“Sit up straight. Put your shoulders back. Don’t slouch.” Chances are good that you’ve heard nags like these from your mother more than a few times in your life.

This week, I tested a gadget that might give mothers a rest. It’s a $150 sensor called LumoBack, from a company called Lumo BodyTech, that straps around your lower waist to track your posture and vibrates whenever you slouch. It also tracks steps while walking and running, standing time, sitting time, sleep positions and sleep time.

I’m nearly eight months pregnant and I’ve been wearing this gadget on and off for several days and nights. If it fits me, one of the band’s two sizes will likely fit you. LumoBack uses Bluetooth to correspond with a free app that runs on Apple’s iOS devices. (An Android version of the app is planned before the end of this year.) The company released a new version of its app on Tuesday, as well as a smaller version of its sensor band, both of which I’ve been testing. The band is sold at LumoBack.com.

LumoBack was comfortable enough that I frequently forgot I was wearing it — at least until it vibrated because I wasn’t sitting up straight. This happened while I typed on my computer keyboard, ate a meal, drove a car, sat in a meeting or while standing and talking to someone. After a week, I viewed LumoBack like eating broccoli: I know it’s good for me, but I don’t necessarily enjoy it. One upside is LumoBack is worn under clothing so you aren’t announcing to the world you’re tracking your movements by wearing something visible, like a sporty wristband.

LumoBack’s app with its armless stick figure that reflects your moves on an iOS device’s screen is lovable. The figure turns yellow and frowns when your posture is poor, and turns green and smiles when your posture is good. This illustration helps you know how to adjust your sitting or standing position. But it’s unrealistic to constantly look at a screen to check your posture so most of the times I felt these vibrating nags, I had to guess how to improve my posture.

The first nagging buzz comes after you’ve been exhibiting bad posture for four seconds, then it buzzes again at 11 seconds. If you don’t correct your posture after that, LumoBack assumes you don’t want to be corrected again and stops buzzing. When you change positions, it restarts this count. You can turn the vibrations off, but you’ll probably forget you’re wearing it and your posture will suffer, defeating the purpose. In the next few months, Lumo BodyTech plans to release a coaching component for its app that aims to make the sensor more of a motivational gadget.

For example, notifications will appear on your iPhone that say things like, “You’ve been sitting in that chair for 30 minutes. Stand up for yourself!” Another notification could tell you your posture score is 48 out of 100, encouraging you to try harder to hit your goal of 50 for the day. Speaking of goals, the coming version of the app will set personalized goals for each user. For now, goals are more broad, set using general numbers that combine all users’ statistics.

The band is of stretchy nylon and the sensor, which rests at the small of your back, measures 3.9 inches by 1.6 inches and is 0.3 inch thick. I set up my LumoBack with an iPod Touch by entering my name and calibrated the app specifically for me, which took a few minutes of walking, sitting and standing. I entered my height and weight to help determine calories burned and miles walked in the app’s Steps section.

I was surprised to see how much time I sit during work so I started standing during phone calls to clock more “stands” to get to my daily goal. I was interested to see my sleep positions reflected in the app, which showed how long I slept each night and how much time I spent on my back, right side, left side and front. Data like this is helpful if you’re trying to avoid sleeping in a certain position, like how I’m avoiding sleeping on my back during pregnancy.

I was concerned about wearing a Bluetooth device close to my body for so long. Lumo BodyTech’s co-founder, Andrew Chang, said LumoBack transmits activity around 1 percent to 2 percent of the time during typical usage, and its radiation levels are about 25 times less than a Bluetooth headset.

The LumoBack can store up to a month’s worth of data, so even if you aren’t near your iOS device for transmitting stats, the data recorded by this sensor will be saved for a while, then transmitted when you’re back in range. The sensor’s battery lasts five to seven days, depending on usage, and recharges via an included USB cord that plugs into a computer.

LumoBack or no LumoBack, your mother will probably still bug you about your posture. So save her the trouble: Keep your shoulders back.

Email Katie at katie.boehret@wsj.com.