The molecular basis of the myotonic dystrophy (MD) kinase gene is expansion of the CTG repeat at the 3-untranslated region of the MD gene. Variability of the CTG repeat size in different tissues of affected individuals has been demonstrated. The objective of this report was to examine and review the feasibility of prenatal diagnosis of congenital myotonic dystrophy (CMD) in pregnant women with MD using CTG repeat sizes in amniocytes or villi. We present a case of a pregnant woman with MD who underwent prenatal diagnosis of MD using amniocytes. The repeat size in the amniocytes was smaller than the repeat size in the maternal leukocytes and smaller than the repeat size in the infant blood. The infant had CMD. We also reviewed the literature for reports on MD cases that were prenatally tested for CTG repeat size using amniocytes or chorionic villi. Data were tabulated based on the number of maternal CTG repeats, prenatal procedure [amniocentesis or chorionic villus sampling (CVS)], CTG repeat size in fetal tissue, fetal/infant blood, and pregnancy outcome. Twenty-seven pregnancies at risk for MD that underwent prenatal diagnosis were reported. Eleven (40.7%) of the 27 pregnancies underwent amniocentesis, and 16 (59.3%) underwent CVS. Fourteen patients (61%) demonstrated an increase in CTG repeat size in the amniocytes or villi compared with the maternal repeat size. Nine (33%) of the 27 pregnancies were terminated because of CMD risk. The outcomes of 11 (40.7%) pregnancies were consistent with diagnosis of CMD. CMD was diagnosed in fetuses demonstrating expansion or contraction of the CTG mutation in the amniocytes. Prenatal diagnosis of MD is possible by using mutation analysis on maternal and fetal DNA and detection of the CTG repeat expansion. Prenatal diagnosis of CMD is more complex. The possible lack of correlation between CTG repeat size in amniocytes, villi, and other fetal tissues is a potential limitation in prenatal diagnosis and counseling of CMD using CTG repeat size. Thus, prenatal diagnosis of CMD should be based on a combination of factors, including maternal pregnancy history, clinical findings, and cautious interpretation of maternal and fetal DNA analysis. Am.