Complex sentence: The largest twin registry in the United States is the Mid-Atlantic Twin Registry (MATR) at Virginia Commonwealth University, which has more than 51,000 registered participants, with approximately 46,000 of these representing intact twin pairs.
Split into simpler sentences: The largest twin registry in the United States is the Mid-Atlantic Twin Registry (MATR) at Virginia Commonwealth University. Registered participants number more than 51,000 with approximately 46,000 of these representing intact twin pairs.