What You Need to Know About SMA Carrier Testing

My husband and I conceived our second child just four months after we lost our first child to spinal muscular atrophy.

The decision to try to have a baby so soon after our son Andy’s passing was a difficult one. However, we hated being “childless parents” and felt ready to introduce another life to our family. After weighing all of our options, we conceived the baby naturally, and we prayed that the odds would be in our favor this time. It was assumed that, since there was no question about Andy’s SMA diagnosis, both my husband and I were carriers of the inherited gene that causes the disease, which gave us a one in four chance of having another SMA-affected child.

We had to wait until I was 12 weeks pregnant with our second child before the pre-natal testing for SMA could be done and three additional weeks for the test results. It was an excruciatingly stressful and long wait. As a condition of the pre-natal testing, my husband and I also had to have blood tests to determine our carrier status.

What the blood test does is count the number of the Survivor Motor Neuron 1 or SMN1 genes a person has. Typically, non-carriers have two copies of this gene, one passed from their mother and one from their father. SMA carriers have only one copy of the SMN1 gene, and their other gene is deleted. When two carriers each pass that deletion to their child, the child’s carrier test will show that he/she does not have any copies of the SMN1 gene at all and will confirm the diagnosis of SMA.

So we were surprised to receive the result that, while I definitely was an SMA carrier with one copy of SMN1, my husband appeared to not be a carrier at all or have any type of mutated gene that would cause the disease.  His test showed that he had two copies of SMN1. We also learned that the new baby, a girl, was an SMA carrier like me, and would be unaffected by the disease. While we celebrated the health of our daughter Lucy, we were left with many questions. The most pressing being that, if my husband wasn’t a carrier, how did Andy have SMA?

Our geneticist gave us two potential answers. Either my husband WAS NOT a carrier, and the gene he passed to Andy mutated upon conception (called a “de novo” mutation), or he WAS a “hidden” SMA carrier with a genotype that the blood test could not detect. She said that de novo mutations have occurred in a small percentage (approximately 2-3 percent) of the population of people diagnosed with SMA. Also, she explained that there is a subset of approximately 6 percent of carriers that can not be identified by the current blood test.

In order to find out more, our family recently participated in what is called a “Linkage Study.” Blood samples were taken from each of our parents, my husband’s siblings, each of us, and Lucy to determine who passed what gene to whom. Luckily, we had Andy’s cord blood stored as well, so we could get a complete picture of how the SMA-causing deleted gene had traveled through our family to him. Scientists at the University of Pennsylvania conducted the study.

We received the results yesterday, learning that my husband is indeed a carrier of SMA. In his case, he received two copies of the SMN1 gene from one parent, and a deletion of the gene from the other. It is that deletion that both he and I passed on to our son.

So why am I sharing this story with you? Because if we’d had carrier testing before we had children, the test results would have indicated that my husband wasn’t a carrier – that he was “normal.” I would bet that the doctor explaining those results to us would have said that, even though I was a carrier, we had nothing to worry about. AND WE STILL WOULD HAVE UNKNOWINGLY HAD A CHILD WITH SMA.

It is critical to understand that the SMA carrier test, while very important, is not 100 percent accurate. If you received a “normal” result from an SMA carrier test, but it is confirmed that your partner is a carrier, there still is a possibility that you might have a child with SMA. That said, we are recommending that everyone in our family who plans to have children test for their carrier status. And, if any of them are carriers – regardless of their partners’ test results – that they also elect to have pre-natal testing done with any pregnancy.

This piece of knowledge is Andy’s gift to us all.

For more information on SMA carrier or pre-natal testing:

–Audra Butler

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21 Responses to What You Need to Know About SMA Carrier Testing

  1. Rebekka says:

    Once again, Audra, you have done a wonderful job of explaining this. Thank you for sharing this info. It is so very important!

  2. Rosaleen Butler nee Garry says:

    Just to clarify, SMA carrier is on the Garry side so all related to Garry/Duffy need testing. Unknown if deletion comes from Garry or Duffy. Butlers are clear.

  3. Stephanie Humes says:

    What an amazing story! It is so important to let people know that just because you are not a carrier, you should do further testing if one parent is a carrier. My husband and I are both carriers of the SMN1 gene and were fortunate enough to find out before starting a family. I hope one day it is the norm to have this important genetic test offered by your OB. Thank you so much for sharing!

  4. Stephanie says:

    My husband was worried about our risk of having an affected child and so after meeting with a genetic counselor, they started by testing me first. My blood test came back with 2 copies of the SMN1 gene. When speaking with the genetic counselor, their interpretation was that a guarantee that you are not a carrier would be to have 3 copies of the gene (I’m not sure of the % of the population who has this). Because I have 2 copies, I am not a definite carrier (only one copy), but there is a very small chance that both copies of the gene can be on one chromosome leaving the other chromosome with a deletion of the gene. That would then make me a carrier. This is very rare she explained but technically possible and the current test cannot detect this difference. I’m wondering if this is the case with your husband or if you have any further information about this idea. How was your pregnancy determined to be a carrier? Was this done with CVS? Because if my husband gets tested and he has 2 copies, I guess that still is no guarantee that we won’t have an affected child. And since we have already had CVS done (I’m 14 weeks pregnant), I’m wondering if it is possible to test the baby directly although the genetic counselors did not offer this.

  5. admin says:


    That absolutely is the case for my husband. He has 2 copies of SMN1 that he inherited from one parent, and he inherited the deleted gene (which makes him a carrier) from the other parent. With my son, we were not offered genetic testing for SMA, so we did not know that he had the disease until after he was born. We did a CVS when I was 12 weeks pregnant to find out if my daughter was affected – she is a carrier. (My father-in-law has three copies of SMN1 – so that does happen has your geneticist said. I don’t know the percentage either, but I’m sure it’s quite small.)

    My guess if that both you and your husband have test results of 2 copies of SMN1 that you probably are not carriers. There is a pre-natal SMA test for the baby – you can learn more at the links I’ve listed in the post.

    I hope that helps!

    Thanks, Audra

  6. Dorothy says:

    Thank you, Audra, for sharing your story and I’m sorry for the trails you & your family have endured. We just lost our first baby to miscarriage and my bloodwork came back that I have one copy of the SMN1 gene and now my husband has to be tested. It’s nice (if not a little scary) to know that if his bloodwork comes back negative, there’s still a possibility that he’s a carrier. I will talk to my doctor about it when the results are back. Thank you.

  7. CHRISTINA says:

    Hi Audra-sorry for your loss of Andy. Im a carrier of SMA and my husband is a reduced carrier(he has 2 copies)although we were given odds of 1 in 2400 which is better than 1 in 4 we knew this didn’t exactly eliminate the risk of having a child with SMA so we elected to do the CVS test. The result came back my little one has 1 copy of smn1 like me-he’s a carrier. They did say theres still a 1 in 4000 chance he could be affected because the test cant exactly pick up everything(but they also said its very slim). This is my first pregnancy and what i would like to know is how can I eliminate doing the CVS test for further pregnancies? Should I do the linkage study so we exactly know if my husband has 1 copy on each gene? His Dad unfortunately is sick from liver disease and I really don’t know how much longer he will live so I would like to do this before its to late and at least I can continue the rest of my pregnancies with no worries. Thanks!

  8. christina says:

    I posted a prev. message but dont see it here. Anyhow, Im a SMA carrier and my husband is a reduce carrier(2 copies) although are odds were 1 in 2400 which was better than 1 in 4..we still elected to do the CVS test because the risk was still there. The result came back as my son is a carrier like me…thats good news but they also said they still give you a risk of 1 in 4000 that he can have it since the bloodtest cant exactly be 100% perfect. This is my first pregnancy and I dont want any further CVS testing for future pregnancies..if I do the linkage analysis to see what gene its on for my husband would that finally make this at ease about ever worrying?

  9. admin says:

    Hi, Christina! Your other comment is here – I just didn’t have a chance to moderate the website until this morning. (Sorry about that!) I’m glad that you are educated about SMA, had the carrier tests done, and had the CVS. And, I’m thrilled that your baby doesn’t have the disease!!

    The linkage study is expensive and generally not recommended for families without an incidence of SMA. For us, it was a way to determine how Andy got the disease. For you, I think that the assumption is that your husband is a normal non-carrier, with one copy of SMN1 from each parent. In that case, I don’t think the linkage study could tell you anything you don’t already know. It definitely can not predict where the gene will show up.

    Because you are a carrier, there is no absolute way to avoid worry in future pregnancies. With every new baby, there is the same chance of the occurrence of SMA. It’s up to you if you want to take CVS tests with future pregnancies. I know this probably isn’t the answer you wanted, but I hope it helps.


  10. toshena scale says:

    what if the mother is the carrier of sma but the farher is not

  11. doughboy says:

    I thought this thread was informative. My wife and I had this screening done as her brother passed away from SMA type I in the mid-1980’s before the SMN gene was recognized. As a sib, her carrier chance was 67%. With no previous family history, my carrier rate was roughly 3%. Overall, are chance without testing was less than 1%. Our results came back, and she tested as having 1 copy of SMN1. I tested as having 2 copies. This classified me as a reduced carrier. So our results are similar to yours. However, the chances are rare as a reduced carrier is now classified to have a 1/632 chance of carring the deletion. The overall odds for a person testing as a carrier and a person testing as a none carrier are now 1/2528 which is closer to the overall risk of the population which is 1/6400. What I am trying to say is that just is in this case, there are never any guarantees. Even if both tested as reduced carrier, there will never be a chance of 0%. I am sorry for your loss and thank your for your insight and information to remind people that nothing is ever gauranteed.
    Experience: PhD Biomedical Sciences
    Medical Research Experience: 8 years

  12. Tim says:

    Hi Audra,
    Just wanted to thank you for writing this. My wife and I are in the exact same situation. We just lost our first child to SMA, he lived for four days. We took the test, and she came back as a definite carrier, but my test came back with two SMN1 genes. We are now going to have my parents tested for more information.
    After reading your write up, I have a clearer understanding of exactly what is going on.

  13. Pingback: In the Family | Andy's Army

  14. Jessica says:

    Hi Audra,
    I am so thankful I am across this post. My husband and I lost our 2 month old littel boy, Marco, to SMA type 1 this past October. We was born what we thought prefectly healthy, but started exhibiting signs at 7 weeks. We spend 9 days in the childrens hospital, before the diagnosis came back. We had never heard of SMA and have no known family history. We have since had genetic testing, and my husband and I are both “Carriers” with the deleted gene and only two back up genes. We have discussed trying for a baby, and having the CVS test done. It is so nice to read your blog and to see you also took the same path as us. Thank you for sharing your story. God Bless!

  15. Allie says:

    Hi Audra,

    Thank you so much for sharing your story. I am 13 weeks pregnant and am scheduled for an amnio on 8/21/13. I opted to have the amnio over cvs to reduce the miscarriage rate and have a more accurate test result of the baby. I tested as a carrier of SMA and my husband as a reduced carrier ( 2 copies cannot confirm if they are on 1 chromosone). The genetic counselor gave us a 1 in 3 ,330 chance of having a baby with SMA and did not recommend further invasive testing. I of course have not been able to sleep since hearing this since there is no definitive answer on my husbands findings. I am curious if you have any opinion on the cvs vs the amnio? My doctor does not perform the cvs until 13-14 weeks and the amnio at 16 so there was not much of a time difference. I am so worried about the risks of harming a healthy baby but am just not comfortable with the uncertainty of the test. This is our first baby and I want to make all of the right decisions so any input you have is appreciated. Thank you again for sharing your story.

  16. admin says:

    Hi, Allie! Congratulations on your baby! I opted for the CVS with both of my pregnancies after we lost Andy, because we wanted to know as early as possible if the babies were SMA-affected. There certainly is a risk of miscarriage with the test, but – for us – not knowing if they had SMA was riskier. Luckily, Lucy and Will are healthy! However, I totally understand wanting to wait for the amnio too. Whatever choice you make is the right one for your family – just trust that!

  17. Vanessa says:


    Maybe you can help me I was informed yesterday that my husband is a reduced carrier and prior to that they diagnosed me as a carrier..I cant sleep my doctor told me to go to a genetics counselor which is going to take time to get an appointment.. Im lost please let me know any advice im scared to hurt the baby with cvs,,, im 13 weeks

  18. Vanessa says:


    Maybe you can help me I was informed yesterday that my husband is a reduced carrier and prior to that they diagnosed me as a carrier..I cant sleep my doctor told me to go to a genetics counselor which is going to take time to get an appointment.. Im lost please let me know any advice im scared to hurt the baby with cvs,,, im 13 weeks

  19. admin says:

    Hi, Vanessa! I’m e-mailing you!

  20. Trista says:

    I am not sure if you are still getting messages on this site. My husband and I both just found out we are both carriers of SMN1 with 2 copies. From what I am reading if both parents have 2 copies you are reduced risk. Is it still 1 out 4 children will be affected. I am currently 20 weeks.

  21. Tiffany says:

    Hello I’m so scared . I am a reduced risk carrier with 2 copies. And my husband is a carrier. Any advice?

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