Premature births are one of America\'s most serious health problems with more than 10 of U.S. births in this category. Multiple births, largely the result of infertility treatments, are also a big factor. \"A rule of thumb is that each time there\'s another baby in a pregnancy, you subtract three weeks from a full-term pregnancy, \" said Dr. Mitchell Dombrowski, chief of obstetrics and gynecology at St. John Hospital, and professor of ob-gyn at the WSU School of Medicine.