Final answer:
The transition in midlife where fertility declines is called menopause in women and andropause in men. Menopause signifies when a woman can no longer conceive naturally, occurring typically around age 52, while andropause, though less clearly marked, affects men with similar age-related declines in fertility.
Step-by-step explanation:
The term for the midlife transition in which fertility declines is known as menopause for women and andropause for men. Female fertility peaks in the twenties and begins to decline gradually until around 35 years of age, after which the decline accelerates until menopause, typically occurring around age 52. Menopause is defined by the cessation of the menstrual cycle, signaling the end of natural fertility for women. In men, a decline in Leydig cell activity starting around 40 to 50 years of age can lead to symptoms of andropause, which includes reduced muscle mass, fatigue, and a decline in fertility. It's essential to note that while menopause is a clear marker in women, andropause in men does not have such a definitive sign.
During middle adulthood, which extends from the 40s to the 60s, both men and women may experience physical changes. For women, the end of the menstrual cycle is a significant change, while men may face reductions in testosterone levels and lowered fertility. As women approach their mid-40s to mid-50s, menopause sets in as their ovaries begin to lose sensitivity to follicle-stimulating hormone (FSH) and luteinizing hormone (LH), resulting in decreased frequency and eventually the cessation of menstrual periods.