and I still believe that you are fine, BUT it may be reassuring to you to read something with harder numbers.

And to the pp who think that Down's is no big deal, you are aware that 90% of Downs fetus' are miscarried or stillborn. 30% of those born alive, die shortly after birth.

Not everyone wants to wait it out.

Whatever you do Bea, is fine with me, no judgement ever.

Sarah said...

forget the logic. the point is, it seems pretty clear which option you're most comfortable with, and that's ultimately all that matters. there is no right answer, there is only your answer, and in typical bea fashion you've reasoned it all out very well, so i think you can feel extremely good about it.

Meg said...

Hey Bea - Just got here - Congratulations on the scan! rapt for you.

Don't feel you have to justify not getting screening. If I had my time again I would never have done it. It just messed with me, after getting a 1:270 at the age of 29.

I've seen the false positives happen so many times.

travelise said...

I did the same research and came up with the same conclusion, the NT scan/bloods results are so inconclusive as to be absolutly no use at all.

However, I decided I needed to have some reasurance, or I would worry the whole pg (I am 43 so high risk automaticaly). I went and saw 2 genetic councelors to discuss the pros and cons of Amnio, and talked to my FS, obs and gp, and anyone else I could draw into a discussion. I worried and worried about the possibility of miscariage, despite the fact that I knew it is operator dependant, and if you go to the best docs, the rate of miscarriage is now no greater than your background rate at that stage of pg.

In the end I went for the amnio in a near catatonic state of fear (about miscarriage, possible negative results had been compleatly pushed to the back of my mind by then), but after couldn't imagine why I had been so worried. It was done in about 5 mins, and just didn't seem like a big deal.

Of course everyones decision is based on their different circumstances and concerns, if I hadn't been so old I probably wouldn't have done it. But thought I would share, in case it is useful to anyone else who is having a similar dilema.

blondie said...

Go with your gut instincts. Deep down you and Mr Bea know what is right for you... :)

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