Chromosomes. Does the word remind you of high school biology? Punnett squares? Am I making you sweat? Keep reading...I won’t make you take a pop quiz.
The purpose of this post is to refresh your memory about chromosomes and talk about why they matter now.
If you are a women over age 35, you may have been told that you better hurry up and have a baby because your eggs are “getting old.” Facebook and Apple recently announced that they would pay for egg freezing for their female employees. Women who are having their babies over age 35 are considered to be of “Advanced Maternal Age,” or AMA. This is all connected to chromosomes.
Chromosomes are the structures inside cells on which the DNA resides. DNA is the alphabet for genes, the instructions for our body. Humans have 46 chromosomes, and they come in 23 pairs. One of each pair comes from your mother, and the other from your father. These include 22 pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes. So, for the women reading this, you have 46 chromosomes, including two X chromosomes: you are 46,XX. Men reading this have 46 chromosomes, including an X and a Y chromosome, which makes you male: you are 46,XY.
Here's a picture of the chromosomes of a female (46,XX). This broad look at the chromosomes all matched up in pairs is called a karyotype:
Image: www.dnalc.org
Each egg and each sperm has only 23 chromosomes. The combination of a sperm and an egg will result in the expected 46 chromosomes. Women will contribute an X chromosome (because that’s all they have), and men will contribute an X chromosome half the time, and a Y chromosome the other half of the time. Ever wonder what determines the gender of your baby? Now you know: It’s determined at conception based on which sperm penetrates the egg. If that sperm has an X chromosome, you’re having a girl. If it’s a Y, you’re having a boy.
Women are born with all of their eggs and they age along with her. Men make new sperm constantly. When we talk about women’s eggs “getting older,” we mean that the ability to release a mature egg at ovulation with exactly 23 chromosomes gets more difficult. Around age 35, it becomes increasingly difficult.
Most pregnancies conceived with a chromosomal abnormality will result in a miscarriage, usually very early, sometimes even before a woman knows she’s pregnant. As a women ages, her chances of conceiving a pregnancy with a chromosomal abnormality increases, as does her risk for miscarriage. There are some chromosomal conditions, however, that may not miscarry, including Down Syndrome (which is a condition in which there is an extra, third copy of chromosome 21), Trisomy 13 (a third copy of Chromosome 13), and Trisomy 18 (a third copy of Chromosome 18). Babies with Trisomy 13 and Trisomy 18 usually have significant medical problems and are not able to survive more than a few days or months. Individuals with Down Syndrome can often live into their 50s or 60s or longer, but may have learning disabilities and health problems, such as heart defects.
Here is the karyotype of a male with Down Syndrome (47,XY, +21) . See the extra copy of Chromosome 21?
Image: http://openi.nlm.nih.gov/detailedresult.php?img=3232533_IPC-4-3-g004&req=
So, what are the chances of having a baby with Down Syndrome, Trisomy 18 or Trisomy 13?
- At age 35, the chance of having a baby with Down Syndrome is about 1/338 (0.3%); by age 40, that chance increases to about 1/84 (1.2%)
- At age 35, the chance of having a baby with Trisomy 13 is about 1/10,000 (0.01%); by age 40, that chance increases to 1/2700 (0.03%)
- At age 35, the chance of having a baby with Trisomy 18 is about 1/4200 (0.02%); by age 40, that chance increases to 1/1140 (0.09%)
So, as you can see, the chance increases as a woman ages. Trisomy 13 and 18 are much more rare than Down Syndrome, but risk still increases as a woman ages.
What are the chances of having a pregnancy that has any chromosome abnormality, including those that are likely to miscarry?
- At age 35, the chance of a pregnancy with any chromosome abnormality is approximately 1/115 (0.9%)
- At age 40, the chance of a pregnancy with any chromosome abnormality is approximately 1/30 (3.5%)*
Each person views these risk numbers differently. Some people may see these numbers and think: “This is a low risk; what’s the big deal?” Some people may see these numbers as high risk, and want to get more information for themselves, beyond the general statistics for their age group.
In particular, some people, especially those who have already experienced several miscarriages or have had a child with one of these conditions, may want to know if they have a predisposition towards conceiving a pregnancy with a chromosomal condition.
So what are the options for screening and testing?
Should everyone get screening?
What can you do with the testing/screening results?
Stay tuned for future blog posts!
*Risk numbers from: Gardner & Sutherland. Chromosome Abnormalities in Genetic Counseling, 3rd Edition, 2004. Risk statistics for a pregnancy with any chromosome abnormality are those detected at 10-11 weeks gestation, so may be higher at conception when considering those conceptions that would have miscarried earlier due to the chromosome abnormalities.