A 42-year-old African American comes to your office for her annual wellness examination. She has no medical problems today. However, she reports a family history of uterine leiomyomas. She exercises regularly in a local gym. She smokes one pack of cigarettes daily. She is married and has four children. Her menarche was at 18 and her menopause was at 40. Which of the following is a protective factor against uterine leiomyomas in this patient?