is repurposed as a tibialis anterior muscle, a version of this being called the Bridle procedure. Severe scoliosis caused by FSHD can be corrected with spinal fusion; however, since scoliosis is a compensatory change in response to muscle weakness, correction of spinal alignment can result in further impaired muscle function. Prognosis Genetics partially predicts prognosis. Those with large D4Z4 repeat deletions (with a remaining D4Z4 repeat array size of 10-20 kbp, or 1-4 repeats) are more likely to have severe and early disease, as well as non-muscular symptoms. Those who have the genetic mutations of both FSHD1 and FSHD2 are more likely to have severe disease. It has also been observed that disease manifestation is milder when a prominent family history is present, as opposed to a new mutation. In some large families, 30% of those with the mutation do not show symptoms, and 30% of those with symptoms do not progress beyond facial and shoulder weakness. Women are less likely to be symptomatic, and if symptomatic have less severe manifestations. Remaining variations in disease course are attributed to unknown environmental factors. A single study found that disease course is not worsened by tobacco smoking or alcohol consumption, common risk factors for other diseases. Pregnancy Pregnancy outcomes are overall good in mothers with FSHD; there is no difference in rate of preterm labor, rate of miscarriage, and infant outcomes. However, weakness can increase the need for assisted delivery. A single review found that weakness worsens, without recovery, in 12% of mothers with FSHD during pregnancy, although this might be due to weight gain or deconditioning. Epidemiology The prevalence of FSHD ranges from 1 in 8,333 to 1 in 15,000. The Netherlands reports a prevalence of 1 in 8,333, after accounting for the undiagnosed. The prevalence in the United States is commonly quoted as 1 in 15,000. After genetic testing became possible in 1992, average prevalence was found to be around 1 in 20,000, a large increase compared to before 1992. However, 1 in 20,000 is likely an underestimation, since many with FSHD have mild symptoms and are never diagnosed, or they are siblings of affected individuals and never seek definitive diagnosis. Race and ethnicity have not been shown to affect FSHD incidence or severity. Although the inheritance of FSHD shows no predilection for biological sex, the disease manifests less often in women, and even when it manifests in women, they