Will My Child Get a Worse Form of Myotonic Dystrophy?

The short answer is yes, there is a significant risk that your children could inherit a more severe version of myotonic dystrophy type 1 (DM1) than you have, and they may start showing symptoms at an earlier age ^[1][2]. This happens because of a genetic phenomenon known as anticipation, which is a core biological feature of DM1 ^[1]. Understanding how anticipation works can be frightening, but having this knowledge is a critical step in making informed choices about family planning and your health.

The “Stutter” in the Gene

To understand genetic anticipation, it helps to look at the specific biology of DM1. The disease is caused by a mutation in the DMPK gene ^[1]. Inside this gene, there is a specific sequence of DNA building blocks—represented by the letters CTG—that repeats itself, almost like a genetic stutter ^[1].

When the body creates sperm or egg cells, this repeating sequence is highly unstable ^[3]. As the gene is passed down from a parent to a child, the stutter often expands, meaning the number of CTG repeats increases ^[1][2]. The larger the repeat expansion, the more severe the symptoms tend to be, and the earlier in life they typically appear ^[1][4].

To understand your own lab report, it helps to know the typical ranges for these repeat numbers. While there is no rigid, absolute threshold that perfectly predicts the disease course, the sizes generally fall into these categories ^[1][5]:

• Normal: 5 to 37 repeats
• Premutation (no symptoms but risk of expansion in children): 38 to 49 repeats
• Mild / Late-onset: 50 to 100 repeats
• Classic / Adult-onset: 100 to 1,000 repeats
• Congenital / Childhood-onset: Over 1,000 repeats

This expansion across generations is the definition of genetic anticipation: as the disease is passed down, the mutation “grows,” anticipating an earlier and often more severe disease presentation ^[1][6].

• Generation 1: A grandparent might have 70 repeats and develop mild cataracts in their 60s.
• Generation 2: Their child might have 300 repeats and develop muscle weakness and heart issues in their 30s.
• Generation 3: Their grandchild might inherit over 1,000 repeats and be born with congenital myotonic dystrophy, a severe form of the condition present from birth that causes life-threatening muscle weakness and breathing difficulties ^[1][4].

(Note: While expansion is the primary concern, it is also possible—though much less common—for the repeat sequence to contract, or shrink, when passed down, potentially leading to a milder presentation in the child ^[2][1].)

The Role of the Passing Parent

The parent who passes on the gene influences the risk of a rapid CTG expansion. While the repeat sequence can expand when passed down from either parent, research shows that massive expansions—the kind that lead to the severe congenital form of DM1 in babies—are significantly more common when the mother passes on the gene ^[7][6]. If the father has DM1, the repeat often still expands in his children, but the increase is usually less dramatic ^[7].

What about Myotonic Dystrophy Type 2 (DM2)?

If you have myotonic dystrophy type 2 (DM2) rather than type 1, genetic anticipation works very differently. DM2 is caused by a different genetic stutter (a CCTG repeat) in a different gene called the CNBP gene ^[8]. Unlike the gene in DM1, the DM2 mutation is relatively stable across generations ^[8][9]. As a result, genetic anticipation is generally not observed in DM2, and people with DM2 do not face the same risk of having a baby with a severe congenital form of the disease ^[10][11].

Taking Control: The Role of Genetic Counseling

Given the high risks of genetic anticipation in DM1, meeting with a genetic counselor is a highly recommended step for anyone considering having children ^[12][9]. A genetic counselor can help you understand your specific risks based on your precise genetic test results ^[12]. Because DM1 is passed in an autosomal dominant pattern, only one parent needs to have the mutated gene for the child to be at risk of inheriting it.

A counselor can walk you through proactive reproductive options to build your family safely. These may include:

• Preimplantation Genetic Testing (PGT): Using in vitro fertilization (IVF) to create embryos in a lab. The embryos are tested for the DM1 mutation, and only those without the mutation are implanted ^[13].
• Prenatal Testing: Testing a fetus during pregnancy (via amniocentesis or chorionic villus sampling) to see if it has inherited the expanded gene ^[14]. It is important to know that while prenatal testing can confirm if the gene is present and its approximate size, it cannot perfectly predict the exact clinical severity or age of onset for the child.
• Alternative Options: Many families also explore alternative ways to build their family without passing on the gene, such as using donor eggs, donor sperm, or adoption.

Furthermore, women with DM1 face unique health risks during pregnancy, such as higher rates of miscarriage and a potential worsening of their own DM1 symptoms ^[14][15]. If you have DM1 and plan to become pregnant, establishing a specialized medical team before conception is vital for ensuring both your safety and your baby’s health.