【单选题】40-year-old G2P1001 presents to your office for a routine OB visit at 30 weeks gestational age. Her first pregnancy was delivered 10 years ago and was uncomplicated. She had a normal vaginal delivery at 40 weeks and the baby weighed 7 lb. During this present pregnancy, she has not had any complications, and she reports no significant medical history. She is a nonsmoker and has gained about 25 lb to date. Despite being of advanced maternal age, she declined any screening or diagnostic testing for Down syndrome. Her blood pressure range has been 100 to 120/60 to 70. During her examination, you note that her fundal height measures only 25 cm. Which of the following is a likely explanation for this patient's decreased fundal height?