miscarriage For our purpose in the book, the loss of an early embryo at or before the expected time of a period. Not noticeable without specially measuring levels of serum human chorionic gonadotropin One form of so-called biochemical pregnancy. miscarriage rate The percentage chance that a pregnancy will end as a miscarriage. The normal rate of clinically apparent miscarriage for young women is about 12 percent. The rate rises independently with age, the number of previous pregnancies, and especially the number of previous miscarriage s experienced. The rate is also higher with many causes of infertility when conception occurs with or without treatment.