Which of the following factors increases sex hormone binding globulin?

Sex hormone binding globulin (SHBG) is a protein produced by the liver that binds to sex hormones, specifically testosterone and estradiol, regulating their bioavailability in the bloodstream. Pregnancy is a condition marked by significant hormonal changes that impact SHBG levels.

During pregnancy, increased levels of estrogen stimulate the liver to produce more SHBG. This elevation in SHBG enhances the binding of sex hormones, which is critical for various physiological processes related to maternal health and fetal development. Higher SHBG levels during pregnancy are essential for regulating the active forms of testosterone and estrogen, ensuring that both the mother and developing fetus have adequate hormonal support.

In contrast, other factors such as obesity, menopause, and smoking typically do not lead to an increase in SHBG levels. Obesity is often associated with lower SHBG due to increased levels of insulin and androgens, which can inhibit SHBG production. Menopause is characterized by a decrease in estrogen levels, leading to lower SHBG levels as well. Smoking has been linked to reduced SHBG levels too, possibly due to its effects on estrogen metabolism.

Thus, the condition of pregnancy is the notable factor that leads to an increase in SHBG, making it the correct choice in this context.