The next scan coming up on Dec 7 is the one that tests for chromosomal abnormalities such as Down Syndrome and T18. Since my first cousin has downs, naturally I am wondering if this increases my risk. With the scan, they visually measure the fluid in the back of the baby’s neck via ultrasound. They take blood tests and match all that info up including maternal age and it spits out a numeric risk value like 1:15 or 1:10,000. Depending on if I was high risk, I would then have the option to do an amniocentesis where they sample some of the amniotic fluid and can give an actual diagnosis. But this test carries a risk of miscarriage — which is often why couples choose not to tell the world until these tests come back as low risk.
Luckily I have a friend who is a retired Genetic Counselor. So the below very detailed info is courtesy of Zoey Trueblood Wilson. Thanks Zo!!
i can now be the bearer of good news… family history of down syndrome does NOT influence your risk. this is because down syndrome is a sporadic occurrence which happens by chance, it is not a heritable trait in a family. back to highschool biology… every cell in our body has 46 chromosomes, 23 pairs – you got one set from your mom and one from your dad. in down syndrome, there are 3 of chromosome #21 (it is also called trisomy 21). this happens because when the body is making eggs or sperm (which each have 23 chromosomes) an extra #21 gets in. scientifically speaking, the homologous pairs fail to separate in meiosis II in an event called nondisjunction. so an egg that is supposed to have 1 chromosome #21 gets 2, and when it combines with the sperm, there are 3 in total. again, this just happens by chance. the risk for nondisjunction increases as a woman’s age increases… this is because when we are born, all of our eggs are already formed, so as we get older, so do the eggs. an older egg is more prone to errors of chromosome separation. the fact that your cousin has down syndrome has no bearing on your own personal risk.
That being said, there are VERY RARE cases where there is heritable down syndrome in a family. this is because of something called a robertsonian translocation where the 2 chromosome #21s are fused together. someone could be what we call a balanced carrier where they have the right number of chromosomes, but they are stuck together… because they travel together, then it’s hard to make eggs or sperm with the right number of chromosomes and the risk for having a baby with down syndrome would be comparatively high. this is very rare and we usually see with it very high rates of miscarriage in many different family members as well as more than one case of down syndrome in a family. if you are at all worried about this (let me say again IT IS VERY RARE), you could ask if your cousin ever had a karyotype done. a karyotype is basically a photo of a person’s chromosomes. if he has three #21s standing in a row (also called 47,XY,+21) then that is NOT a translocation and your risk would be simply that due to your age alone – see paragraph above. the nomenclature for down syndrome with a robertsonian translocation would look like this: 46,XY,der(21;21)(q10;q10). if he has this one (VERY RARE) then you could get a karyotype of your own chromosomes to make sure you’re not a balanced carrier… only if you found out you were a carrier would you get the amnio.
Preggers (and fam) in Sydney 