Imagine a 57-year-old woman, let’s call her Maria, who’s just opened a letter about her mammography results. She’s had several mammograms before, but this time reads new information: “Your breasts are dense”.

While the letter assures her that dense breasts are common, it also indicates it could make it harder to see breast cancer on the mammogram.

Maria is confused about what to do next and wonders if she should be worried. Does she need to see her GP?

Maria may be fictional but she reflects the findings from the first trial of its kind we publish today.

We show women notified they have dense breasts alongside their mammogram result are more confused and anxious, do not feel more informed, and have greater intentions to see their GP for advice.

Remind me, what is breast density?

Dense breast tissue appears white on a mammogram and can hide (or mask) a cancer, which also appears white.

Dense breasts are very common. About 25–40% of women are considered to have dense breasts.

Breast density is one of several independent risk factors for breast cancer. After years of consumer advocacy, more women are being told about their breast density when they get their results from breast cancer screening.

The idea is simple: let women know if they have dense breasts – something that can raise cancer risk and make mammograms harder to read – so they can decide whether to get extra testing, such as an ultrasound or MRI.

Notifying women about their breast density is now legislated in the United States, recommended in Australia, and is being considered in other jurisdictions, such as the United Kingdom.

This is despite the lack of robust evidence on whether the benefits of notifying breast density at screening outweigh potential harms for women, and the impact on health services.

What we did and what we found

Our trial was co-designed with BreastScreen Queensland. From September 2023 to July 2024 we randomised 2,401 women (average age 57) who had a clear mammogram (their mammogram didn’t show cancer), but had dense breasts, into three categories:

1. Control: no notification of dense breasts (standard care)
2. Intervention 1: notification of breast density as part of the screening results letter plus extra written information in a leaflet
3. Intervention 2: notification of breast density as part of the screening results letter plus a link to extra information in an online video.

Eight weeks after screening, we found women notified they had dense breasts felt more anxious and confused about what to do about their breast health compared to the control group.

They also did not feel more informed to make decisions about their breast health, and had greater intentions to discuss this with their GP.

We haven’t followed participants for long enough yet, nor was the trial specifically designed to see if notifying women about their breast density led to extra cancers being detected.

The trial also had some limitations. For example there was a low proportion of women from non-English speaking backgrounds.

However, this is the first randomised trial world-wide to evaluate the immediate impact of breast density notification on women in the context of mammography screening.

It provides evidence for breast screening programs internationally to carefully consider the potential impact of such notification.

What next?

In Australia, where breast density notification is now recommended, it is important we acknowledge that the topic of breast density may be confusing and some women may be worried.

Communicating about breast density, including public messaging, should be focused on density being one of many risk factors for breast cancer and that there are other potentially modifiable ways to reduce a woman’s overall risk.

This includes maintaining a healthy weight, being physically active, reducing alcohol intake, and not smoking. Messaging should also emphasise that mammograms remain the best way to screen for breast cancer in most women even if they have dense breasts.

GPs need to be prepared to have conversations with women about breast density and their overall risk of breast cancer. This includes discussing the benefits and harms of extra screening (via ultrasound, MRI or contrast-enhanced mammograms) that can detect cancers not found on mammograms.

But even that’s not straight forward. For instance, while there is evidence extra screening will detect more cancers, there’s currently no evidence on whether it will reduce advanced-stage breast cancers or death from breast cancer.

Extra screening may lead to adverse effects such as false-positives – apparent abnormalities that, after further evaluation, are found not to be cancer.

Extra screening is also not equitable for all women due to out-of-pocket costs and limited availability through public services.

We need better pathways for evidence-based, equitable care in Australia so the benefits of notifying women about their breast density indeed outweigh any adverse consequences for women and the health system. These pathways need to be evaluated to ensure they are feasible, acceptable, effective and equitable.

Brooke Nickel, NHMRC Emerging Leader Research Fellow, University of Sydney and Nehmat Houssami, Professor of Public Health, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

As the number of people with allergies grows worldwide, scientists are trying to work out precisely how and why these conditions – such as asthma and eczema – develop.

One long-standing idea is the “hygiene hypothesis”. This suggests our modern indoor lifestyles are to blame, as they limit our early exposure to germs and allergens which help train the immune system.

But growing evidence suggests having a pet may counter this effect. As any pet owner knows, our furry friends bring a lot of mess, germs and fur into our homes – along with the cuddles.

So, does spending time with animals lower children’s risk of allergies? Here’s what we know.

How allergies develop

During early childhood, our immune systems learn what to attack and what to ignore to stop us getting sick.

Evidence suggests early exposure – to family members, food, germs, dust, dirt, pollen and pet dander (skin flakes) – shapes this immune response.

Allergic conditions develop when the immune system overreacts to harmless substances, such as dust, pollen or certain foods. These reactions can affect the skin, airways and gut.

However, we still don’t fully understand why some people develop allergies while others don’t.

Scientists have identified genes linked to allergic conditions. But most have subtle effects on the immune system and act as “risk factors” – they increase the chance of disease but don’t cause it outright.

Recent research suggests exposure to bacteria in our environment could be another major factor.

From birth, our bodies are colonised by bacteria, especially in the gut. This community of microorganisms is known as the microbiome.

Ongoing “crosstalk” between the microbiome and immune system is crucial for healthy immune function. When this balance is disturbed, it can contribute to inflammation and disease.

The effect of our early environment

In the last few decades, studies of children raised on farms gave us some of the first clues that early environments can affect allergy risk.

Compared to children raised in cities, children on farms are less likely to have allergic conditions such as eczema and asthma. This is especially true of those in close contact with animals.

Notably, farm-raised children tend to develop a more diverse microbiome than children raised in urban environments. This may help make their immune system more tolerant to foreign substances (such as bacteria and dirt) and less likely to develop allergies.

However, across the world children are increasingly living in urban areas.

This means a pet may be the closest contact they have with animals. So, does this still lower their risk of developing allergies?

What the studies show in eczema

Some studies indicate children with pets may be less likely to have allergies.

However this evidence hasn’t always been easy to interpret.

It can be difficult to tell whether lower allergy rates are due to the pets themselves or other factors, such as location, lifestyle or a family history of allergies.

A review of results from 23 studies found children exposed to dogs early in life were significantly less likely to develop eczema.

Another 2025 study analysed genetic data from more than 270,000 people. It found a gene linked to eczema only increased risk of eczema in children who hadn’t been exposed to dogs.

This suggests early dog exposure may help protect children who are genetically more likely to develop eczema.

What about asthma?

When it comes to asthma, the story gets trickier.

One 2001 study followed more than 1,000 children in the United States from birth to age 13. It found those living with dogs indoors were less likely to develop frequent wheezing – a common asthma symptom – but only if they didn’t have a family history of asthma.

A Korean study from 2021 found those who had dogs during childhood were less likely to develop allergies. But they had a slightly higher risk of non-allergic wheeze — a type of breathing difficulty usually caused by airway irritation or infections (not allergens).

This suggests while growing up with a dog may protect against allergic conditions, such as asthma, it may increase the chance of certain non-allergic respiratory symptoms.

What about cats?

It’s challenging to tease apart the specific effects of cats versus dogs, since many early studies grouped all furry pets together.

But in studies that have looked at them separately, living with cats didn’t seem to reduce allergy risk.

One potential reason is cats and dogs carry very different microbes, which may influence how they shape the household environment.

So, should you get a pet?

If you’re already thinking about getting a dog, there’s decent evidence early exposure could reduce your child’s risk of eczema, and possibly other allergic conditions too.

It’s not a guarantee, but a potential bonus – alongside companionship, joy and never having to worry about what to do with leftovers.

And if a dog’s not on the cards, don’t worry. Spending time outdoors, encouraging messy play, and avoiding overuse of disinfectants can all help build a more resilient immune response.

Samantha Chan, Immunology and Allergy Lead, Snow Centre for Immune Health, WEHI (Walter and Eliza Hall Institute of Medical Research) and Jo Douglass, Professor of Medicine University of Melbourne and Director of Research, Royal Melbourne Hospital., The University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.