Dear Doctor: Several studies have shown a correlation between marijuana use and low birth weight, and I understand that marijuana today has been cultivated to be much stronger than what was used in earlier years. Should this be a concern for women who are pregnant?

Dear Reader: We’ve written before about the use of marijuana, also referred to as cannabis, during pregnancy, specifically in relation to morning sickness. And while it’s true that cannabis is effective in easing nausea, we recommend against its use during pregnancy.

Cannabis is a complex plant that contains hundreds of chemical compounds, including those with psychoactive effects. Science’s understanding of cannabis’s properties, as well as its long-term effects on the human body, is in its early stages. What we do know is that chemicals in cannabis can pass through a woman’s body and into her fetus, and that may affect development.

Several recent studies have drawn a connection between cannabis use during pregnancy and low birth weight, a term that refers to infants born weighing less than 5 pounds, 8 ounces. Although low birth weight infants can be healthy, they are also at increased risk of complications such as an inability to fight infection, having trouble eating and not gaining weight.

Cannabis use in pregnancy also has been linked to increased risk of preterm birth. A recent study from the Colorado School of Public Health analyzed health data from 3,200 women and found that cannabis use during pregnancy was associated with a 50% higher risk of low birth weight. Close to 90% of women in the survey who had used cannabis during pregnancy also breastfed their babies, adding another unknown to the equation.

As you mention, cannabis products today are 6 to 7 times more potent that they were in the 1970s, according to some estimates. Meanwhile, cannabis use among women who are pregnant has increased more than 60% between 2002 and 2014, according to the National Survey on Drug Use and Health. Current research suggests that 4% to 7% of pregnant women now use cannabis, often in the early months of pregnancy to deal with morning sickness. In a study published earlier this year, which also found a link between cannabis use and low birth weight, 22% of the 2,173 pregnant women screened positive for cannabis use.

Our advice against cannabis use during pregnancy is not based on any specific study. Rather, it’s because too little is known about how the chemical compounds in cannabis may affect a developing fetus or a child’s later development. In our opinion, that lack of knowledge makes the potential risks of cannabis use during pregnancy too great at this time.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I’m in my 70s, and I work about 35 hours a week doing shipping, cleaning and light lifting. I’ve stopped visiting the gym because I’m too tired after work. My back hurts, and I’ve had heart issues recently, leaving me with a pacemaker and defibrillator. Are the 10,000 steps I walk at work each day enough exercise?

Dear Reader: The 10,000 steps you are logging each day are to be celebrated at any age. Add in a physically active job and your wish to stay connected to your work community, and we think you are making many good choices. The interactions of a workplace offer purpose and camaraderie. And the health benefits of human connection -- particularly among the elderly -- are well-documented. These include a reduced risk of cardiovascular problems, inflammatory issues and cognitive decline.

In discussing the scope and rigor of exercise with our older patients, we encourage them to listen to their bodies. They themselves are the best teachers when it comes to what types of exercise to take part in, and how much to do. Our approach is to break physical exercise into four basic categories -- endurance, resistance, flexibility and balance. In terms of plain old numbers, the American Academy of Family Physicians recommends at least 2.5 hours of moderate aerobic exercise spread out over the course of a week. This is exercise that raises the resting heart rate, such as brisk walking.

Your work, which in addition to those 10,000 daily steps has some strenuous elements, is contributing to that recommendation. However, to balance things out, you might consider adding resistance exercises, like lifting free weights. Resistance exercise helps prevent osteoporosis and frailty, builds and maintains muscle mass, improves balance and improves mood and cognition.

Flexibility exercises such as yoga or tai chi are also an important element of regular exercise, and they also help with balance. Senior and community centers often offer these and other flexibility classes targeted to older bodies. Just be sure to let the instructors know about any physical problems you have, including the back issues you mentioned in your letter.

You wrote of having heart problems in the recent past. Since you have both a pacemaker and a defibrillator, we strongly recommend that you see your primary care physician to discuss your current and any future exercise regimens. While moderate exercise with either of those devices is generally considered safe, certain adjustments may need to be made to accommodate the implanted devices.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My boyfriend laughs when I turn down the car stereo when I’m trying to find a new address, but I’ve seen his dad do the same thing when he’s changing lanes on the freeway. Why does the quiet make you feel like you can see better?

Dear Reader: Your boyfriend may have fun teasing you, but we’d be surprised if there haven’t been similar instances in his own life.

The scenarios you describe, which require you to evaluate incoming data and make split-second decisions, suck up a lot of mental energy. Researchers refer to this as “cognitive load.” Eliminating a distraction such as sound can help with concentration by reducing the cognitive load. That’s because, much like a computer, our brain’s capacity to engage in multiple tasks is finite.

To understand why, it helps to appreciate the complexities of the human brain. It’s made up of three major parts -- the cerebrum, cerebellum and brainstem. The largest and topmost part is the cerebrum, which is divided into left and right hemispheres. It deals with learning, emotions, reasoning and fine motor skills, and it interprets the flood of data from the five senses. Tucked beneath is the much smaller cerebellum, which oversees muscle movement, balance and posture. Acting as a relay center between the two and the spinal cord is the brainstem, which also oversees the staying-alive stuff we don’t think about but can’t live without. This includes breath, heartbeat, swallowing, sleep cycles and temperature control.

The three areas of the brain, plus the spinal cord, make up the central nervous system. It regulates movement, thought and emotion. It’s why you’re able to find that new address. But while it seems as though we can concentrate on multiple things at once, the brain handles tasks sequentially. It’s just making the switch so swiftly, in mere nanoseconds, that it feels like we’re multi-tasking. Once you’re hunting for a street sign, though, or gauging car lengths in fast-moving traffic, all of the collecting and interpreting of data by the central nervous system reaches critical mass. That cognitive load we talked about earlier gets to be too great. Turning down the radio removes enough incoming data to make it easier to concentrate.

A small study in Sweden looked into this phenomenon several years ago. Researchers had each of 32 participants perform an easy visual task and a challenging visual task. They were also instructed to ignore the audio -- a random sequence of sounds -- that was playing. The subsequent MRI tests revealed that when participants concentrated on a visual task, there was a decrease in the responsiveness of the auditory nerves. Basically, the brain itself was turning down the volume.

The takeaway here is that when our inner mute button can’t adequately remove the auditory distraction, our brain kicks in and we decide to turn down the car stereo. We’re adjusting our environment so the most important task -- finding the address or safely merging onto the freeway -- has a better chance of getting accomplished.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)