I researched everywhere I can think of on-line (including PubMed) and can’t seem to find an answer to my question. Trisomy 18 is a genetic disorder that usually results in a fetus dying before birth or shortly afterward. Depending on the source you go by, it affects females from 2 to 4 times as often as males. My question is this: Why does it affect females more? Since this is usually an error in maternal meiosis (meaning the defect is present in the egg when fertilized), it doesn’t make sense to me that more t-18 affected eggs would be fertilized by X-bearing sperm than Y-bearing sperm. I have seen some articles that say t-18 males are more severely affected than t-18 females. I’m thinking that equal numbers of t-18 eggs are fertilized by x or y bearing sperm, but because male embyros are more badly affected (for some reason) more of them tend to die and be miscarried before the second trimester, which is when maternal screening serum tests and ultrasounds start to detect problems.
The reason I ask is that my maternal screening came back higher risk for Trisomy 18. Based on the fact that the ultrasound showed no anatomical abnormalities whatsoever, my husband and I decided not to have an amnio. We didn’t want to take the risk of the amnio causing me to miscarry a healthy fetus. I’m still second-guessing a bit, though. If I could find out that it would be highly unlikely for a male fetus with Trisomy 18 to survive to 18 weeks with no abnormalities, that would make me feel better, even though we’re fairly sure that the fetus is normal, and the test was a false positive.
You are probably correct in your supposition of increased spontaneous abortion in male conceptuses. Most aneuploidy syndromes have a very large rate of spontaneous abortion and only a tiny fraction make it through until birth. I don’t think that there are any ways to test this supposition, though – so many factors are at play and there are no good models for this kind of disorder.
Good luck with your pregnancy. Now for some unheeded opinion – please feel free to ignore me. My wife is going to have her anatomical ultrasound on Friday. I don’t know what test you had (was it your triple screen AFP?), but I think if I were in the same situation, I would ask her to have an amnio. I respect your decision and you have obviously done the research and I don’t aim to convince you, as I have no idea of your situation. But the procedure has only gotten safer and there is usually far less than 1% chance of miscarriage when it is done in a reputable center. Trisomy 18 is a bad thing – 90% of children die by age 1, and I would want to be absolutely sure. But again, that’s me and you are you.
There may be some other less invasive tests for fetal aneuploidy. I would try asking around, perhaps maybe the in the UC genetics department. I have met some people over there and they seem top-notch.