Dear Doctor: I just entered my second trimester without any nausea at all. Considering a recent study linking morning sickness to a successful pregnancy, should I be worried?

Dear Reader: Nausea occurs in up to 80 percent of pregnancies. The cause appears to be related to a rapid increase of the pregnancy hormone beta hCG (BHCG) as well as the rise of estrogen. The nausea can be distressing for many women and can lead to worries about a future miscarriage.

Paradoxically, nausea may be an indication of a successful pregnancy. Preliminary studies have found this correlation, but they could not differentiate if the lack of nausea was related only to an early miscarriage or to an overall successful pregnancy.

A 2016 study in JAMA Internal Medicine looked at women between the ages of 18 and 40 who had a history of one or two miscarriages and who were trying to conceive. Of those women who were deemed to be pregnant based on a measurement of BHCG, 797 eventually became pregnant. These women kept daily diaries, recording instances of nausea and vomiting from the second week to the eighth week of their pregnancy. If a woman had nausea once, she was considered to have nausea in the pregnancy.

In week two of their pregnancy, only 20 percent of the women reported nausea, but by week eight of their pregnancy, nearly 85 percent of women had reported nausea. After the eighth week of the pregnancy, nausea was measured at monthly intervals. Also, to confirm a viable pregnancy, the women underwent an ultrasound at week six or week seven.

Investigators found that women who experienced nausea at some point during their pregnancy had a 50 percent reduction in the risk of a miscarriage. Those women who were afflicted with both nausea and vomiting at some point in their pregnancy had a 75 percent decrease in the chance of having a miscarriage. Moreover, based on the week-six and week-seven ultrasounds, the lack of nausea in the women who miscarried was not related to a lack of a viable pregnancy.

The authors proposed that nausea during pregnancy may have a protective effect by alerting a woman about her condition so as to alter her diet and avoid toxic substances, which could increase nausea or harm a pregnancy. I'm not so sure about this, but it is a possibility.

In regards to this study, I can understand the worry about your lack of nausea as you enter the second trimester of your pregnancy. But when you look further at the study, you find that nearly 94 percent of miscarriages happened in the first trimester of the pregnancy. So as you enter the second trimester, there is significantly less likelihood that you will have a miscarriage. Also, this study focused on women who had previously had a miscarriage, so its findings may be less applicable to you.

In summary, I wouldn't be concerned about your lack of nausea. But please do schedule follow-ups with your obstetrician for your regular prenatal care.

(Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles.)

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095.)

Dear Doctor: My parents, who are in their late 70s, are both now using hearing aids. This makes me wonder -- when should a hearing test become part of an annual checkup?

Dear Reader: Our ears connect us to the world, and yet until something goes wrong, hearing is one of the senses we seem to take for granted.

It's a good idea to have a hearing test as a baseline when you're a relatively young adult so that if you develop a hearing issue later, you will have a useful comparison. You can talk to your family doctor about giving you a hearing test or seek out the help of a hearing professional, known as an audiologist.

Hearing loss usually happens so gradually that you may not even be aware of it. It's not until you run into problems such as frequently asking people to repeat themselves or cranking up the volume on your TV that you realize something has changed.

Hearing is a complex process. When you hear a sound, your brain is interpreting electrical signals that it receives via the auditory nerve. These signals originate in the delicate structures within your inner ear, which receive sounds -- that is, vibrations -- and turn them into nerve impulses.

It's important to identify hearing loss as soon as possible. Studies suggest a link between hearing loss and serious conditions such as depression or dementia. An early diagnosis and successful intervention can reduce symptoms of depression and help preserve cognitive function.

If you think you may have trouble with your hearing, you're not alone. About 15 percent of American adults report some form of hearing problem. Signs that you could be experiencing hearing loss may include:

-- Frequently asking people to repeat themselves;

-- Having trouble pinpointing the source or direction of a loud noise;

-- Trouble hearing telephone conversations;

-- Being told the volume of your TV or radio is excessively loud;

-- Difficulty understanding a person who isn't facing you as he speaks;

-- Struggling to hear conversations in noisy environments, like a restaurant;

-- Difficulty understanding high-pitched voices;

-- Being told by others that your hearing seems impaired.

If you've experienced these situations, a hearing test is a good idea. It will reveal whether you have hearing loss in either ear, and pinpoint the type of hearing loss and to what degree it has progressed.

There are several types of hearing tests.

A physical exam with an instrument called an otoscope reveals any problems in your ear canal or eardrum. Additional tests include a pure tone test, which reveals how well you can hear -- you guessed it -- a variety of pure tones.

A speech test evaluates your ability to understand the spoken word. Tympanometry tests reveal any problems in the middle ear and evaluate the mobility of your eardrum.

These hearing tests are painless. Taken together, the results offer a detailed picture of your hearing, known as an audiogram. And if a problem should be uncovered, your audiogram gives you and your doctor the information needed to move forward.

(Eve Glazier, M.D., MBA, is an internist and assistant professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and primary care physician at UCLA Health.)

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095.)

Dear Doctor: We live near a freeway with a lot of traffic. Do I need to be worried about my kids developing asthma?

Dear Reader: This is a valid question. Cars release a number of small particles into the air that can irritate the lungs. This particulate matter, in addition to nitric oxide and black carbon, has been associated with asthma. Children are more vulnerable to the effects of pollution because their lungs and immune system are still developing.

An analysis of many studies, published in the journal Allergy, found a significant association between traffic-related air pollution and asthma. The association was seen as early as the age of 1 and increased as the child became older. The rise in asthma as kids get older seems to be related both to structural changes that pollution creates in the lungs and also to the extent of time that older children spend outside compared to younger children.

One other important aspect of asthma is that it can be exacerbated by stress. Noise pollution and the danger of cars in close proximity may worsen asthma symptoms in children because they can create a more stressful environment. This is an important factor if your child already has asthma.

A recent study in Minnesota evaluated the number of asthma cases in the emergency room, in the hospital and in a clinical setting. The researchers then looked at the proximity of people to a major source of traffic. After taking poverty out of the equation, which is in itself a risk factor for asthma, the researchers found that, as traffic density increased, so did the number of asthma exacerbations, or worsening of asthma symptoms. In this study, for every 10 percent increase in traffic density, there was a 15 percent increase in asthma-related visits to the emergency room, hospital or outpatient clinic.

Clearly, if you have a child with asthma, moving him or her away from traffic would indeed reduce the chance of a severe asthma attack.

I understand your concern. It is also a societal concern. Despite the evidence linking pollution to asthma, the levels of pollutants that are associated with an increased risk of asthma are well below the guidelines from the World Health Organization.

This doesn't mean that if you live near a freeway, you have to move. The majority of children who live near a freeway do not get asthma. But if your child is already showing signs of asthma at a young age, then moving to an area with fewer pollutants is perhaps something you should consider, if it is at all possible.

It took many years for our society to accept the ills of secondhand smoke from cigarettes. When it did, smoking was then banned from many public places, even casinos. It is more difficult to ban housing near freeways or heavily trafficked areas, especially when that housing already exists.

(Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles.)

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095.)