Does regular upper-body workout lead to an increase in the circumference of the wrist? I'm thinking of exercises like bench press, curls, etc.
In short, Yes!
There are two aspects to this:
Any exercise that places regular stress on a given bone will increase bone density overtime. For the wrist specifically, strengthening the forearm and hand muscles (in the forearm) will have the greatest effect. When you do these other exercises these muscles are still indirectly strengthened as supporting muscles and, in the case of the bench press for example, there is a lot of stress placed on the bones when in the starting and ending positions.
The process of bone density and growth is a relatively slow process and depends on many factors. Some factors are genetic in nature but the most notable are related to gender, age, hormones and nutrition. If you're a female beyond menopause then your bone density cannot increase and you should limit these high impact activities especially if you're at risk of osteoporosis. This is one reason why women should participate in strength training and high impact activities while they're young as they're bones will become more dense and therefore stronger to prevent osteoporosis later in life. For males bone growth may slow with age but it does not normally stop altogether unless there are other genetic, nutritional, and/or hormonal factors. (See below for more detailed explanation on bone growth.)
The term wrist is literally where the carpus or carpal bones are but many people actually mean the lower forearm when they refer to the wrist. There are also muscles in the forearm, such as the Flexor Carpi Ulnaris and pronator quadratus. The pronator quadratus is in the wrist where you would wear a wrist watch, but not in the wrist proper, (where the arm meets the hand or where handcuffs go). These can be strengthened as well. These muscles can increases in size as well with weighted movements.
Forearm and Wrist Exercises include:
Wrist Curls For Flexion
Reverse Wrist Curls For Extension
Behind The Back Wrist Curls
One-Arm Cable Reverse Curls
To specifically target the pronator quadratus:
Seated forearm pronation
Side lying forearm pronation
Standing forearm pronation with rope attachment
Flexbar forearm pronation
Band seated forearm pronation
Band standing forearm pronation
While you can increase the thickness of your wrist with physical activities, this should be in no way be a goal nor a hindrance unless there are mitigating factors. The reason being is that the process is relatively slow and not significant. Anything that stimulates the production of HGH will have a net effect of bone gain.
Detailed Explanation on Bone Growth:
There are several types of bone cells that preform different functions. Osteocytes are the cell in the bone that maintain the bone mineral concentration. (1) Stimulated as stress is placed on the bone through gravity or exercise by mechanotransduction (3, 10, 11, 12), the Osteocytes will increase mineralization (aka calcification). Bone mineral is initially deposited in “hole” zones between the ends of collagen fibrils of the matrix. As bone matures, hydroxyapatite crystals enlarge and by crystal growth and aggregation. The process of mineralization takes place throughout the entire life. (1) Osteocytes die as a consequence of senescence, degeneration/necrosis, apoptosis (programmed cell death), and/or osteoclastic engulfment. The percentage of dead osteocytes in bone increases with age from less than 1% at birth to 75% after age 80. (12)
Osteoblasts are the cell responsible for bone growth. (1, 2) Osteoblasts are stimulated by Human Growth Hormone (HGH) (3, 4, 5, 6, 7) which is produced in higher amounts during and after strenuous exercise. (3) Osteoblasts are found in the growing portions of bone, including the periosteum and endosteum. After bones are fully developed in adulthood, the bone will not increase in length but can/will increase in diameter given the right conditions of nutrition and hormone stimulation.
HGH is naturally produced by the pituitary gland during sleep. HGH production will wane with age. To both, maximize bone gains as well as muscle, you need to maximize HGH production. HGH is produced by the body every night. Production is increased after anaerobic activities. To maximize production of HGH, you'll need to get good rest (8 hours sleep) and ample nutrition including protein and vitamins. Some people supplement with BCAA's, Vitamin D, Glutamine, GABA, A-GPC, L-arginine and L-lysine to improve HGH performance. Some suggest avoiding sugar after workouts. They say that consuming sugar (especially fructose) within 2 hours post workout will cause your hypothalamus to release somatostatin, which will decrease your production of HGH. Simple sugars that are high-glycemic also spike insulin levels. Not only does this lead to body fat storage, but it severely decreases the release of growth hormone. (13)
Osteoclasts are the cells responsible for breaking down the bone back into raw materials. These are the antithesis to the Osteoblasts. Together they make up a process called bone remodeling and facilitate appositional growth. (2, 8, 9)
"The result is an increase in the overall rate of bone remodeling, with a net effect of bone accumulation. The absence of GH results in a reduced rate of bone remodeling and a gradual loss of bone mineral density." (7)
Finally, here is proof that the introduction of Growth Hormone along with exercise/stress on bone will increase both bone size and density in experiments done on rats. Assuming the biological processes are the same in humans, we can expect the same results give similar circumstances.
"The mild exercise [in control group -without GH] doubled the mineralizing surface but did not influence the bone formation rate significantly. The combination of GH injections plus exercise, however, resulted in a further increase of 39% in bone formation rate, primarily at the anterolateral aspects, and an increase of 5% in cross-sectional area compared with the group injected with GH only. The femur ultimate breaking load was increased by 37% and the stiffness by 42% in the group injected with GH compared with the saline-injected group." (3)
Bone minerilaztion/density increases with any stress placed on the bone including working out. Human Growth Hormone (HGH) and other hormones will help to facilitate this as long as there is simulation by mechanotransduction. Osteocytes are the cells responsible for this and have an average lifespan of 30 years so bone minerilaztion will decrease with age.
Bone size can and will increase as well. This is heavily dependent on Human Growth Hormone levels that increase with high intensity and anaerobic workouts. Overall, HGH decreases with age without HGH treatment. In women post menopause, bone density nor size will not increase due to reduction of various hormone levels, however, this too can be mitigated by HGH treatment and hormone therapy but with possible side effects.