A 22-year-old primagravida comes to the office for a prenatal visit. She is in her second trimester and has had prenatal care since she was 8 weeks pregnant. Her only complaint is a new brownish line straight down her abdomen. Vital signs are unremarkable. Urine has no protein, glucose, or leukocytes. Fetal heart rate is 140; the client's uterus is palpated to the umbilicus. What physical finding is responsible for her new "brown line"?