This means I don't know for sure what is going on. It could still be dwarfism (although you would expect the gap to be widening as growth would continue to slow down). It could just be really short legs.

They also did not see any of the other signs of dwarfism. Now, the radiologist said that lots of those signs are hard to see on ultrasound (narrowing of the pelvis or the spind, for instance). But definitely Erik and I could not see the bulging forehead or depressed nasal bridge, and we couldn't see bowing in the legs.

That doesn't mean it's not dwarfism as those signs could develop late or not at all. They're not always present.

I am grateful for this. Even if the worst is true, there is no way now that it could be a fatal dysplasia (or dwarfism). It is even unlikely that it would be a serious achondroplasia (the most common dysplasia, or dwarfism). Apparently on average achondroplasia is definitively diagnosed before 28 weeks. So for it to be still "ambiguous" at 31 weeks is a good sign--mild achondroplasia, hypochondroplasia (super-mild achondroplasia) or a healthy person with really short legs (and arms). Or a very late developing achondroplasia, but like I said, that's looking unlikely.

I did a shitload of research over the past 6 days (pardon the language). I have a pile of medical abstracts, websites and textbook copies that weighs more than the baby does. I'm glad I did this, because most of the information I've gathered is from those sources. Doctors will answer your questions, but selectively, and only if you know what questions to ask.

One thing I learned is that the genetic mutation that causes dwarfism has been identified and can be analysed prenatally. So I can get a definitive "yes" or "no". This is what I'm going to ask the doctor for tomorrow. I just can't stand the thought of another 9 weeks of weekly or biweekly ultrasounds, and wondering every time--is this the day? Ack ack ack.

I mean, if I hadn't done any research I still wouldn't know the medical names for the illnesses. I wouldn't know what the secondary signs are, to ask if they were present. I wouldn't even know what 3 weeks behind dates means in terms of diagnosing them.

As it turns out, "weeks" is not how they're diagnosed. You look at the actual measurements and calculate the standard deviations below the norm, or the percentile values (and if you've never taken stats and have no idea what I'm talking about, feel free to ignore. This is more for my sake anyway). To definitively diagnose dwarfism in the third trimester, you need to have a "very short" femur, which is defined as 2 standard deviations below the mean minus 5 mm or less. At 31 weeks, that would be a 49 mm femur or shorter. If that were the measurement we were getting, it would definitely be dwarfism.

We're at 53 mm. This is still classified as a "short" femur, which is anywhere from 2 standard deviations below the mean to 2 standard deviations below minus 4 mm. So from 50-53 mm. We're right at the cusp of a "short" femur. "Short" femurs can be "dwarfism of late development" (which tends to be milder), chromosomal anomalies (like Down's or trisomy 18, but there are no other signs of such disorders), mild growth retardation (not likely given the rest of baby's stats are more or less on track), or "constitutional shortness." A short person.

In one study I read, 12 fetuses were diagnosed with "short" femurs in the third trimester. Of those 12, only 1 had dwarfism. One had a chromosomal anomaly. The other 10 were healthy.

I have a pretty good idea of how the parents of those 10 babies felt.

The exception to this rule is when a "short" femur is found in the third trimester and there is a family history of dwarfism. Then you are likely looking at dwarfism again. But that makes sense.

You know I'm just babbling now and no one has to read all of this. There won't be a test. I promise.

But here is something I found interesting. OK, standard deviations off the mean sounds pretty abstract, especially if you've never taken stats or hate numbers. So here is another way to think of it.

By mathematical definition, 34% of all fetuses will fall within one standard deviation below the mean (or average) and the mean. Forty-eight per cent will fall between the mean and two standard deviations below the mean. (The same holds for going above the mean, but of course, this has no bearing on dwarfism.)

This means that by mathemcatical definition, 2.4% of all fetuses will have femurs that are farther than 2 standard deviations from the mean, or in potential dwarfism territory. That's 1/50.

But dwarfism strikes only 1/5000. And that's all forms of dwarfism.

So of the fetuses whose femurs are more than 2 standard deviations below the mean, and thus in a grey area, only 1/100 will have a form of dwarfism. Most of them will be the "very short" as opposed to "short" but anyway, even if it were more even, still, that's a 1% chance of a "short" femur meaning dwarfism.

That's not to say that the other 99% are hunky dory because there are other pontential problems--growth retardation, downs', trisomy 18, complete absence of limbs. And I am not as up on their prevalence. But I'll bet you that all of them put together still account for far less than 2% of fetuses.

This doesn't mean I'm out of the woods. It could still be dwarfism. But everything I've read seems to indicate that dwarfism, at least the serious varieties, become unambiguous before 31 weeks. I so so so so so hope this means everything is ok.

Oh, and another interesting stat to ponder:

We're at 53 mm. The average for the age is 60mm. That's a 7 mm difference between the norm and the second percentile. Not only that, but 53 mm is 88.3% of the normal size for the age. I just find it amazing to think how little natural variation there is in this measurement.

Anyway. That's it. Still scared, but less so. I wish I'd gotten something more definitive.